JAS-39 Gripen Owners/Build Thread
03-25-2015, 01:28 PM
#1201
Gear Mechanism and Drag Link Modification
The drag link on the Gripen landing gear provides support to the gear strut and prevents over-rotation of the strut rearwards. This is similar to the drag link on the F-16, for example. Of course, the drag link has to fold in order for the gear to retract and this produces a bit of a geometry problem. The scale main landing gear for most (all?) F-16 kits such as BVM, Yellow, Skymaster, or Jet Legend is provided in a single large assembly so the builder only has to bolt in the entire unit and doesn't have to deal with locating the drag link pivot, cylinder mounts, etc, etc.
No so with the Airworld Gripen. The builder has to determine where to mount the drag link pivot, and the gear retract cylinders. This is a major PITA. I spent an untold number of hours working on this and trying to get it scale. One thing that I noticed was that the drag links appeared to be too long and there was no way that I could get the gear to work with the forward pivot in an even semi-scale location. At the time, I didn't have measurements of the full scale, but I took a chance and shortened the upper drag link. Here are the pivot-to-pivot measurements of the full scale:
Using a 1/5 scale factor, the model measurements should be 99mm (3.90") for the forward/upper link and 82.6mm (3.25") for the lower/rear drag link. The AW drag links measure 116mm (4.57") and 84mm (3.31"). The lower/rear link is quite close, but the 17mm (0.67") error in the upper link is a major problem. I had shortened the upper/forward by drilling new pivot pin holes which were 109mm (4.29") apart which made it possible to install the pivot block in a near scale location.
The drag link is not fitted with any form of down stop. In other words, when you start the retraction sequence, the drag link can fold downwards (rather than upwards) and the gear will jam. I had this happen several times when I was testing the gear. My short term solution was to simply zip-tie a rubber band to the forward/upper drag link (near the pivot block and thus just inside the closed gear door so it couldn't be seen) and routed the rubber band up into the gear bay, across the tops of the longerons, and down to the drag link for the other strut. It seemed to solve the problem. However, rubber tends to break at the worst times (like during "visits" or contests) so I needed something a bit more robust.
I further modified the gear by drilling a hole through the lower drag link and press-fitting a 0.077" diameter length of music wire. The end of the upper drag link was filed to provide a downstop for the music wire. A slight notch had to be made on the opposite side of the pivot to allow the drag link to fully fold. Shortening the upper drag link to the scale length reduces the folding angle and the notch probably wouldn't be required.
Regards,
Jim
The drag link on the Gripen landing gear provides support to the gear strut and prevents over-rotation of the strut rearwards. This is similar to the drag link on the F-16, for example. Of course, the drag link has to fold in order for the gear to retract and this produces a bit of a geometry problem. The scale main landing gear for most (all?) F-16 kits such as BVM, Yellow, Skymaster, or Jet Legend is provided in a single large assembly so the builder only has to bolt in the entire unit and doesn't have to deal with locating the drag link pivot, cylinder mounts, etc, etc.
No so with the Airworld Gripen. The builder has to determine where to mount the drag link pivot, and the gear retract cylinders. This is a major PITA. I spent an untold number of hours working on this and trying to get it scale. One thing that I noticed was that the drag links appeared to be too long and there was no way that I could get the gear to work with the forward pivot in an even semi-scale location. At the time, I didn't have measurements of the full scale, but I took a chance and shortened the upper drag link. Here are the pivot-to-pivot measurements of the full scale:
Using a 1/5 scale factor, the model measurements should be 99mm (3.90") for the forward/upper link and 82.6mm (3.25") for the lower/rear drag link. The AW drag links measure 116mm (4.57") and 84mm (3.31"). The lower/rear link is quite close, but the 17mm (0.67") error in the upper link is a major problem. I had shortened the upper/forward by drilling new pivot pin holes which were 109mm (4.29") apart which made it possible to install the pivot block in a near scale location.
The drag link is not fitted with any form of down stop. In other words, when you start the retraction sequence, the drag link can fold downwards (rather than upwards) and the gear will jam. I had this happen several times when I was testing the gear. My short term solution was to simply zip-tie a rubber band to the forward/upper drag link (near the pivot block and thus just inside the closed gear door so it couldn't be seen) and routed the rubber band up into the gear bay, across the tops of the longerons, and down to the drag link for the other strut. It seemed to solve the problem. However, rubber tends to break at the worst times (like during "visits" or contests) so I needed something a bit more robust.
I further modified the gear by drilling a hole through the lower drag link and press-fitting a 0.077" diameter length of music wire. The end of the upper drag link was filed to provide a downstop for the music wire. A slight notch had to be made on the opposite side of the pivot to allow the drag link to fully fold. Shortening the upper drag link to the scale length reduces the folding angle and the notch probably wouldn't be required.
Regards,
Jim
Last edited by rcjets_63; 04-02-2015 at 03:35 PM.
03-25-2015, 02:20 PM
#1202
Gear Mechanism - Down Lock (actually lack there-of)
As I mentioned in the previous post, the Gripen gear design relies on hydraulic lock (trapping incompressible oil in the cylinder) to keep the gear in the fully up and fully down positions. Additionally, there is no down lock on the drag link. This is a problem. Here's why.....
In the illustration below, the main landing gear is shown such that the model is sittling level on the ground. The main gear strut (shown in green) is vertical. All pivot points are represented by red dots. The drag link (pink upper, blue lower) is extended & straight. The wheel isn't quite directly under the pivot point but is slightly forward a distance Lw. The weight of the model acting on the strut and the lever arm Lw results in a moment that tries to rotate the gear forward. The drag link has no down lock and therefore it is the gear cylinder which prevents the gear from rotating and collapsing. The rotation is opposed by the force generated by the landing gear cylinder which acts over a lever arm distance of Lc. Since Lc is small (about 1/4 of Lw) the force on the cylinder is only about 1/4 of the weight on the strut or about 4-5 pounds.
While the force on the cylinder is reasonably small when the model is sitting level on the ground, it is a different story when the model touches down on landing. Here is an illustration of that situation. I've rotated the above illustration by 10 degrees to show the model touching down with a 10 degree nose high AOA. The forces and lever arm distances change sustantially.
Now, the gear strut (green) is not vertical but is tilted back 10 degrees. This results is quite the increase in the lever arm Lw. The landing loads (which are much larger than the weight of the model, at least for too many of my landings) are also increased. This results in a large moment attempting to rotate the gear. The drag link has no down lock so only the cylinder can prevent the gear from collapsing. The cylinder lever arm Lc is less than Lw so the force on the cylinder is several times larger than the landing force. To make matters worse, when the gear is down, the cylinder rod is fully extended which makes the rod susceptible to BENDING.
And that is exactly what happened to my left cylinder rod. Whether it happened on the first or second landing, who knows, but it is bent and needs a replacement from MTH in Austria. Many tanks to Roy Maynard for lending me his cylinders to get my Gripen back in the air.
If the landing gear was properly designed and included a down-lock on the drag link, here is what the illustration would be:
Once again, we have the landing load acting over a lever arm Lw which results in a moment which attempts to rotate the gear. However, now the drag link is locked and opposes the rotation. There is a relatively large distance (L-DL, sorry RCU won't do a subscript) between the strut pivot and the drag link lower pivot so only a small force is applied to the drag link and none to the cylinder.
This is why drag links have down locks. You can see the down lock installed at the pivot point of the full scale Gripen drag link.
This isn't all that difficult to do on a model. Indeed, here's some pics of the locking drag link mechanism on the BVM PNP F-16.
Many thanks to Chris Balling and Boli Muentes for the pics of their F-16's.
Regards,
Jim
aaa
As I mentioned in the previous post, the Gripen gear design relies on hydraulic lock (trapping incompressible oil in the cylinder) to keep the gear in the fully up and fully down positions. Additionally, there is no down lock on the drag link. This is a problem. Here's why.....
In the illustration below, the main landing gear is shown such that the model is sittling level on the ground. The main gear strut (shown in green) is vertical. All pivot points are represented by red dots. The drag link (pink upper, blue lower) is extended & straight. The wheel isn't quite directly under the pivot point but is slightly forward a distance Lw. The weight of the model acting on the strut and the lever arm Lw results in a moment that tries to rotate the gear forward. The drag link has no down lock and therefore it is the gear cylinder which prevents the gear from rotating and collapsing. The rotation is opposed by the force generated by the landing gear cylinder which acts over a lever arm distance of Lc. Since Lc is small (about 1/4 of Lw) the force on the cylinder is only about 1/4 of the weight on the strut or about 4-5 pounds.
While the force on the cylinder is reasonably small when the model is sitting level on the ground, it is a different story when the model touches down on landing. Here is an illustration of that situation. I've rotated the above illustration by 10 degrees to show the model touching down with a 10 degree nose high AOA. The forces and lever arm distances change sustantially.
Now, the gear strut (green) is not vertical but is tilted back 10 degrees. This results is quite the increase in the lever arm Lw. The landing loads (which are much larger than the weight of the model, at least for too many of my landings) are also increased. This results in a large moment attempting to rotate the gear. The drag link has no down lock so only the cylinder can prevent the gear from collapsing. The cylinder lever arm Lc is less than Lw so the force on the cylinder is several times larger than the landing force. To make matters worse, when the gear is down, the cylinder rod is fully extended which makes the rod susceptible to BENDING.
And that is exactly what happened to my left cylinder rod. Whether it happened on the first or second landing, who knows, but it is bent and needs a replacement from MTH in Austria. Many tanks to Roy Maynard for lending me his cylinders to get my Gripen back in the air.
If the landing gear was properly designed and included a down-lock on the drag link, here is what the illustration would be:
Once again, we have the landing load acting over a lever arm Lw which results in a moment which attempts to rotate the gear. However, now the drag link is locked and opposes the rotation. There is a relatively large distance (L-DL, sorry RCU won't do a subscript) between the strut pivot and the drag link lower pivot so only a small force is applied to the drag link and none to the cylinder.
This is why drag links have down locks. You can see the down lock installed at the pivot point of the full scale Gripen drag link.
This isn't all that difficult to do on a model. Indeed, here's some pics of the locking drag link mechanism on the BVM PNP F-16.
Many thanks to Chris Balling and Boli Muentes for the pics of their F-16's.
Regards,
Jim
aaa
Last edited by rcjets_63; 03-25-2015 at 02:38 PM.
03-27-2015, 10:50 AM
#1203
Scale Details - Engine Bay Louvers
I guess the new trend is to simply have these 3D printed but I did it the old fashioned way and built the louvers from 0.040" thick G-10 sheet. The circular louver was done by laminating 1/4" balsa to the G-10 using canopy glue, sanding to 80 degrees to put the louvers on an angle, sanding the lamination into a 32mm diameter circle, and then wrapping it with two turns of 0.005" G-10. The balsa is then carefully removed. The straight triple louver is about 7/16" wide x 2" long, with louvers at a 30 degree angle, and used 3/16 balsa to get the louvers at the right spacing.
I also made a change to the heat shield. I previously had installed a ceramic blanket but it got soaked with fuel when I had left the model upside down with a bit of fuel in the tank. As the temp warmed up and the air in the tank expanded, fuel dribbled out of the vent and soaked the blanket. I really do need to make a vent plug as a long-term solution. The blanket was replaced with a piece of G-10 and the upper longeron was wrapped with heat shield aluminum tape.
Regards,
Jim
I guess the new trend is to simply have these 3D printed but I did it the old fashioned way and built the louvers from 0.040" thick G-10 sheet. The circular louver was done by laminating 1/4" balsa to the G-10 using canopy glue, sanding to 80 degrees to put the louvers on an angle, sanding the lamination into a 32mm diameter circle, and then wrapping it with two turns of 0.005" G-10. The balsa is then carefully removed. The straight triple louver is about 7/16" wide x 2" long, with louvers at a 30 degree angle, and used 3/16 balsa to get the louvers at the right spacing.
I also made a change to the heat shield. I previously had installed a ceramic blanket but it got soaked with fuel when I had left the model upside down with a bit of fuel in the tank. As the temp warmed up and the air in the tank expanded, fuel dribbled out of the vent and soaked the blanket. I really do need to make a vent plug as a long-term solution. The blanket was replaced with a piece of G-10 and the upper longeron was wrapped with heat shield aluminum tape.
Regards,
Jim
Last edited by rcjets_63; 03-31-2015 at 11:33 AM.
03-27-2015, 03:26 PM
#1204
Hi Jim,
Cool thread - I check what you're doing every once in a while...
G-10 is great material - I think it's ideal for things like louvers. Since it has plenty of "fiber!", the webs will be stronger than if it were 3D printed or made from SLA, which basically yields you a "resin" part. When is next test flight?
Tucson was great - very nice weather last week... You missed a fun one!
Cool thread - I check what you're doing every once in a while...
G-10 is great material - I think it's ideal for things like louvers. Since it has plenty of "fiber!", the webs will be stronger than if it were 3D printed or made from SLA, which basically yields you a "resin" part. When is next test flight?Tucson was great - very nice weather last week... You missed a fun one!
03-27-2015, 06:03 PM
#1205
Ron,
The next test flights are tomorrow; I'm loading the vehicle up now and am headed to the lakebed early. It's quite the drive but well worth it.
Glad you had fun at Tucson. First one I've missed since '08 but I had to get the Gripen in the air. Top Gun will be here too soon.
Jim
The next test flights are tomorrow; I'm loading the vehicle up now and am headed to the lakebed early. It's quite the drive but well worth it.
Glad you had fun at Tucson. First one I've missed since '08 but I had to get the Gripen in the air. Top Gun will be here too soon.
Jim
03-28-2015, 10:11 PM
#1207
Three more Gripen flights today out at the lake bed. The landing gear retracted/extended without any issues. It was really cool to fly it around and get it trimmed with the gear up. I even did a 10ft "burner" pass that was pretty cool.
I didn't make any major improvements to the Gripen Leap take offs though I went easy on the throttle and throws. Indeed, it was difficult to get the plane to rotate at all. I'll try again next weekend.
The landings were better. The trick seems to be to do a low approach and bring it in with power. Nice gentle touchdowns.
The sequencer over-ride switch made it a lot easier to service the jet.
Regards,
Jim
I didn't make any major improvements to the Gripen Leap take offs though I went easy on the throttle and throws. Indeed, it was difficult to get the plane to rotate at all. I'll try again next weekend.
The landings were better. The trick seems to be to do a low approach and bring it in with power. Nice gentle touchdowns.
The sequencer over-ride switch made it a lot easier to service the jet.
Regards,
Jim
Last edited by rcjets_63; 03-28-2015 at 10:13 PM.
03-29-2015, 04:33 AM
#1208
Good to hear Jim about the gear and more successful flight time. Anxious for you to share the geometry issues. Be nice to see flying footage if you have any recorded from the second time at the lake bed. Hopefully we can get Pedro's concept on the canards working.
Roy
Roy
03-29-2015, 08:33 AM
#1210
I brought a video camera with me hoping that one of the locals would be at the lake bed and could video for me but no joy. I propped it up on a work bench but only got a couple of seconds of a take off run on camera.
I'll be happy to share the control throw movements once I get them finalized. I posted my initial set up and the changes to the canards already.
Jim
I'll be happy to share the control throw movements once I get them finalized. I posted my initial set up and the changes to the canards already.
Jim
03-31-2015, 09:54 AM
#1211
HPU Charge Fitting
I figured that the reason the landing gear did not retract on the second flight was due to low system pressure. I had charged the accumulator to 150psi several days before the flight but did not recharge it before flying. There is very little volume in the accumulator so a small leak could have easily dropped the pressure. This past weekend, I chargered the accumulator a few minutes before takeoff and the gear retracted/extended without a problem.
The HPU comes with a short length of 4mm tubing on the accumulator's barbed fitting. The line is "sealed" by installing a check valve on the opposite end. Because of the short length of 4mm tubing and the use of the check valve, it is necessary to get both hands inside the jet to attach the compressor for charging. This quickly became a pain, not to mention that the check valve my leak, so I wanted to make the charging process a bit more user friendly. My air compressor is fitted with regular 1/16" air line (which is too small to go over the accumulator barbed fitting) so I made a 4mm to 1/16 tubing adaptor from a couple brass Y-fittings (one for fuel, one for air, both had the same nipple threads) from previously built models. I soldered the nipples to the hexagon center hub to eliminate potential air leaks. The airline is connected to a BVM fill valve which is mounted at a convenient location just inside the fuselage at the left inlet. The hydraulic system and now be easily charged
Regards,
Jim
I figured that the reason the landing gear did not retract on the second flight was due to low system pressure. I had charged the accumulator to 150psi several days before the flight but did not recharge it before flying. There is very little volume in the accumulator so a small leak could have easily dropped the pressure. This past weekend, I chargered the accumulator a few minutes before takeoff and the gear retracted/extended without a problem.
The HPU comes with a short length of 4mm tubing on the accumulator's barbed fitting. The line is "sealed" by installing a check valve on the opposite end. Because of the short length of 4mm tubing and the use of the check valve, it is necessary to get both hands inside the jet to attach the compressor for charging. This quickly became a pain, not to mention that the check valve my leak, so I wanted to make the charging process a bit more user friendly. My air compressor is fitted with regular 1/16" air line (which is too small to go over the accumulator barbed fitting) so I made a 4mm to 1/16 tubing adaptor from a couple brass Y-fittings (one for fuel, one for air, both had the same nipple threads) from previously built models. I soldered the nipples to the hexagon center hub to eliminate potential air leaks. The airline is connected to a BVM fill valve which is mounted at a convenient location just inside the fuselage at the left inlet. The hydraulic system and now be easily charged
Regards,
Jim
Last edited by rcjets_63; 03-31-2015 at 11:33 AM.
03-31-2015, 10:12 AM
#1212
Main Landing Gear - Inner Doors
Now that the gear is working, I figured I'd sort out the MLG inner gear doors. As you may recall, I wasn't happy with the fit of the kit-supplied doors and the doors were too small as the main doors had to be trimmed as they were interfering with the drag link which had been installed in a more scale location. I "splashed" the fuselage to make a mold for new inner doors and made new doors using 3 layers of 4oz cloth.
It looked like the main gear door might interfere with the laming lights that I am about to install on the front face of the landing gear so I trimmed away a bit more of the aft edge of the main doors to provide the needed clearance and trimmed the aft inboard edge of the main door to the scale shape. The inner doors were then trimmed to shape.
Here are pics of the doors on the fuselage and a pic of the inner door I molded compared to the kit-supplied inner door.
The new inner gear door, as shown, needs to be stiffened by laminating a layer of 3/32" balsa to the inside face and then glassing the balsa. That's on tonight's to-do list. Then the door can be mounted and the linkage between the door and the strut be developed.
Regards,
Jim
Now that the gear is working, I figured I'd sort out the MLG inner gear doors. As you may recall, I wasn't happy with the fit of the kit-supplied doors and the doors were too small as the main doors had to be trimmed as they were interfering with the drag link which had been installed in a more scale location. I "splashed" the fuselage to make a mold for new inner doors and made new doors using 3 layers of 4oz cloth.
It looked like the main gear door might interfere with the laming lights that I am about to install on the front face of the landing gear so I trimmed away a bit more of the aft edge of the main doors to provide the needed clearance and trimmed the aft inboard edge of the main door to the scale shape. The inner doors were then trimmed to shape.
Here are pics of the doors on the fuselage and a pic of the inner door I molded compared to the kit-supplied inner door.
The new inner gear door, as shown, needs to be stiffened by laminating a layer of 3/32" balsa to the inside face and then glassing the balsa. That's on tonight's to-do list. Then the door can be mounted and the linkage between the door and the strut be developed.
Regards,
Jim
Last edited by rcjets_63; 03-31-2015 at 10:36 AM.
03-31-2015, 10:34 AM
#1213
Super-Light Main Wheel Tires
Thomas White did an outstanding job on the custom scale wheel rims and tires (to replace the less-than-scale kit supplied main wheels and recommended Dubro nose-wheels). The main tires were a bit heavier than the Airworld foam tires so I asked him to make me "Jet World Masters tires" which would be light weight but less durable and would only need to last a limited number of flights. Thomas came through and cut 228 grams or just over a half pound out of the weight of a pair of tires. That gives me another 1/2 pound of margin between the current weight of the plane (41.6 lbs with 200 gram 1/5 scale pilot figure) and the JWM 44.0 lb max dry weight.
Regards,
Jim
Thomas White did an outstanding job on the custom scale wheel rims and tires (to replace the less-than-scale kit supplied main wheels and recommended Dubro nose-wheels). The main tires were a bit heavier than the Airworld foam tires so I asked him to make me "Jet World Masters tires" which would be light weight but less durable and would only need to last a limited number of flights. Thomas came through and cut 228 grams or just over a half pound out of the weight of a pair of tires. That gives me another 1/2 pound of margin between the current weight of the plane (41.6 lbs with 200 gram 1/5 scale pilot figure) and the JWM 44.0 lb max dry weight.
Regards,
Jim
04-01-2015, 08:36 AM
#1214
Main Landing Gear - Inner Gear Doors (Cont'd)
Further to post #1212 above, I finished the construction of the main gear inner doors. The doors are basically a sandwich panel construction with the outer skin being 3 layers of 4 oz cloth/resin. Originally I was going to use 3/32" balsa as a core material but I didn't have any light (and easily bendable) balsa so instead I used blue foam which was cut into thin strips using a bandsaw. The foam was glued to the skin using spray adhesive and the edges were beveled. The inner skin is a layer of 0.5 oz cloth/resin over the foam, beveled edges, and down onto the inner side of the outboard skin. This construction greatly added to the stiffnes of the door yet added only a few grams of weight.
Superfil lightweight 2-part filler material (available from Aircraft Spruce) was used around the periphery of the cutout to blend the fuselage contor into the doors.
Regards,
Jim
Further to post #1212 above, I finished the construction of the main gear inner doors. The doors are basically a sandwich panel construction with the outer skin being 3 layers of 4 oz cloth/resin. Originally I was going to use 3/32" balsa as a core material but I didn't have any light (and easily bendable) balsa so instead I used blue foam which was cut into thin strips using a bandsaw. The foam was glued to the skin using spray adhesive and the edges were beveled. The inner skin is a layer of 0.5 oz cloth/resin over the foam, beveled edges, and down onto the inner side of the outboard skin. This construction greatly added to the stiffnes of the door yet added only a few grams of weight.
Superfil lightweight 2-part filler material (available from Aircraft Spruce) was used around the periphery of the cutout to blend the fuselage contor into the doors.
Regards,
Jim
Last edited by rcjets_63; 04-01-2015 at 08:41 AM.
04-02-2015, 01:40 PM
#1215
Main Landing Gear - Inner Doors (Cont'd) - Hinging
It's pretty tough to get even half decent pics of these hinges on the full-scale Gripen. On the ground, the drag link or gear strut often block the view of the hinges while flight shots generally don't have enough resolution to see any detail of the region. Here are some of my best pics of the hinges:
In the flight shot, the hinges cutouts seem to be blurry rectangular-shaped dark spots of different sizes. The rectangle is longer fore/aft than inboard/outboard. The middle hinge cutout appears to be larger than the others. You can't really tell much more, and some of the dark color may be weathering. However, the shot is great for giving the relative location of the hinges.
Here is a cropped photo of the Gripen up on jacks with the gear retracted. You wont find that pic on Airliners.net or other walkaround sites so you might want to save it. Here you have enough resolution to see that the skin cutouts are retangles but the longer dimension is inboard/outboard (not fore/aft as the flight shot would have you believe). There reinforcement plates around the hinge cutouts with slight blisters on the fore/aft side and some serious weathering, particularly on the middle hinge.
The hinges themselves can be seen in these pics:
The hinges have two tabs (like a clevis) on the door side and a single tab on the fuselage side and the hinges are mounted on the inner surface of the gear door. There really isn't any available hinge that is even close to scale but there seem to be two approaches to this as follows and the builder has their choice as follows:
Using Robart Hi-Point Hinges as done by Janne. All it all this is simple and easy. The cutout is the wrong shape, but it looks like the right shape (at least per the flight shot) so it's genius:
Or, you can make the blade style hinges from G-10 similar to the outer door hinges. I waffled, overthought, etc and finally decided to do the blade style since the slot shape was more correct, the portion of the hinge which is visible on the inside face of the door would more closely resemble the full scale. I made the hinges from .060" thick G-10 sheet which was cut into strips 1/4" wide and 3/4" long. A Sullivan Gold-N-Rod was CA'd to the fuselage skin and a length of of 1/16" music wire is the hinge pin.
Tonight's effort will be to make the door-strut mechanical linkage that closes the door as the gear is raised.
Regards,
Jim
It's pretty tough to get even half decent pics of these hinges on the full-scale Gripen. On the ground, the drag link or gear strut often block the view of the hinges while flight shots generally don't have enough resolution to see any detail of the region. Here are some of my best pics of the hinges:
In the flight shot, the hinges cutouts seem to be blurry rectangular-shaped dark spots of different sizes. The rectangle is longer fore/aft than inboard/outboard. The middle hinge cutout appears to be larger than the others. You can't really tell much more, and some of the dark color may be weathering. However, the shot is great for giving the relative location of the hinges.
Here is a cropped photo of the Gripen up on jacks with the gear retracted. You wont find that pic on Airliners.net or other walkaround sites so you might want to save it. Here you have enough resolution to see that the skin cutouts are retangles but the longer dimension is inboard/outboard (not fore/aft as the flight shot would have you believe). There reinforcement plates around the hinge cutouts with slight blisters on the fore/aft side and some serious weathering, particularly on the middle hinge.
The hinges themselves can be seen in these pics:
The hinges have two tabs (like a clevis) on the door side and a single tab on the fuselage side and the hinges are mounted on the inner surface of the gear door. There really isn't any available hinge that is even close to scale but there seem to be two approaches to this as follows and the builder has their choice as follows:
Using Robart Hi-Point Hinges as done by Janne. All it all this is simple and easy. The cutout is the wrong shape, but it looks like the right shape (at least per the flight shot) so it's genius:
Or, you can make the blade style hinges from G-10 similar to the outer door hinges. I waffled, overthought, etc and finally decided to do the blade style since the slot shape was more correct, the portion of the hinge which is visible on the inside face of the door would more closely resemble the full scale. I made the hinges from .060" thick G-10 sheet which was cut into strips 1/4" wide and 3/4" long. A Sullivan Gold-N-Rod was CA'd to the fuselage skin and a length of of 1/16" music wire is the hinge pin.
Tonight's effort will be to make the door-strut mechanical linkage that closes the door as the gear is raised.
Regards,
Jim
Last edited by rcjets_63; 04-02-2015 at 02:46 PM.
04-03-2015, 09:10 AM
#1216
Main Landing Gear - Inner Doors (Cont'd) - Linkage
While the large main gear door is operated by a cylinder, the smaller inner gear door has a mechanical linkage between the door and the landing gear strut. The geometry involved is pretty complex as the strut rotates along one axis at about 45 degrees on roll on the station line plane while the door rotates on a mostly fore/aft axis. The trick is to come up with a linkage that doesn't bind at any point in the movement. Here are some pics of the full scale Gripen linkage:
As for the model, there certainly is no guarantee that the landing gear strut size/shape/geometry is exactly scale, or precisely at the scale location, or that the gear door and hinge point is exactly scale, and all of these factors will influence where the linkage needs to be installed. The strut has no ready-made attachment point for the linkage so you have basically nothing to go on, and then there is the issue of what fittings (clevis, rod ends, ball links) to use on each end.
While the full-scale is likely different from the model, I needed to start somewhere. The full scale uses rod ends but I didn't have any micro rod ends so I chose ball links as which are shorter and provide a greater range of movement than clevises. I installed 1/16" Dubro ball links on some small pieces of G-10, spot glued them into near scale positions on the door and strut, linked them with a 2-56 threaded rod, and manually swung the gear. Wow, it worked...almost. There was a bit of binding at on link and the door didn't quite close all the way. OK, more door movement needed so relocate the attachment point on the strut a bit further down, lengthen the threaded rod, and swing the gear again. Too much movement! OK, split the difference, move the strut attachment point up half the distance it was moved down, adjust the threaded rod, swing the gear....OMG, it worked!
Now that I had a working geometry, I needed to strengthen up the whole assembly. I spot glued strips of G-10 to the strut to act as a dam/mold and simply filled the area behind the ball link mounting plate with V-poxy. After the glue hardened the G-10 strips were removed and the glue was sanded to shape. The process was repeated for the door attachment, but foam behind the ball link mounting plate was cut away so the V-poxy would tie the plate directly to the door skin.
Here is a video showing the operation of the door:
https://www.youtube.com/watch?app=desktop&persist_app=1&v=tMMACc63Rbc Regards,
Jim
.
While the large main gear door is operated by a cylinder, the smaller inner gear door has a mechanical linkage between the door and the landing gear strut. The geometry involved is pretty complex as the strut rotates along one axis at about 45 degrees on roll on the station line plane while the door rotates on a mostly fore/aft axis. The trick is to come up with a linkage that doesn't bind at any point in the movement. Here are some pics of the full scale Gripen linkage:
As for the model, there certainly is no guarantee that the landing gear strut size/shape/geometry is exactly scale, or precisely at the scale location, or that the gear door and hinge point is exactly scale, and all of these factors will influence where the linkage needs to be installed. The strut has no ready-made attachment point for the linkage so you have basically nothing to go on, and then there is the issue of what fittings (clevis, rod ends, ball links) to use on each end.
While the full-scale is likely different from the model, I needed to start somewhere. The full scale uses rod ends but I didn't have any micro rod ends so I chose ball links as which are shorter and provide a greater range of movement than clevises. I installed 1/16" Dubro ball links on some small pieces of G-10, spot glued them into near scale positions on the door and strut, linked them with a 2-56 threaded rod, and manually swung the gear. Wow, it worked...almost. There was a bit of binding at on link and the door didn't quite close all the way. OK, more door movement needed so relocate the attachment point on the strut a bit further down, lengthen the threaded rod, and swing the gear again. Too much movement! OK, split the difference, move the strut attachment point up half the distance it was moved down, adjust the threaded rod, swing the gear....OMG, it worked!
Now that I had a working geometry, I needed to strengthen up the whole assembly. I spot glued strips of G-10 to the strut to act as a dam/mold and simply filled the area behind the ball link mounting plate with V-poxy. After the glue hardened the G-10 strips were removed and the glue was sanded to shape. The process was repeated for the door attachment, but foam behind the ball link mounting plate was cut away so the V-poxy would tie the plate directly to the door skin.
Here is a video showing the operation of the door:
https://www.youtube.com/watch?app=desktop&persist_app=1&v=tMMACc63Rbc Regards,
Jim
.
Last edited by rcjets_63; 04-03-2015 at 09:16 AM.
04-04-2015, 08:44 AM
#1218
Hi Sean,
Thanks very much. It was a challenge but fortunately it was the last bit of engineering that needed to be done on this build. The plane is pretty much done except for the sanding, filling, and painting which is what I'll be doing all weekend.
To answer your question, nope, no wax was needed. I carefully peeled the G-10 away from the V-poxy when the glue was still a bit "rubbery".
Regards,
Jim
Thanks very much. It was a challenge but fortunately it was the last bit of engineering that needed to be done on this build. The plane is pretty much done except for the sanding, filling, and painting which is what I'll be doing all weekend.
To answer your question, nope, no wax was needed. I carefully peeled the G-10 away from the V-poxy when the glue was still a bit "rubbery".
Regards,
Jim
04-06-2015, 10:19 AM
#1219
Scale Details - Gun Exhaust Louvers
It was a long long weekend of puttying, sanding, priming, and repeat but I took a little break to make the louvers on the underside of the gun blister. I had received a PM asking me how to make louvers so here is the step by step. Warning, some math is required....
The first thing you will need is a good photo of the full scale louvers and a measurement to know how big they will be on the model. Here is a pic of the gun louvers and the location on the full scale. I was able to scale the photo to determine that the cutout in the model needed to be 7/8" x 2-7/8".
Looking at the photo of the louvers, you will see that there is the front plate, 15 spaces, and 15 angled louvers. The louvers appear to be on approximately a 25 degree angle. The pitch of the louvers is 2.875" / 15 = 0.192" measured along the fore/aft axis. However, the louvers are at a 25 degree angle which gives 0.191 x cosine (25 degrees) = 0.192 x 0.906 = .174". On other words, the thickness of the G-10 and spacer need to add up to 0.174". I estimated the thickness of the full scale louver at 0.150" so I used .030" thick G-10. This leaves 0.144" left over as the thickness of the balsa AND the GLUE. Using .125" balsa and estimating the thickness of a layer of glue (on each side of the balsa) as .005" gives a stack of .030" G-10 louver + .005" glue + .125" balsa + .005" glue which adds up to 0.165" which is .009" shy of the required .174". That would probably be close enough for most louvers but since this assembly has 15 louvers, an error of .009" made 15 times would result in a .135" overall error in length. As such, I needed a thicker spacer than .125" balsa. I added a layer of .010" G-10 which then requires another layer of glue, but changed the estimate to .003" for a thin layer of glue. This gives a stack of .030" G-10 louver + .003" glue + .125" balsa + .003" glue + .010" G-10 spacer + .003" glue = .174"
Next, I cut out the pieces of G-10 and balsa in the stack. The .030" pieces of G-10 were cut into strips 1-1/4" long (not 7/8") and 3/8" wide. The balsa and .010" G-10 was cut into strips 1-1/4" long and 1/2" wide (G-10 spaces should have been ~3/4" long to simplify gluing). The long end of the balsa strips was then sanded to a 25 degree angle using a disc sander
Next, all the pieces were glued together using canopy glue (which can be later scraped away).
Once the glue had dried, the laminate is sanded to the correct width using a disc sander. The a Dremel is used to grind away the ends of the balsa to expose the ends of the G-10 for gluing with CA. I wish I'd remembered to cut the .010" thick G-10 spacers shorter (~3/4") as I was likely going to get some CA on the G-10 spacers.
Strips of .030" thick G-10 x 3" long x 3/8" wide were then glued to the sides of the laminate using THICK CA. This gives a lot of strength to the assembly.
The face of the assembly is then sanded to even out the edges of the louvers. The balsa spacers are then removed via Dremel and X-acto. The .010" thick G-10 spacers (which were cut wider than the louvers so they could be grabbed with needle nose pliers) were snapped off and any canopy glue residue was scraped off the louvers.
The gun louvers are a bit more complicated as the ends of each louver have been notched. I sanded the notch in the louvers, sanded away part of the side pieces to allow the louvers to stick up through thickness of the fiberglass fusleage, and trial fitted the assembly to the model. The gun louvers also have two fore/aft louvers and a horizontal piece at the front. In order to support these front louvers while they were being cut, a couple of pieces of scrap balsa were sanded to thickness and glued into the spaces between the front louvers
The assembly was then cut using a band saw and the front fore/aft louvers and horizontal pieces were glued into postion. The forward end was then sanded even and the front end piece was CA'd in place. Beads of thick CA were added to simulate the visible welds at the front of the full-scale louvers
Finally, the completed louvers were glued into place on the model.
Regards,
Jim
It was a long long weekend of puttying, sanding, priming, and repeat but I took a little break to make the louvers on the underside of the gun blister. I had received a PM asking me how to make louvers so here is the step by step. Warning, some math is required....
The first thing you will need is a good photo of the full scale louvers and a measurement to know how big they will be on the model. Here is a pic of the gun louvers and the location on the full scale. I was able to scale the photo to determine that the cutout in the model needed to be 7/8" x 2-7/8".
Looking at the photo of the louvers, you will see that there is the front plate, 15 spaces, and 15 angled louvers. The louvers appear to be on approximately a 25 degree angle. The pitch of the louvers is 2.875" / 15 = 0.192" measured along the fore/aft axis. However, the louvers are at a 25 degree angle which gives 0.191 x cosine (25 degrees) = 0.192 x 0.906 = .174". On other words, the thickness of the G-10 and spacer need to add up to 0.174". I estimated the thickness of the full scale louver at 0.150" so I used .030" thick G-10. This leaves 0.144" left over as the thickness of the balsa AND the GLUE. Using .125" balsa and estimating the thickness of a layer of glue (on each side of the balsa) as .005" gives a stack of .030" G-10 louver + .005" glue + .125" balsa + .005" glue which adds up to 0.165" which is .009" shy of the required .174". That would probably be close enough for most louvers but since this assembly has 15 louvers, an error of .009" made 15 times would result in a .135" overall error in length. As such, I needed a thicker spacer than .125" balsa. I added a layer of .010" G-10 which then requires another layer of glue, but changed the estimate to .003" for a thin layer of glue. This gives a stack of .030" G-10 louver + .003" glue + .125" balsa + .003" glue + .010" G-10 spacer + .003" glue = .174"
Next, I cut out the pieces of G-10 and balsa in the stack. The .030" pieces of G-10 were cut into strips 1-1/4" long (not 7/8") and 3/8" wide. The balsa and .010" G-10 was cut into strips 1-1/4" long and 1/2" wide (G-10 spaces should have been ~3/4" long to simplify gluing). The long end of the balsa strips was then sanded to a 25 degree angle using a disc sander
Next, all the pieces were glued together using canopy glue (which can be later scraped away).
Once the glue had dried, the laminate is sanded to the correct width using a disc sander. The a Dremel is used to grind away the ends of the balsa to expose the ends of the G-10 for gluing with CA. I wish I'd remembered to cut the .010" thick G-10 spacers shorter (~3/4") as I was likely going to get some CA on the G-10 spacers.
Strips of .030" thick G-10 x 3" long x 3/8" wide were then glued to the sides of the laminate using THICK CA. This gives a lot of strength to the assembly.
The face of the assembly is then sanded to even out the edges of the louvers. The balsa spacers are then removed via Dremel and X-acto. The .010" thick G-10 spacers (which were cut wider than the louvers so they could be grabbed with needle nose pliers) were snapped off and any canopy glue residue was scraped off the louvers.
The gun louvers are a bit more complicated as the ends of each louver have been notched. I sanded the notch in the louvers, sanded away part of the side pieces to allow the louvers to stick up through thickness of the fiberglass fusleage, and trial fitted the assembly to the model. The gun louvers also have two fore/aft louvers and a horizontal piece at the front. In order to support these front louvers while they were being cut, a couple of pieces of scrap balsa were sanded to thickness and glued into the spaces between the front louvers
The assembly was then cut using a band saw and the front fore/aft louvers and horizontal pieces were glued into postion. The forward end was then sanded even and the front end piece was CA'd in place. Beads of thick CA were added to simulate the visible welds at the front of the full-scale louvers
Finally, the completed louvers were glued into place on the model.
Regards,
Jim
Last edited by rcjets_63; 04-06-2015 at 10:23 AM.
04-06-2015, 10:36 AM
#1220
In other weekend work, I managed to get the speedbrakes blended into the cutout and fuselage contor. This took a combination of Superfil, 2 part Bondo putty, and darn near infinite patience. The upper hinge fairing was a particular challenge.
I also got the backup pitot tube mounted. The rear of the pitot has a short 4-40 rod which threads into an aluminum cylinder glued into the fin. This both allows the pitot to be safely removed and also allows the fiberglass to be sanded down to a feather edge to blend into the metal pitot tube.
Regards,
Jim
I also got the backup pitot tube mounted. The rear of the pitot has a short 4-40 rod which threads into an aluminum cylinder glued into the fin. This both allows the pitot to be safely removed and also allows the fiberglass to be sanded down to a feather edge to blend into the metal pitot tube.
Regards,
Jim
Last edited by rcjets_63; 04-06-2015 at 10:47 AM.
04-06-2015, 10:44 AM
#1221
Pilot Figure
Having a pilot figure in the cockpit is mandatory at both Top Gun and the JWM. It makes sense as the plane just doesn't look right without one. I tend to see the 12" tall Blue Box Elite pilot figures used which is great if you have a 1/6th scale plane. The AW Gripen is 1/5th scale and the Blue Box pilot would look like a dwarf. However, YT International makes a 1/5 scale Euro jet pilot. I warn you though, they are not cheap and the lead time is about 3-4 months so plan you order well in advance. However, Wilson at YT does as great job and the pilots look fantasic. My guy even had a custom shoulder patch and name tag (Roman Svoboda - Czech Air Force) based on photos I sent Wilson.
Roman will be shipped tomorrow by courier and I will have him strapped into the Gripen in time to fly this coming weekend. Gotta get in some practice for Top Gun.
Regards,
Jim
Having a pilot figure in the cockpit is mandatory at both Top Gun and the JWM. It makes sense as the plane just doesn't look right without one. I tend to see the 12" tall Blue Box Elite pilot figures used which is great if you have a 1/6th scale plane. The AW Gripen is 1/5th scale and the Blue Box pilot would look like a dwarf. However, YT International makes a 1/5 scale Euro jet pilot. I warn you though, they are not cheap and the lead time is about 3-4 months so plan you order well in advance. However, Wilson at YT does as great job and the pilots look fantasic. My guy even had a custom shoulder patch and name tag (Roman Svoboda - Czech Air Force) based on photos I sent Wilson.
Roman will be shipped tomorrow by courier and I will have him strapped into the Gripen in time to fly this coming weekend. Gotta get in some practice for Top Gun.
Regards,
Jim
Last edited by rcjets_63; 04-06-2015 at 10:53 AM.
04-09-2015, 09:18 AM
#1225
Elevator/Fuselage Junction
The forward inboard corner of the elevator blends into a fairing molded into the fuselage. In the full-scale Gripen, the fairing houses the elevator hydraulic actuator. The blister on the elevator serves as the attachment point for the actuator rod and the front edge of the blister is open with the edge visible when the hydraulics are off and the elevator is deflected down. Here's a pic:
In the case of the model, this junction is quite a challenging feature. The elevator and fairing have to be carefully trimmed to provide just enough of a gap (when the elevator is deflected down) that you can get the wings on/off but no so much as to provide a large & unsightly gap. When installing the elevator hingeson the wings, the builder needs to be careful to first mark the required inboard/outboard location of the elevator such that the blister is perectly centered in the fairing. The rear edge of the fairing needs to be filed to an angle to minimize the elevator hinge gap. Unfortunately, the gel-coat and fiberglass fuselage is thin & brittle and the edge will easily chip.
To repair the damage and provide a stronger egde, I CA'd a 1/2" wide x .010 thick G-10 strip to the fuselage. Another piece of G-10 was used as a spacer during the gluing process.
The G-10 was trimmed to the scale shape (per the first photo) and, in order to improve the fit between the elevator blister and the fairing some Superfil (blue filler seen in photo above) was added to the sides of the blister. More filler needed to be added to the front of the blister where it fits into the fairing. Once it was primed, it looked much better than the original fit.
Aft the primer dried, a bit more filler was added to the inboard side of the blister to further improve the look of the junction.
.
Regards,
Jim
The forward inboard corner of the elevator blends into a fairing molded into the fuselage. In the full-scale Gripen, the fairing houses the elevator hydraulic actuator. The blister on the elevator serves as the attachment point for the actuator rod and the front edge of the blister is open with the edge visible when the hydraulics are off and the elevator is deflected down. Here's a pic:
In the case of the model, this junction is quite a challenging feature. The elevator and fairing have to be carefully trimmed to provide just enough of a gap (when the elevator is deflected down) that you can get the wings on/off but no so much as to provide a large & unsightly gap. When installing the elevator hingeson the wings, the builder needs to be careful to first mark the required inboard/outboard location of the elevator such that the blister is perectly centered in the fairing. The rear edge of the fairing needs to be filed to an angle to minimize the elevator hinge gap. Unfortunately, the gel-coat and fiberglass fuselage is thin & brittle and the edge will easily chip.
To repair the damage and provide a stronger egde, I CA'd a 1/2" wide x .010 thick G-10 strip to the fuselage. Another piece of G-10 was used as a spacer during the gluing process.
The G-10 was trimmed to the scale shape (per the first photo) and, in order to improve the fit between the elevator blister and the fairing some Superfil (blue filler seen in photo above) was added to the sides of the blister. More filler needed to be added to the front of the blister where it fits into the fairing. Once it was primed, it looked much better than the original fit.
Aft the primer dried, a bit more filler was added to the inboard side of the blister to further improve the look of the junction.
.
Regards,
Jim