another Babcock Aeronca
12-13-2009, 12:59 PM
#26
Thread Starter
RE: another Babcock Aeronca
This turns out to be a real nostalgia project for me. Not only that the model is from a time when I was not yet able to build it. I'm digging up some old tools I kept from the sixties and they are of good use here. Remember when the fine-lead mechanical pencils came out? Today I'd use one any time to mark the glueing position of the parts, but we didn't have them in the fifties. The clamp is very modern, of course, we used to use clothes-pegs. But the contact cement already existed (even this brand) and was an exciting new invention, as we know from the Babcock assembly instructions.
The stab is glued and turned out well, even though there are some boo-boos, too. I didn't manage to pull the main spar completely forward to the "hinge edge", there was just no point to pull on. It's a bit aslant but hardly visible. Another misalignment is that the grooves (bracings) on stab and elevator are not exactly in-line. The main spar had been fettled and glued in exactly centered, so I put that down to Babcock. Both misalignments are not harmful, anyway.
Found a 30 years old hairdryer (good old German quality) and tried for the bending job. It's OK and doesn't need temperature checking since it produces the right temperature, anyway. It's too hot for my fingers (used a wooden gripper to hold the rudder) but just right for the plastic. It got soft so I could pre-bend the rudder. I had cut two slits into the leading edge and glued in two CA hinges before closing the two halves. There is a "step" on the rudder's tip where not only bending but also stretching would have been needed, and the plastic cracked there. I found the color of the plastic too lightened and was afraid it would crack despite heating, so I decided to try plastic cement to soften the bending line in addition. That turned out very well.
By the way, found a matching color Humbrol enamel in my plastic modeling inventory. Will be of good use for the main landing gear. 1-3/4" wheels are replaced by 2" ones, still only 0.25 oz each.
It got dark early and there's weird white stuff on my roof deck, so no outdoors glueing. Thought a bit CA on the rudder won't hurt, but it did. Oh well, I should wait. Tried CA for the rudder since it's so small and clamping is no problem. Again I found the contact cement would have been even better. It's really convenient to apply the CA and have the parts fix-glued shortly after, but the problem is to align and straighten the parts during the short setting time.
With contact cement, I just applied it to both halves of the stab (the seams around, the reinforcement, and the main spar) and immediately started joining them. I had planned the process before and marked some key positions, and I laid something between the halves to hold them apart. One after another, I attached the edges, starting with the main spar at the elevator side, took out the distance holders one after the other and finally closed the whole stab. Checked alignment and straightness and corrected by slightly bending the whole thing. Finally fastened the seams by pressing them, holding them on the edge of the building board and pressing with my thumbnail.
It would have been not that hard to build a rig from three balsa spars supporting the left and right leading edge and the left trailing edge. Glueing with the super CA glue would leave enough time to align the parts. But it would need three other spars to press the upper half onto the lower half while the glue sets or the seams (leading/trailing edges) would come out wavy due to the internal stress of the plastic. That's not a bad sign, though, since it shows that the plastic is still good after 50 years. When it still has internal stress it can't be very brittle, either. Anyway, you may see for which reasons I start to prefer the contact cement, except for less noxiousness.
The stab is glued and turned out well, even though there are some boo-boos, too. I didn't manage to pull the main spar completely forward to the "hinge edge", there was just no point to pull on. It's a bit aslant but hardly visible. Another misalignment is that the grooves (bracings) on stab and elevator are not exactly in-line. The main spar had been fettled and glued in exactly centered, so I put that down to Babcock. Both misalignments are not harmful, anyway.
Found a 30 years old hairdryer (good old German quality) and tried for the bending job. It's OK and doesn't need temperature checking since it produces the right temperature, anyway. It's too hot for my fingers (used a wooden gripper to hold the rudder) but just right for the plastic. It got soft so I could pre-bend the rudder. I had cut two slits into the leading edge and glued in two CA hinges before closing the two halves. There is a "step" on the rudder's tip where not only bending but also stretching would have been needed, and the plastic cracked there. I found the color of the plastic too lightened and was afraid it would crack despite heating, so I decided to try plastic cement to soften the bending line in addition. That turned out very well.
By the way, found a matching color Humbrol enamel in my plastic modeling inventory. Will be of good use for the main landing gear. 1-3/4" wheels are replaced by 2" ones, still only 0.25 oz each.
It got dark early and there's weird white stuff on my roof deck, so no outdoors glueing. Thought a bit CA on the rudder won't hurt, but it did. Oh well, I should wait. Tried CA for the rudder since it's so small and clamping is no problem. Again I found the contact cement would have been even better. It's really convenient to apply the CA and have the parts fix-glued shortly after, but the problem is to align and straighten the parts during the short setting time.
With contact cement, I just applied it to both halves of the stab (the seams around, the reinforcement, and the main spar) and immediately started joining them. I had planned the process before and marked some key positions, and I laid something between the halves to hold them apart. One after another, I attached the edges, starting with the main spar at the elevator side, took out the distance holders one after the other and finally closed the whole stab. Checked alignment and straightness and corrected by slightly bending the whole thing. Finally fastened the seams by pressing them, holding them on the edge of the building board and pressing with my thumbnail.
It would have been not that hard to build a rig from three balsa spars supporting the left and right leading edge and the left trailing edge. Glueing with the super CA glue would leave enough time to align the parts. But it would need three other spars to press the upper half onto the lower half while the glue sets or the seams (leading/trailing edges) would come out wavy due to the internal stress of the plastic. That's not a bad sign, though, since it shows that the plastic is still good after 50 years. When it still has internal stress it can't be very brittle, either. Anyway, you may see for which reasons I start to prefer the contact cement, except for less noxiousness.
12-14-2009, 12:23 PM
#27
Thread Starter
RE: another Babcock Aeronca
Finally, a drive had to be ordered. Pondering what it should be, I narrowed it down by assuming 7" maximum prop diameter and two cells LiFePo, the former due to ground clearance, the latter because I want to try them. Problem is that I'm not sure about the cell voltage. Graupner says 3.3 V but others specify 3.1 V, so I have to see myself. That's why I ordered two cells to make a pack and not a ready-made pack from Graupner. Downside of the cells is their weight. Especially the small 1100 mAh A123 cells weigh as much as a 2200 mAh LiPo. But that means a fall-back solution since I might replace the two A123 cells by two normal LiPo cells any time, getting even more power and capacity.
Anyway, a low-voltage motor is needed to get by with 6.2 V (worst case) or 6.6 V (Graupner spec) or even 7.4 V (LiPo). It has to have a high kv value since the "small" propeller has to turn quite fast to deliver the power needed. I prefer AXI just because I know them quite well, and from their brushless outrunner line the [link=http://www.modelmotors.cz/index.php?page=61&product=2208&serie=20&line=GOLD]2208/20[/link] fits the bill. Based on educated guesses, I assumed 25 oz AUW and a cruise power of 15 W electric.
There's only a limited number of choices both of motors and propellers, so it's sufficient to get in the ballpark. Using Martin Hepperle's [link=http://www.mh-aerotools.de/airfoils/javaprop.htm]JavaProp[/link] web program, a generic 7x4" prop was "designed" to get its coefficients. Together with the motor coefficients, they allow calculating the whole drive's characteristics. It turned out that the 7x4" prop is quite right, but a 7x5 is not bad either. There are several choices in case the battery voltage is changed so the drive may be adapted without problem.
My calculation spreadsheets are attached for those interested (rename from pdf to xls), and the characteristics curves are shown for both full and cruise power. The variable names should be self-explanatory. The kink in the curves comes from flow separations on the "calculated" propeller. Assuming 10 m/s (22 mph) as cruise and climb speed, the drive operates not nearly with maximum efficiency at full power but at peak efficiency in cruise flight, limiting the current draw to 3 A and extending the flight time to about 20 minutes. That's how I lay out a drive for such a model, long cruise flight at the expense of slow (maybe even bad) climb.
Anyway, a low-voltage motor is needed to get by with 6.2 V (worst case) or 6.6 V (Graupner spec) or even 7.4 V (LiPo). It has to have a high kv value since the "small" propeller has to turn quite fast to deliver the power needed. I prefer AXI just because I know them quite well, and from their brushless outrunner line the [link=http://www.modelmotors.cz/index.php?page=61&product=2208&serie=20&line=GOLD]2208/20[/link] fits the bill. Based on educated guesses, I assumed 25 oz AUW and a cruise power of 15 W electric.
There's only a limited number of choices both of motors and propellers, so it's sufficient to get in the ballpark. Using Martin Hepperle's [link=http://www.mh-aerotools.de/airfoils/javaprop.htm]JavaProp[/link] web program, a generic 7x4" prop was "designed" to get its coefficients. Together with the motor coefficients, they allow calculating the whole drive's characteristics. It turned out that the 7x4" prop is quite right, but a 7x5 is not bad either. There are several choices in case the battery voltage is changed so the drive may be adapted without problem.
My calculation spreadsheets are attached for those interested (rename from pdf to xls), and the characteristics curves are shown for both full and cruise power. The variable names should be self-explanatory. The kink in the curves comes from flow separations on the "calculated" propeller. Assuming 10 m/s (22 mph) as cruise and climb speed, the drive operates not nearly with maximum efficiency at full power but at peak efficiency in cruise flight, limiting the current draw to 3 A and extending the flight time to about 20 minutes. That's how I lay out a drive for such a model, long cruise flight at the expense of slow (maybe even bad) climb.
12-18-2009, 12:14 PM
#28
Thread Starter
RE: another Babcock Aeronca
Finally some cruicial building progress: I had the heart to glue the wings. It helped to do it step by step, as announced in [link=http://www.rcuniverse.com/forum/fb.asp?m=9316357]post #18[/link].
The first picture shows the whole mess with the left wing glued and the right wing blank prepared. I used the hairdryer to soften the bending (leading) edge, but obviously it's not hot enough. Bending is easy but after a while the thing swings open again. So I put a bit Revell plastic cement (which basically seems to be a dissolver) into the groove, and that worked well again. Of course, now the wing has to dry/harden for quite a while, but it has to with the contact cement, anyway.
There were some things to learn about contact cement. All instructions (cement and model) recommend to apply it sparingly, but they don't say what that means. I'd say rather too much than too little! When I applied really sparingly there were always spots where the cement didn't bond, so I had to spread the parts and apply contact cement into the slit, disperse it, and press for final bonding. That's messy, not applying too much cement in the first place! You have to apply the cement with the tube nozzle and then disperse it, for what I use my index finger. I had to apply bit by bit since the cement dries so fast and had to make sure that the whole bonding surface was wetted. Any excess cement was automatically slided to an edge and wiped off there or adhered to my finger and was wiped (or peeled) off there easily.
Anyway, the wing feels sturdy enough to carry all flight loads and it turned out reasonably straight. (It will look even better once the seams are fettled.) With one wing lying flat on the building bord, trailing and leading edge are quite straight and, even more important, parallel, so there's no wash-out (or -in). The tip is 2" above the board when the center section is flat on the bord, so the dihedral is as specified.
The crack in the root LE was simply glued and especially pressed to have the "triangle" fixed. A drop of CA will seal the whole area.
There is a problem with the parts fit, supposedly caused by Babcock, but maybe also my misunderstanding. Anyway, the center section is just too thick to get the wing "shells" (upper and lower sides) closed completely. The bottom sheet under the center section is not allowed for in the "step" near the TE of the upper side. Besides, the center section is a bit too long so even though the LE is glued tightly the TE is hardly closed. It wouldn't help cutting off a bit (lengthwise) from the rear center section since it would still be too thick. Oh well...
Where the bending (leading edge) ends at the wing tip, a small gap is unavoidable. A drop of CA will fill it nicely, though.
The leading edge turned out very, very sharp. The plastic seemed to nearly crack despite the dissolver, and the bending "groove" didn't turn into a nice LE radius. Instead it stayed a groove, even sticking out of the LE in forward and down direction. I can only hope that this will work as an airflow "cutter" when the wing is flown at some angle of attack.
The right wing was just glued as well but has to dry overnight, as mentioned above, to let both the plastic cement and the contact cement harden. All looks OK so far, it's straight and has the right dihedral as well. The exact alignment of the center section paid off now, and the flat bottom wing helps as well in adjusting and straightening the wing halves. Again I like the contact cement because it adheres immediately but still stays "adjustable", even for a few hours. It just moves only if I want it to. All I had to do was straighten the glued wings and slightly loading them flat on the building board over night.
The drive has just arrived, and they even ready-made the A123 battery pack for me. Great service and it weighs only 3.0 oz!
The first picture shows the whole mess with the left wing glued and the right wing blank prepared. I used the hairdryer to soften the bending (leading) edge, but obviously it's not hot enough. Bending is easy but after a while the thing swings open again. So I put a bit Revell plastic cement (which basically seems to be a dissolver) into the groove, and that worked well again. Of course, now the wing has to dry/harden for quite a while, but it has to with the contact cement, anyway.
There were some things to learn about contact cement. All instructions (cement and model) recommend to apply it sparingly, but they don't say what that means. I'd say rather too much than too little! When I applied really sparingly there were always spots where the cement didn't bond, so I had to spread the parts and apply contact cement into the slit, disperse it, and press for final bonding. That's messy, not applying too much cement in the first place! You have to apply the cement with the tube nozzle and then disperse it, for what I use my index finger. I had to apply bit by bit since the cement dries so fast and had to make sure that the whole bonding surface was wetted. Any excess cement was automatically slided to an edge and wiped off there or adhered to my finger and was wiped (or peeled) off there easily.
Anyway, the wing feels sturdy enough to carry all flight loads and it turned out reasonably straight. (It will look even better once the seams are fettled.) With one wing lying flat on the building bord, trailing and leading edge are quite straight and, even more important, parallel, so there's no wash-out (or -in). The tip is 2" above the board when the center section is flat on the bord, so the dihedral is as specified.
The crack in the root LE was simply glued and especially pressed to have the "triangle" fixed. A drop of CA will seal the whole area.
There is a problem with the parts fit, supposedly caused by Babcock, but maybe also my misunderstanding. Anyway, the center section is just too thick to get the wing "shells" (upper and lower sides) closed completely. The bottom sheet under the center section is not allowed for in the "step" near the TE of the upper side. Besides, the center section is a bit too long so even though the LE is glued tightly the TE is hardly closed. It wouldn't help cutting off a bit (lengthwise) from the rear center section since it would still be too thick. Oh well...
Where the bending (leading edge) ends at the wing tip, a small gap is unavoidable. A drop of CA will fill it nicely, though.
The leading edge turned out very, very sharp. The plastic seemed to nearly crack despite the dissolver, and the bending "groove" didn't turn into a nice LE radius. Instead it stayed a groove, even sticking out of the LE in forward and down direction. I can only hope that this will work as an airflow "cutter" when the wing is flown at some angle of attack.
The right wing was just glued as well but has to dry overnight, as mentioned above, to let both the plastic cement and the contact cement harden. All looks OK so far, it's straight and has the right dihedral as well. The exact alignment of the center section paid off now, and the flat bottom wing helps as well in adjusting and straightening the wing halves. Again I like the contact cement because it adheres immediately but still stays "adjustable", even for a few hours. It just moves only if I want it to. All I had to do was straighten the glued wings and slightly loading them flat on the building board over night.
The drive has just arrived, and they even ready-made the A123 battery pack for me. Great service and it weighs only 3.0 oz!
12-18-2009, 05:21 PM
#30
Thread Starter
RE: another Babcock Aeronca
Oh, savor I do, and I thought I'm slow. 
More than one week for the wing consisting of 6 parts is not exactly fast, isn't it? At least it's not "Build it today - fly it tomorrow" like advertized. Well, probably people were more practiced assembling and adjusting wobbly parts back in the fifties. And I don't remember savoring the contact cement as much in the sixties as I do now. I don't even need CA, the contact cement is enough to make my head hurt... [&:]
Otherwise it's already like Christmas for me. The Aeronca is a nice new toy, and several new toys (eh, technologies) are tried with it. Yesterday my first 2.4 receiver, today my first A123 battery, still not enough so a tiny coax heli, I'm just insatiable right now.
More than one week for the wing consisting of 6 parts is not exactly fast, isn't it? At least it's not "Build it today - fly it tomorrow" like advertized. Well, probably people were more practiced assembling and adjusting wobbly parts back in the fifties. And I don't remember savoring the contact cement as much in the sixties as I do now. I don't even need CA, the contact cement is enough to make my head hurt... [&:]Otherwise it's already like Christmas for me. The Aeronca is a nice new toy, and several new toys (eh, technologies) are tried with it. Yesterday my first 2.4 receiver, today my first A123 battery, still not enough so a tiny coax heli, I'm just insatiable right now.
12-19-2009, 11:04 AM
#31
Thread Starter
RE: another Babcock Aeronca
The drive! (The wing seams have been fettled but there were some spots that needed repair after that.)
The tiny AXI 2208/20 on the right is familiar to me, as actually is the Jeti ESC. But while the motors have not been evolved (or have they?), the ESC was improved in steps from the plain 18 to the Advance 18 and now the Advance 18 Pro. Each generation was adapted to new technologies, especially batteries. While the Advance 18 knew the correct LiPo cut-off voltage, the Pro now knows the LiFePo cut-off voltage, even in three levels (2.8/2.5/2.2V) and as a hard cut-off or a slow reduction of power. I chose the latter and 2.8V to begin with as well as no brake. It's all easily set with the programming card, shown below the ESC.
I ordered the motor mount parts, but it seems back mount is not possible since the prop adaptor has too much diameter for the hole in the Aeronca's cowl. I think I'll use front mount and the Graupner adapter with the spinner-shaped nut.
For later testing, several propellers had been ordered. The APC 7x4 SloFly might be the best fit, but I have also 7x5 and 7x6 for less voltage (or more speed). I don't like the APC props that much so I ordered the aeronaut Carbon Electric props because they look so cool. The 6.5x4 may be even useable but the 7x7 seems to be a racing prop. Both are just for playing, though. I like the good old Graupner Super Nylon (glass filled) best, at least they seem to match the Aeronca's look. Of Course, that's my taste and I'll just try the 7x5 and maybe even the 7x6.
The battery pack was made by the supplier just because I said in my order it would be nice if he would do. These are the power and balancer plugs I need. Amazing. The cells are the original (?) A123 cells type APR18650M1A - made in China. Found a technical data sheet in the Web and there is the 3.3V nominal voltage, so it's the manufacturer's specification. I checked with my trusty Schulze charger.
The cells were full, and during a first 1A discharge it looked like 3.15V at 50% capacity. (My 1100 mAh NiCd packs have their 1.2V nominal voltage at 50% capacity when discharged with 1A.) First charge was with 1.5A (standard) and the second discharge gave about 3.25V at 50%, as well as a third discharge. The second and third charge were intended to be at 3A and 5A (maximum fast charge) but the charger limited to 2.6A in both cases. Seems Schulze disagrees about that.
Anyway, there was no noticeable rise in temperature on the battery and it seems to have a rather low internal resistance (impedance). The discharge curve is rather flat, not only on the spec sheet but also in reality. But - according to the data sheet the battery must be already 500 cycles old. The capacity is only 1000 mAh, not more. Besides, the two cells are noticeably different in voltage and capacity, so a balancer is really needed. Not surprising for a new technology, though.
By the way, it's 8°F today, white powder coming from the sky, but my neighbor is riding his bicycle and the sun is looking through the mess and laughing at us. Weird.
The tiny AXI 2208/20 on the right is familiar to me, as actually is the Jeti ESC. But while the motors have not been evolved (or have they?), the ESC was improved in steps from the plain 18 to the Advance 18 and now the Advance 18 Pro. Each generation was adapted to new technologies, especially batteries. While the Advance 18 knew the correct LiPo cut-off voltage, the Pro now knows the LiFePo cut-off voltage, even in three levels (2.8/2.5/2.2V) and as a hard cut-off or a slow reduction of power. I chose the latter and 2.8V to begin with as well as no brake. It's all easily set with the programming card, shown below the ESC.
I ordered the motor mount parts, but it seems back mount is not possible since the prop adaptor has too much diameter for the hole in the Aeronca's cowl. I think I'll use front mount and the Graupner adapter with the spinner-shaped nut.
For later testing, several propellers had been ordered. The APC 7x4 SloFly might be the best fit, but I have also 7x5 and 7x6 for less voltage (or more speed). I don't like the APC props that much so I ordered the aeronaut Carbon Electric props because they look so cool. The 6.5x4 may be even useable but the 7x7 seems to be a racing prop. Both are just for playing, though. I like the good old Graupner Super Nylon (glass filled) best, at least they seem to match the Aeronca's look. Of Course, that's my taste and I'll just try the 7x5 and maybe even the 7x6.
The battery pack was made by the supplier just because I said in my order it would be nice if he would do. These are the power and balancer plugs I need. Amazing. The cells are the original (?) A123 cells type APR18650M1A - made in China. Found a technical data sheet in the Web and there is the 3.3V nominal voltage, so it's the manufacturer's specification. I checked with my trusty Schulze charger.
The cells were full, and during a first 1A discharge it looked like 3.15V at 50% capacity. (My 1100 mAh NiCd packs have their 1.2V nominal voltage at 50% capacity when discharged with 1A.) First charge was with 1.5A (standard) and the second discharge gave about 3.25V at 50%, as well as a third discharge. The second and third charge were intended to be at 3A and 5A (maximum fast charge) but the charger limited to 2.6A in both cases. Seems Schulze disagrees about that.
Anyway, there was no noticeable rise in temperature on the battery and it seems to have a rather low internal resistance (impedance). The discharge curve is rather flat, not only on the spec sheet but also in reality. But - according to the data sheet the battery must be already 500 cycles old. The capacity is only 1000 mAh, not more. Besides, the two cells are noticeably different in voltage and capacity, so a balancer is really needed. Not surprising for a new technology, though.
By the way, it's 8°F today, white powder coming from the sky, but my neighbor is riding his bicycle and the sun is looking through the mess and laughing at us. Weird.
12-20-2009, 06:00 AM
#32
Thread Starter
RE: another Babcock Aeronca
OK, I call it a wing. It could become an endless (and useless) saga otherwise. I have to stop work anyway since I've accumulated far too much of that allergenic cement and dissolver. I need the next two weeks vacation to recover.
Again some words about the contact cement. When fettling the seams it turned out that some spots on the left wing trailing edge were not bonded. I had learned my lesson when glueing the right wing so that was OK. But on the left wing I had to spread the upper and lower sheeting, fill in contact cement, disperse it and press for bonding. Now also the contact cement dissolves the plastic at least a bit, that's why they recommend to apply it sparingly. Liberally and completely greasing is sparingly enough, though, and won't soften the plastic too much. On the other hand, repair-glueing requires excess glue to fill the formerly unbonded spots and noticeably softens the plastic. That's why the trailing edge had to be loaded over night to come out straight again. By the way, clamping would have made it kinky. Finally, it needs several days for the cement or the dissolver, respectively, to diffuse out of the plastic and let it harden again. (Even though this PS plastic is always kind of soft.)
Actually, I'm not satisfied how the wing turned out, but I shouldn't complain. There are several flaws and mistakes. The right sheeting has slightly different color, probably the plastic blanks were from different production lots. I don't care. The cracks in both root leading edges could be repaired. The root trailing edges (seams) could be closed (bonded) even though the parts didn't fit. Still I don't know if it was my misunderstanding, but I think the center section appendage is too long and too thick for the wing "shells" (sheetings). Even though the sheeting was glued tightly around the leading edge, the trailing edge had to be closed and glued forcibly. There are "steps" both on top and bottom where the softened plastic was pulled around the rear tip of the center section. If I would build another Babcock model, I'd probably glue the wing sheetings before glueing the center section bottom B-5. That would cure the problem and the overlap would be hardly visible since it's on the bottom side. A good overlap and bonding is essential since the sheeting and the center section have to bear the bending forces. Russ, how did it turn out on your model and can you spot my mistake?
Another mistake was on the wing tips. The right one has some twist or washout, looks like intentionally, the more so as it fell into place this way. On the left tip I struggled to get it reasonably straight, and I had to struggle. This is one of several points where the instructions (including drawings) are not clear or not comprehensive enough. At least I didn't find out what was intended, maybe due to my thick head (from inhaling too much dissolver, of course). Anyway, I'll have to trim out a yawing/ rolling tendency later. The picture of the right wing tip is additionally flipped so you may compare it to the left one.
Duh. Maybe I'm looking just too close or being too critical. Nobody is perfect (except the captain, they say), and the wing isn't bad at all. Apart from the flaws mentioned it's quite OK. Both panels are straight and parallel (not twisted thanks to the accurately aligned center section and the flat bottom) so it will fly reasonably straight. The dihedral is as specified (2" at the tip, maybe a tad less) and equal on both sides, giving only 5 degrees, but the low C/G will help stability. The wing seems to be strong enough to carry the flight loads and it weighs 5.4 oz (154 g) what is not exactly lightweight but not really heavy, either. The weight is nearly equal on both sides, so no lead on one tip required. Wing span (tip to tip) is 38.6" (980 mm) and chord length is 6.5" (166 mm).
Again some words about the contact cement. When fettling the seams it turned out that some spots on the left wing trailing edge were not bonded. I had learned my lesson when glueing the right wing so that was OK. But on the left wing I had to spread the upper and lower sheeting, fill in contact cement, disperse it and press for bonding. Now also the contact cement dissolves the plastic at least a bit, that's why they recommend to apply it sparingly. Liberally and completely greasing is sparingly enough, though, and won't soften the plastic too much. On the other hand, repair-glueing requires excess glue to fill the formerly unbonded spots and noticeably softens the plastic. That's why the trailing edge had to be loaded over night to come out straight again. By the way, clamping would have made it kinky. Finally, it needs several days for the cement or the dissolver, respectively, to diffuse out of the plastic and let it harden again. (Even though this PS plastic is always kind of soft.)
Actually, I'm not satisfied how the wing turned out, but I shouldn't complain. There are several flaws and mistakes. The right sheeting has slightly different color, probably the plastic blanks were from different production lots. I don't care. The cracks in both root leading edges could be repaired. The root trailing edges (seams) could be closed (bonded) even though the parts didn't fit. Still I don't know if it was my misunderstanding, but I think the center section appendage is too long and too thick for the wing "shells" (sheetings). Even though the sheeting was glued tightly around the leading edge, the trailing edge had to be closed and glued forcibly. There are "steps" both on top and bottom where the softened plastic was pulled around the rear tip of the center section. If I would build another Babcock model, I'd probably glue the wing sheetings before glueing the center section bottom B-5. That would cure the problem and the overlap would be hardly visible since it's on the bottom side. A good overlap and bonding is essential since the sheeting and the center section have to bear the bending forces. Russ, how did it turn out on your model and can you spot my mistake?
Another mistake was on the wing tips. The right one has some twist or washout, looks like intentionally, the more so as it fell into place this way. On the left tip I struggled to get it reasonably straight, and I had to struggle. This is one of several points where the instructions (including drawings) are not clear or not comprehensive enough. At least I didn't find out what was intended, maybe due to my thick head (from inhaling too much dissolver, of course). Anyway, I'll have to trim out a yawing/ rolling tendency later. The picture of the right wing tip is additionally flipped so you may compare it to the left one.
Duh. Maybe I'm looking just too close or being too critical. Nobody is perfect (except the captain, they say), and the wing isn't bad at all. Apart from the flaws mentioned it's quite OK. Both panels are straight and parallel (not twisted thanks to the accurately aligned center section and the flat bottom) so it will fly reasonably straight. The dihedral is as specified (2" at the tip, maybe a tad less) and equal on both sides, giving only 5 degrees, but the low C/G will help stability. The wing seems to be strong enough to carry the flight loads and it weighs 5.4 oz (154 g) what is not exactly lightweight but not really heavy, either. The weight is nearly equal on both sides, so no lead on one tip required. Wing span (tip to tip) is 38.6" (980 mm) and chord length is 6.5" (166 mm).
12-20-2009, 06:02 AM
#33
Thread Starter
RE: another Babcock Aeronca
Horizontal stab and rudder are complete. The provided control horn was glued on even though it's not a tight fit and leaves gaps all around. I just used the medium CA and filled the gaps with it, then applied activator to let it set. Turned out quite well and above all reinforces the inner corner of the elevater, what was really needed. The horizontal stab is now ready to be snapped into the fuselage cutout below the fin. Maybe the inner seams have to be trimmed more to stay clear of the fuselage, but that is best done later. In the end, the elevator will be mounted using the provided small pins.
On the rudder, the hinge gaps were closed with CA, unfortunately a bit too much so there will be a gap between fin and rudder. I may stuff it with a strip of foam plastic. A Kavan slow flyer control horn was glued to the rudder. It has a pin going through the rudder so its quite sturdy. I mounted it so the inner hole is "abeam" of the hinge line since I assume the inner hole is the right match to the small servo horn. I didn't check it yet, though. The control horn was mounted on the left side to avoid collision of horn and linkage with the elevator.
I plan to use the linkage exit provided in the rear fuse tip, but to make a push rod instead of a torque rod. I'll use plastic tube with thin piano wire (1/32", 0.8 mm) and will let the tube come out of the fuse tip and bend the wire "back" to the rudder control horn. The wire will be snaked there and threaded into the control horn hole. For the elevator, I'll use the hole provided in the right fuselage as exit for the linkage, coming from the left "cockpit" (where the C/L bellcrank would be) going to the elevator control horn.
The rudder control horn is white plastic and is now painted yellow to make it unobtrusive. The same yellow Humbrol enamel no. 69 has been used to paint the main landing gear. I used a simple brush and tried to get by with one coating. That turned out not quite well since there is some smear and thin paint, but I may paint over the visible parts any time later. By the way, the yellow seems to be the right one, commonly used for vintage airplanes, and reminds me of a nice story. A clubmate had an Aeronca Champ (the 65 hp version, unfortunately had to sell it recently) and refurbished it the DIY way, completely stripping and re-covering it. One of the biggest problems was to find aviation-approved yellow paint. After much searching he found out what we all found hilarious: Yellow is the color of the national mail system in Central Europe, and the German Mail had their trucks, cars, bicycles, callboxes, and mailboxes painted in "mail yellow". Of course they used huge quantities so the paint was even quite cheap, and it was virtually the right shade of yellow and even aero-approved. Bingo! Of course the Champ was nicknamed "the mailbox".
The original wheels of the model are lost, only three hub halves are there. The 2" Kavan wheels look quite nice and weigh only 0.25 oz each, so they are a good replacement, anyway. The tailwheel was still there but was replaced by a better-looking Graupner wheel of same diameter. Fresh 1/32" piano wire was used instead of the rusty original one to bend the tail landing gear, later glued into place in the fuselage. Wire end sleeves were used to border the wheel on the wire. I didn't bother making a steerable tail wheel, again building stock to a large extent. The main landing gear weighs 1.3 oz (37 g) and the tail landing gear about 0.05 oz (1g).
Now it's time to pause for a while and ponder on the details of the linkage and motor installation to see if my ideas are feasable.
On the rudder, the hinge gaps were closed with CA, unfortunately a bit too much so there will be a gap between fin and rudder. I may stuff it with a strip of foam plastic. A Kavan slow flyer control horn was glued to the rudder. It has a pin going through the rudder so its quite sturdy. I mounted it so the inner hole is "abeam" of the hinge line since I assume the inner hole is the right match to the small servo horn. I didn't check it yet, though. The control horn was mounted on the left side to avoid collision of horn and linkage with the elevator.
I plan to use the linkage exit provided in the rear fuse tip, but to make a push rod instead of a torque rod. I'll use plastic tube with thin piano wire (1/32", 0.8 mm) and will let the tube come out of the fuse tip and bend the wire "back" to the rudder control horn. The wire will be snaked there and threaded into the control horn hole. For the elevator, I'll use the hole provided in the right fuselage as exit for the linkage, coming from the left "cockpit" (where the C/L bellcrank would be) going to the elevator control horn.
The rudder control horn is white plastic and is now painted yellow to make it unobtrusive. The same yellow Humbrol enamel no. 69 has been used to paint the main landing gear. I used a simple brush and tried to get by with one coating. That turned out not quite well since there is some smear and thin paint, but I may paint over the visible parts any time later. By the way, the yellow seems to be the right one, commonly used for vintage airplanes, and reminds me of a nice story. A clubmate had an Aeronca Champ (the 65 hp version, unfortunately had to sell it recently) and refurbished it the DIY way, completely stripping and re-covering it. One of the biggest problems was to find aviation-approved yellow paint. After much searching he found out what we all found hilarious: Yellow is the color of the national mail system in Central Europe, and the German Mail had their trucks, cars, bicycles, callboxes, and mailboxes painted in "mail yellow". Of course they used huge quantities so the paint was even quite cheap, and it was virtually the right shade of yellow and even aero-approved. Bingo! Of course the Champ was nicknamed "the mailbox".
The original wheels of the model are lost, only three hub halves are there. The 2" Kavan wheels look quite nice and weigh only 0.25 oz each, so they are a good replacement, anyway. The tailwheel was still there but was replaced by a better-looking Graupner wheel of same diameter. Fresh 1/32" piano wire was used instead of the rusty original one to bend the tail landing gear, later glued into place in the fuselage. Wire end sleeves were used to border the wheel on the wire. I didn't bother making a steerable tail wheel, again building stock to a large extent. The main landing gear weighs 1.3 oz (37 g) and the tail landing gear about 0.05 oz (1g).
Now it's time to pause for a while and ponder on the details of the linkage and motor installation to see if my ideas are feasable.
12-23-2009, 02:08 AM
#34
Thread Starter
RE: another Babcock Aeronca
Next year the wing will go to a coordinate measuring machine to discover its airfoil.
Teddy and I wish you a merry Christmas and a happy new year!
Teddy and I wish you a merry Christmas and a happy new year!
01-10-2010, 04:46 AM
#35
Thread Starter
RE: another Babcock Aeronca
We're back, but not much time for modeling. The wing rested on two bars for two weeks, each wing was supported about in its C/G so no bending force (instead of resting on the center section and both wings hanging "down" ). Still the whole thing warped a bit so the right wing has now 3/16" less dihedral. Dang. Maybe it was the glue curing completely, but who knows. Should be not that important, though, and is correctible by shimming. It's just the second major building flaw.
The Graupner Super Nylon 7x5 prop is going to be a perfect match. Vintage model - vintage prop (they made them since the sixties), that's one aspect. Actually it's made for glow engines (with its big hub), but it matches the electric motor as well. Its geometry is "vintage", and was measured by scanning a front/top view and a side view on a scanner. Martin Herpperle's programs [link=http://www.mh-aerotools.de/airfoils/index.htm]PropellerScanner[/link] and [link=http://www.mh-aerotools.de/airfoils/javaprop.htm]JavaProp[/link] have been used to measure its geometry and compute the aerodynamic coefficients.
This prop shows a nice smooth blade stall at low speeds and no stall from about 7 m/s (16 mph) on. The high-kv motor turns fast enough to let the prop give enough thrust even at only 7" diameter. And 5" pitch is still so much that the prop is working with maximum efficiency at much slower cruise rpm.
If the figures are correct, the drive will have 47% overall efficiency at the expected 10 m/s (22 mph) cruise speed. Estimated current draw is 3.75 A, giving 15 minutes flight time from the rather small 1 Ah real battery capacity (instead of 1.1 Ah nominal capacity). At full power in a 10 m/s-flight-speed climb, the drive is far from maximum efficiency but the 40% are still not bad at all.
Static thrust should be 3.8 N (13.7 oz), and at 10 m/s flight speed thrust should be still 3.25 N (11.7 oz), giving 0.55 and 0.47 thrust/weight ratios, respectively. Both are by far suffient.
The figures have to be checked/tweaked with measurements of the real drive including ESC and battery. The calculation spreadsheets are attached, just rename pdf to xls.
The Graupner Super Nylon 7x5 prop is going to be a perfect match. Vintage model - vintage prop (they made them since the sixties), that's one aspect. Actually it's made for glow engines (with its big hub), but it matches the electric motor as well. Its geometry is "vintage", and was measured by scanning a front/top view and a side view on a scanner. Martin Herpperle's programs [link=http://www.mh-aerotools.de/airfoils/index.htm]PropellerScanner[/link] and [link=http://www.mh-aerotools.de/airfoils/javaprop.htm]JavaProp[/link] have been used to measure its geometry and compute the aerodynamic coefficients.
This prop shows a nice smooth blade stall at low speeds and no stall from about 7 m/s (16 mph) on. The high-kv motor turns fast enough to let the prop give enough thrust even at only 7" diameter. And 5" pitch is still so much that the prop is working with maximum efficiency at much slower cruise rpm.
If the figures are correct, the drive will have 47% overall efficiency at the expected 10 m/s (22 mph) cruise speed. Estimated current draw is 3.75 A, giving 15 minutes flight time from the rather small 1 Ah real battery capacity (instead of 1.1 Ah nominal capacity). At full power in a 10 m/s-flight-speed climb, the drive is far from maximum efficiency but the 40% are still not bad at all.
Static thrust should be 3.8 N (13.7 oz), and at 10 m/s flight speed thrust should be still 3.25 N (11.7 oz), giving 0.55 and 0.47 thrust/weight ratios, respectively. Both are by far suffient.
The figures have to be checked/tweaked with measurements of the real drive including ESC and battery. The calculation spreadsheets are attached, just rename pdf to xls.
02-21-2010, 09:53 PM
#36
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RE: another Babcock Aeronca
OK, UStik how's the progress on your Aeronca? I was looking at mine today and almost brought it to the field since it was 65f/17c out and calm winds. Instead I flew my Parkzone T-28, GWS C-130, Robbe Seabee and Grumman Panther EDF - just ran out of room in the SUV! Russ Farris
02-22-2010, 02:35 PM
#37
Thread Starter
RE: another Babcock Aeronca
Great to hear that! Not quite that nice weather here (7 C, windy, snow till yesterday) so still waiting for better conditions to glue outdoors. Linkages are prepared, motor is mounted, parts are cut, now CA is needed to repair cracks and assemble the fuse. Strange is that Multiplex forgot to ship the USB cable I need to upgrade my tx to 2.4 GHz. Meanwhile set up a rough copy of the model in the simulator. Can't copy the nasty behavior, but it's a pretty fast little beast at 25 oz weight. Think I have to search for a big paved area to land it.
04-20-2010, 12:51 AM
#38
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RE: another Babcock Aeronca
Progress report on the Aeronca, UStik? I'm about to return mine to the skies again soon. (Knees shaking!) One thought I have is to reduce the rudder throw even more.
That volcanic ash cloud over Europe isn't keeping out of the air is it?
Russ Farris
That volcanic ash cloud over Europe isn't keeping out of the air is it?
Russ Farris
04-20-2010, 03:36 PM
#39
Thread Starter
RE: another Babcock Aeronca
No progress to my regret. In early March I took my annual flu and still couldn't get rid of it. I'm just afraid I would damage some of the irreplaceable parts of the Aeronca and the glueing is too finicky.
Reducing the rudder throw is a good idea I think. If the plane is on the verge of getting out of control it seems to have too much of it. Rather fly big turns, what you're doing anyway. I plan setting a low dual rate, but my knees will be shaking, too.
That ash, oh well, I can't make it out so I wonder if it's really that much over here. Where is our daring air force to check it out?
Reducing the rudder throw is a good idea I think. If the plane is on the verge of getting out of control it seems to have too much of it. Rather fly big turns, what you're doing anyway. I plan setting a low dual rate, but my knees will be shaking, too.
That ash, oh well, I can't make it out so I wonder if it's really that much over here. Where is our daring air force to check it out?
04-21-2011, 08:03 PM
#40
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RE: another Babcock Aeronca
ORIGINAL: UStik
No progress to my regret. In early March I took my annual flu and still couldn't get rid of it. I'm just afraid I would damage some of the irreplaceable parts of the Aeronca and the glueing is too finicky.
Reducing the rudder throw is a good idea I think. If the plane is on the verge of getting out of control it seems to have too much of it. Rather fly big turns, what you're doing anyway. I plan setting a low dual rate, but my knees will be shaking, too.
That ash, oh well, I can't make it out so I wonder if it's really that much over here. Where is our daring air force to check it out?
No progress to my regret. In early March I took my annual flu and still couldn't get rid of it. I'm just afraid I would damage some of the irreplaceable parts of the Aeronca and the glueing is too finicky.
Reducing the rudder throw is a good idea I think. If the plane is on the verge of getting out of control it seems to have too much of it. Rather fly big turns, what you're doing anyway. I plan setting a low dual rate, but my knees will be shaking, too.
That ash, oh well, I can't make it out so I wonder if it's really that much over here. Where is our daring air force to check it out?
04-21-2011, 11:20 PM
#41
Thread Starter
RE: another Babcock Aeronca
Thanks for asking Russ, but no progress. I just can't work with glue anymore, that is it's not worth to bear the consequences. I looked for a friend who would do the glue work for me but no success so far. I made a simulator model of the Aeronca that is hard enough to fly due to the high wing loading but does not even show that terrible habits. Though I'm sure you, as a professional pilot, will master the flights I wish you good luck. I think one can always use it with this model.
04-21-2011, 11:52 PM
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RE: another Babcock Aeronca
Sorry to hear about your glue allergy, UStik. I'll keep trying to refine the Aeronca - less rudder throw is in the works. Yes, it's a challenge to fly, but showing others a 1958 plastic ARF in the air is worth it. Hope you can finish yours soon! Russ Farris
