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Jemco 40 Fun Scale Mustang

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Old 01-14-2018, 11:03 AM
  #1  
Sandnat
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Default Jemco 40 Fun Scale Mustang

Been wanting to revisit this bird for a long time. Jay (Big Stickflyer) has a thread up about plans for it and it appears there are a lot of guys out there that built one back in the day.

C/P from the post I put on the thread Jay started:

Great to find a tribe of Mark's 40 size Mustang fans! I found a kit of it a year or so ago and just started a counterfeit version of it. Traced and cut all the parts and got a cabinet shop buddy to duplicate that kinda oddball gear bock out of a piece of ash. Going to hand off the original kit off to my son and he is going to build an electric version. It was my second build back in the 80's I loved that little airplane. 40 FP in it and I flew the snot out of it. Didn't fuel proof the nose well enough and it was nearly rotted off when I clocked a tree with it. Re built it, with a new nose, sheeted most of the wing, and added mechanical retracts. Then flew the crap out of it for several more years before I moved on. Still have it's carcass up in the attic. Couldn't bare throwing it away. Figured a survivor like that deserves better than a garbage can
I plan to do a bolt on wing and power it OS 46 FX. All I know for sure on mods so far..

Sooo....Here is the starting point. My complete counterfeit kit is on the left less the canopy. The wood used for the copy came out 54 grams lighter than the kit parts when I compared the totals for the two parts groups

Steve
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Old 01-15-2018, 02:01 AM
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acemcduck
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I'm in. I've had one of these in the kit stash for a long time.


Dave
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Old 01-15-2018, 05:32 PM
  #3  
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Default Pictures and mods

I built one of these back in the 80's. Well I built the one pictured below. I think it's the same as yours but I don't remember what my box looked like.
The one I built was actually designed for a 29 size engine as I recall. Or maybe it was for a 19. The wing was a flat bottom and tons of down thrust in the engine set up.

Well, I did just a couple mods. First and foremost, I made rib extensions (half ribs for the bottom) if you will. Pieces that I glued on to the bottom of the existing ribs to make a semi-symmetrical airfoil. And that wing came out thick at the root. Kind of an out of the box approach but it worked. Then of coarse the next thing was to get rid of all that down thrust and bolt a OS 45 FSR and eventually adding a pipe on it.

I loved that plane. It was pretty fast too. One day I was doing a spin and left it in a little too long.
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Old 01-15-2018, 10:38 PM
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Hey Dave hopefully this might get you ginned up and start gluing it together!
Originally Posted by acemcduck View Post
I'm in. I've had one of these in the kit stash for a long time.


Dave
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Old 01-15-2018, 10:56 PM
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Hey Rick, You said the near universal comment of about everyone that built one. 'I loved that plane' Yellow and black and red. Mine was yellow and black.

That was pretty wild about the rib mod. I went up in the loft and took a picture of the wing of my old one I mentioned in the opening comment. lol Still has the 30 year old futaba retract servo in it. The retracts is kinda why I quit flying it. Everywhere I flew was off grass and it didn't do that well. Would nose over in a heartbeat

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Old 01-15-2018, 10:57 PM
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I am having problems with getting pics to attach..
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Old 01-15-2018, 10:59 PM
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Old 01-15-2018, 11:44 PM
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[img]Am further along than the first post would allude to... lol Starting to look like an airplane. Had a senior moment and inadvertently put in about a degree of right thrust in the firewall by misaligning the firewall key on the fuse jig. Thought , what the heck probably needs it anyway so just leave it in.. so I curved the nose to the right at f1/spinner backplate to get everything matched up. Started getting the wing sorted out todayClick image for larger version

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Old 01-16-2018, 10:08 AM
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I'll check this out... big fan of these planes here as well. I built 2 of these years ago. I've got the DynaFlite .60 Mustang here to build too. These style Fun Scales though are different then the newer DynaFlite versions... that crutch in the middle sets them apart I think... ... one of my favorite classics. Every time I see one it jogs my memory to good times.

Rick, that was a very cool looking Mustang man. Great scheme there!
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Old 01-17-2018, 01:28 AM
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Think you said you were going to put a 90FX in it on Jay's thread. That should be pretty swift. And you have built two 40s.

You ever have any concerns about wing strength? I know I about folded mine up one day in a high speed panic pullout one day. This was after I had sheet most of the wing and put the retracts in. It was coming almost head on to me and I swear I could see the tips flex up and the covering wrinkle. The wing structure is a bit strange on the design wit that secondary spar. Fairly strong as far as the front of the wing and gear structure but lacking behind the main spar.. Long ribs held together by monocote till you get to the rather weak trailing edge structure. I had been thinking about sheeting the leading and trailing edge structures and capping the spars but didn't want to get off into all that work. Even though it could be argued it would kinda improve that scalloped kinda flimsy looking leading edge.

Then I had an eureka moment tonight. I finished getting ready to lock everything together on the right wing tonight but it is COLD out in my shop. Sleeting here in Gulf Shores AL tonight and I didn't feel like dealing with CA with freezing air for ventilation. Was fixing to head in the house for the evening and the solution hit me. Don't know why I didn't see it before. A one 1/4 square lower rear spar. (see picture...stick is laying across the top in the approx position). Let the front of the hatches for the wing servos but up against the spar. Will be easy to do the cuts in the ribs before I glue them in Roll the wing over and and put it in from the bottom to make sure everything is nice and flush . Yay! That should go a long ways toward shutting down any twisting and/or flexing.

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Old 01-17-2018, 10:43 AM
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I never thought about any warpage factors back in the day.. it was all about "build and fly"... by the seat of the pants more often then not. Even if it might have flexed I never knew it. I put these planes through a ringer too.

Years and years ago, on one of my old Fun Scale .40's, I did have half of the rear horizontal stab/elevator break off one day though... right up next to the fuselage. That's a vivid memory! I had the plane nose-bleed high and was cranking it like mad.. and then something flew off (the plane was so high I couldn't see what it was..).. so in a slight panic, I throttled back and coasted down... and the plane was somewhat erratic.. then I realized half that hor.stab and elev was gone!.. Fortunately the half that was left there had the pushrod/elevator linkage still intact operating the remaining elevator. Landing was actually uneventful... I searched but never found the parts that came off the plane... So ... moral.. make sure you're rear stabs are secured.. ha. I built a new stab-half/elev and scabbed it in there somehow.. as that wasn't the end of that plane by a long shot.

The only other area that I remember being marginal was that main-spar/landing-gear mount set-up. I remember on one my Mustangs that spar actually split from a less then perfect landing... and it torqued the gear wire, and just split that spar quite a bit, and it originated in the hole the landing-gear wire inserts into that spar/wing, and basically cracked at that slot the LG wire seats in. I remember doing surgery/repair on it, and having to cram epoxy into the crack, and just hoping it would hold... I don't remember it ever coming loose again. I lost both of Mustangs to dumb-thumbs and taking chances... oh what fun though!

Yes, I've got a .91FX sitting here with my DynaFlite FS .60 Mustang. I think the newer DynaFlites do have sheeting on the top/leading edge of the wing...(both the .40 and .60).. which might be to alleviate flex.. not sure. but the early generations like the one you're building are a bit different in many ways... but still have a very similar outer shape.

I like you're spar idea... and it just sitting there in your pic it just looks right too. So carry on... and have a great day!
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Old 01-17-2018, 08:36 PM
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I've never built a Dynaflite Mustang of any size but I did build a Jemco FS Mustang 40 back around 83. I thought the wing seemed a little weak and flexed to much for my liking. I sheeted the top front half and the center section of the wing. I then used a spray foam to fill the pockets in front of the main spar. Once it's dry it's very light and makes the wing really strong. It's also easy to do. I didn't make any other mods and the plane gave my many hours of pleasure flying it.
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Old 01-18-2018, 01:06 PM
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Hey Guys, thanks for the thoughts and stories. The day I flexed the wing on mine or around that time frame I was playing the high speed dive zoom pass game pretty hard. Buzzed a flight control in the dive and fortunately nothing fell off. I had heard they would do that and chopped the throttle in time even though I buzzed it twice before it dawned on me what I was hearing . I think it was an aileron, Bunch of slack in the aileron torque hole.

Dgrant mentioned the tail flutter. I am not real happy with what I am seeing om my build at the elevator deck . Looked to me the die cut main side skins tail deck has a lot of stabilizer built in nose down , which I copied as accurately as I could, based off the assumption the main horizontal fuselage jig is the zero reference. The top spine runs out almost to the end of the stab deck also and when seated properly down the entire fuselage, wound up about an eighth of an inch below the stabilizer mount in my kit copy. I will see if I can figure out what is going on there by getting down my old fuselage and taking a look

Did a search on the subject flutter and twist and this post by Littlera popped up:


Aileron flutter can progress to flutter of the wing, depending on the stiffness (mostly torsional) of the wing and control surfaces and mechanical interplay between the wing and offending aileron. It can actually happen the other way around, with the wing driving flutter of the control surface.

This kind of interplay is what is suspected to have caused the wing to fail in the Gee Bee crash from the early 1930's that was captured on film and has probably been seen by most of us. Keep in mind, however, that a WING can actually flutter independently of the control surface. It is rare, but can and does happen occasionally. I have had it happen wilth glider wings with high aspect ratios and relatively inadequate torsional stiffness. You can make a yard stick (wooden only) do this by whipping it like a sword in your hand. Try it! You will also hear the sound of flutter, which of course has come to be known as the death rattle of any airplane that is experiencing it.

Any wing or aileron (or other control surface for that matter) can flutter under the right conditions. Mass balance and proper stiffness are employed on full size aircraft and by modelers if they are smart. The stiffness and balance serve to keep the flutter "envelope" outside of the operational envelope (mostly speed, but load and other factors to a smaller extent) of the airplane.

Keep in mind that on a typical light aircraft the control systems are "free floating." They are tied to movable controls. Our models controls are tied to servos that are not easily moved by aerodynamic inputs from the control surfaces. When connected by properly stiff control rods and cables, this arrangement tends to prevent (damp) the onset of flutter in MOST (NOT ALL) cases. This is why we typically get by with improperly hinged, improperly balanced, and improperly aligned control surfaces without disaster.



Soo....Where it stands now I have cut the notches in the ribs for the 1/4 square bottom spar. Got to thinking after reading the above and Yall's comments. Laid thee sticks across the top of the wing and thought....Why not three small spars across the top? Will cut down the scalloping on the top of the nose of the wing and add a bunch of torsional strength. Will be easy to do and won't add a lot of weight. Nothing is glued yet.

Last edited by Sandnat; 01-18-2018 at 01:19 PM. Reason: clarity
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Old 01-18-2018, 01:07 PM
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lol Going stick happy.

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Will think about this over the next couple of days..


Would think that I need to put at minimum a 3/16 one on top of the wing over the new bottom one.

Dgrant, about what you said about the landing gear block, the pine blocks for the gear looked like it was cut from lumberyard number two pine stud scraps so I am not surprised it split. My buddy cut mine out of ash. Which, as far as I know is good for tool handle so it should resist splitting. Might need to to do a bit of research there and if it looks iffy. Through bolt it with 2-56 bolts and a couple small metal plates on each side will be cheap insurance. Then it will have to twist the whole block out.

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Old 01-18-2018, 08:49 PM
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Originally Posted by Big Stikflyer View Post
I've never built a Dynaflite Mustang of any size but I did build a Jemco FS Mustang 40 back around 83. I thought the wing seemed a little weak and flexed to much for my liking. I sheeted the top front half and the center section of the wing. I then used a spray foam to fill the pockets in front of the main spar. Once it's dry it's very light and makes the wing really strong. It's also easy to do. I didn't make any other mods and the plane gave my many hours of pleasure flying it.
This is a great idea and I have never tried it or seen the results. I will look on Youtube and see if I can find a video on the subject. I can see where the end result would be as strong as a D box.
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