New design: KMD Stunt

[steven yampolsky]

The flying season is over(for me at least). Time to look back and remember things that made this season special and to begin work on the next world beater. This post is about looking back.

Around mid February I was going over my engine collection(It's a huge one: 14 motors) and picked up a Soviet era motor of the 80's: KMD 2.5. I have very special memories with this motor. It powered a model that introduced me to stunt. As I handled the motor, nostalgia took over and on a whim, I decided to recreate from memory that model. I would close my eyes and try to remember that gorgeous model that was given to me by a kid who just turned 18 and was getting shipped to Afganistan. I cherished that model and flew it gingerly for two years(straight and level with occasional wingover). After two years, the kid came back and showed me what that model could really do. At some point, he took my hand and showed me how to loop. Right then and there I got bitten by a stunt bug. Fast forward almost 30 years and I am sitting in my basement with my eyes closed trying to relieve those memories so I could draw the shape of the model that got me started. I am happy to say that I was able to recreate every detail except for the canopy. For some reason it came out too small. Oh well, it looks distinct.

Some of you are already thinking: "Why is this schmuck is talking nostalgia? What is the point of his rambling!?". Well, here's the point: the model turned out to be a REALLY good flier! Now, I am not a NATS of WC champion, nor do I consider myself an especially good pilot but I do manage to give local experts a run for their money.  Anyhow, the model flew at a level where I am confident I could compete with it in expert and not finish last! In other words, I think this model will work for anyone starting from Beginner all the way through Advanced and lower ranks of Expert!

The model exibits similar behavior expected of much larger models: it is not squirly like a typical .15 size model, it tracks extremely well and turns predictably. It doesn't care for the wind much either. The airfoil and wing shape is what gives this model its extremely good characteristics. The airfoil is a modified NACA 0018( mod-83, see attached picture ) that has blunt leading edge which makes the model extremely easy to steer. The planform has a very high taper which helps the model stability in hard corners and high wind.

When it comes to construction, the model has several interesting construction methods:

• Fuselage does not use ANY plywood. It is made out of 3/8 sheet, laminated with 1/16 balsa with carbon veil in between. The nose has an 3/8 side cheek for rigidity. The outcome was a very stiff and light fuse
• Nose gear is a russian plug-in style which allows me to remove it for hand launch. It is my opinions .15 size model should only be hand launched otherwise you loose 50% of the fun!
• I used leather fillets for the first time and fell in love with how great they work! I dreaded making fillets the old fashion way with epoxy based conckoctions

Wing

Wing root chord	11.00
Wing tip chord	6.00
Wingspan (in.)	39.50
MAC (in.)	8.50
Area (sq. in.)	335.75
A/R	4.65

Stab & Elevator

Stab root chord	2 7/8
Stab tip chord	1 1/2
Elevator root chord	2 1/4
Elevator tip chord	1 1/4
Span	15.2500
Area	60.0469
Elevator/Stab %	44%
Stab/Wing %	18%

      
Other dimensions

Spinner backplate to LE	7  3/16
Cylinder centerline to LE	5 5/8
Wing TE to Stab LE	9 5/8
Motor weight (oz.)	6.7

            
      

[GregArdill]

Looks good Steve.

Can't wait to see it finished.

Greg

[Paul Taylor]

Looks Good Steve.

[steven yampolsky]

Can't wait to see it finished.

Oh no, this one already flew and already crashed when we started having waay too much fun on a extremely windy day. The model is so light(24oz full up) the wing loading is only in mid 10's. This obviously makes it a very good wind flier.
So we did. The winds were blowing over tents and chairs and we were having fun! Until we got onto upwind side by mistake....   

The picture I posted was in the ready to fly form. I did not want to paint the model because I wasn't sure if it will fly.

I'm gearing up for the next iteration! Instead of .15-.25 motor size(OS 20 setup as per Brett Buck instructions would be PERFECT for this model), I want to scale it up to .25-.35 and stick a Brodak .40 in it. On top of that, I'll be making it full fuse with a couple of "enhancements". Once this gets going, I'll start posting updates.

[Allen Brickhaus]

Interesting, very interesting.

Allen

[john e. holliday]

I like it.  No flaps. 

[Steve Fitton]

You need to draft that intermediate size for those with the FP 20s out there 

[steven yampolsky]

You need to draft that intermediate size for those with the FP 20s out there 

The dimensions listed are perfect for FP 20. All you need to do is take the dimensions and give the profile view your own "look". If anyone interested in ribs, please send me private message via this forum with your email address and I will email you the .DXF file.

[ChrisSarnowski]

Hi Steve,

That model looks like fun, but you have to do something about your engine collection. Only 14? I'm not sure that counts as a collection.

-Chris

[Dave Denison]

The flying season is over(for me at least). Time to look back and remember things that made this season special and to begin work on the next world beater. This post is about looking back.

Around mid February I was going over my engine collection(It's a huge one: 14 motors)

Steven.

You've got to work that collection up in numbers...right now it's just an  affliction. Keep it growing!

I do like the looks and ideas on that new plane.  The numbers look "very right", especially the airfoil.  Here in the Northwest we have a couple fliers that use the 18% NACA airfoil to very good use.  Seems the windy conditions cause many design changes. Do you have any photos of the wing during construction?
Lets hope you can do a "build" write up on the next one.

  Regards.
  Dave.

ama41041

[GregArdill]

Sorry to hear that it's gone Steve.

I fly a lot of 15 size models, and 24 oz is heavier than anything I'd consider flyable.

The model I flew yesterday (in the wind) has porked up since it was built. It now weighs 16 oz. Absolutely loved the wind.

Greg

[Geoff Goodworth]

The airfoil that you show is much more blunt than NACA 0018. What are the other parameters of the section and is it the same at the root and tip?

[dave siegler]

What does "Russian style plug in gear" look like?

[steven yampolsky]

Sorry to hear that it's gone Steve.

I fly a lot of 15 size models, and 24 oz is heavier than anything I'd consider flyable.

The model I flew yesterday (in the wind) has porked up since it was built. It now weighs 16 oz. Absolutely loved the wind.

Greg

This is not your average .15! KMD's are diesels. On top of that, they are extremely powerful. Power wise it is closer to a Fox 35 than a typical .15 glow. I would look past my powerplant choice and see the model as a strong .20-.25 or a lightweight .35 type of model.

I would also not get hung up on model weight. model weight by itself is a bad indicator. It has to be always correlated with wing area. A better way of determining appropriate airframe weight it to look at wing loading. For a typical stunt model, 11-13 oz/sq. ft. The formula for calculating the wing loading is 144*(weight in oz.)/(wing area). For example, KMD Stunt weighs 24oz and the wing area is 335sq in. Wing loading comes out to 144*24/335 = 10.42 oz/sq ft. For this wing area,based on suggested wing loading of 11-13, target weight can vary anywhere from 11*335/144 = 25.6oz and 13*335/144 = 30.25oz. Since my model had no paint and only one wheel landing gear, it came out lighter than designed weight goals. Anyone who builds this model with proper landing gear and paint will be within proper weight envelope.

[FLOYD CARTER]

NACA 0018 is perhaps the best all around airfoil for a stunter.  And, that diagram looks quite a accurate to me.

Floyd

[steven yampolsky]

NACA 0018 is perhaps the best all around airfoil for a stunter.  And, that diagram looks quite a accurate to me.

Yes and no. It is a good starting point but just like everything else in stunt, it's a balance of things one strives for.

For example, 0018 has low drag coefficient at low angles of attack when compared to a 0022. If I where to use a high aspect ratio wing, the model would have a tendency to wind up in round loops. To offset the wind up tendencies, one could use a high RPM, low pitch power setup and thus achieve "the balance". Another way to eliminate wind up is to use fatter airfoil such as 0022. Fatter airfoils have higher drag coefficient and can help eliminate the wind up tendency. Of course, fatter airfoils also need more power to overcome the higher drag and so the re-balancing game continues.

[steven yampolsky]

I've started assembling the photos I took during airplane construction. The photos are scattered between four computers and two phones 
I just loaded the first batch:

http://syampolsky.fotki.com/model-airplanes/kmd-stunt/

[Steve Hines]

Like the looks of it. It is allmost the same as the Gotch Streak III that I building now from core house. It has foam wings and the rudder does not go behind the fuse.

Steve

[Chris Wilson]

The only thing that worries me is the extreme difference between the outboard fuel feed (made worse by the hopper style tank) and much further inboard spray bar (made worse by centralized reverse drum set up) .

That engine is going to have pull the fuel against centrifugal  forces quite a way only to have it fall back down again in the overheads.

Good luck!

[Geoff Goodworth]

I missed the note that the airfoil had the 83 modifications. Agree that it looks correct but is quite different to the standard 0018.

Regarding thickness in general, quite a few designs use the standard 4-digit NACA sections in thicknesses around 22–24% before the TE is truncated and a flap added.

With the flap added—which I think it should be when considering the complete wing—the thickness finishes around in 18% or slightly thicker—up to 20%. The Europeans tend to be up to 2% thinner.

[Brian Hampton]

This is not your average .15! KMD's are diesels.

Your KMD (black head) is the first, and best, version. There's an engine test report of a black head at http://dkd.net/clmodels/acln/acln81.pdf

I have one of the later plain finished versions that's NIB. Probably the major difference is that it uses a one piece cast venturi unlike yours with 3 different venturi inserts.

[Chris Wilson]

I read somewhere that the later white head KMD's were done under contract and in new premises, and thus the quality control was not as good as the state made ones.

[steven yampolsky]

Your KMD (black head) is the first, and best, version.

I believe there were three "versions" of it: the dark blue, the black and the plain/gray versions. the dark blue was the original and is the most desirable of the three verisons. The gray/plain one is the worst in terms of quality. What amazes me is that these motors fetch $100+ on eBay. I know these motors are good but one hundred dollars good?

[steven yampolsky]

What does "Russian style plug in gear" look like?

I finally dug up some pictures on this...

The stuff written on the first photo rouphly traslates: "Use 1/64th plywood on the sides with grain aligned horizontally". Basically, the grain has to be perpendicular to the landing gear wire.  The middle piece is cut out as shown on the drawing is also made out of plywood. It needs to be as thick as the landing gear wire itself. Once the pocket is built, you are supposed to wrap in in kevlar string but I didn't have it and didn't use it. The thinking behind kevla wire is to prevent the pocket from disintegrating during side loads. Basically, the landing gear has to go straight down in order to avoid any lateral forces.

[Tim Wescott]

But, but -- 1mm is 0.039 inches, or a bit more than 1/32".

(Not that I understood any of the writing other than the "1mm"!)

[steven yampolsky]

But, but -- 1mm is 0.039 inches, or a bit more than 1/32".

(Not that I understood any of the writing other than the "1mm"!)

Well, I should have read it better then! I saw pictures how others have built it and they used really thin stock(wrapped in kevlar string) so I used 1/64th sides and had no problem.

[Garf]

Hi Steve,

That model looks like fun, but you have to do something about your engine collection. Only 14? I'm not sure that counts as a collection.

-Chris

14? Collection?.....Large???&$@^#^)*%$#!!!!!!!!!! I have over 80, and that's not even considered a large collection. I don't even call it a collection at all.

[Garf]

This model reminds me of the Interceptor 35 by Berkley. I like it.

[Bob Kruger]

Steve;

Neat looking little ship.

What glue did you use for the fuselage lamination?

V/r

Bob

Fuselage does not use ANY plywood. It is made out of 3/8 sheet, laminated with 1/16 balsa with carbon veil in between. The nose has an 3/8 side cheek for rigidity. The outcome was a very stiff and light fuse
      

[steven yampolsky]

What glue did you use for the fuselage lamination?

I used West System epoxy. Regular hobby epoxies won't work because they are too thick. With a thin epoxy, I can get the CF wetted out and remove excess by dabbing the surface with a paper towel. Another added benefit of thin epoxies is they can get absorbed by the balsa grain in a way similar to thin CAs which makes it for a stronger bond.

Laminating with CF veil and epoxy was also part of the experiment. I heard so many times that epoxy is heavy and should be avoided. For this project I laminated the fuse, stab and elevators. One would think that the model would come out heavy but it didn't. Final weight was just 24oz with super light wing loading of just 10.4oz/sq ft. A typical wing loading for stunt models is between 11 and 13 oz/sq ft.

Another part of the experiment was to see if one can build a profile without any plywood doublers and have it survive typical motor vibrations. Plywood is heavy and it always looked strange to me when people would go through great lengths to build light wing and aft fuse but slap heavy plywood on the nose "for rigidity". Randy Smith stopped using plywood on his full fuse models loong time ago and his models do not fall apart from vibration. Some of his models that are built using this method are probably approaching 20 years and 1000 flights! I've also flown a profile model built in 1991 that had no plywood doublers that flew amazingly well and had no problems with vibration. I had to test this theory out myself and glad I did. I'm never building another model with plywood doublers!

Steve

[Geoff Goodworth]

Steve, can you explain how you fitted the engine bearers, please?

Are they 3/8" sq? Is the balsa side cut away? What would you do for a larger version of the same construction?

[Eric Viglione]

Steve - I've heard a lot of good things about West System epoxies. I still use Z-Poxy finishing resin for my laminations, which has worked great and is pretty thin and will penetrate most cloth/veil I've tried... but as for Z-Poxy's 30 min slow cure for other stuff, I don't care for as much, the 30 minute gives you about 5 before it starts to thicken.

Do you have a feel for how West Systems compares? Do they have an equivalent finishing resin and a slow cure epoxy glue? Every time I've thought about ordering some West Systems epoxy to try from places like aircraft spruce, I see all the huge gallon containers, too many part numbers to choose from and give up. If you have a part number for small quantities of the stuff you use, could you post it?
Thanks in advance,
EricV

I used West System epoxy. Regular hobby epoxies won't work because they are too thick. With a thin epoxy, I can get the CF wetted out and remove excess by dabbing the surface with a paper towel. Another added benefit of thin epoxies is they can get absorbed by the balsa grain in a way similar to thin CAs which makes it for a stronger bond.<snip>
Steve

[steven yampolsky]

Steve - I've heard a lot of good things about West System epoxies. I still use Z-Poxy finishing resin for my laminations, which has worked great and is pretty thin and will penetrate most cloth/veil I've tried... but as for Z-Poxy's 30 min slow cure for other stuff, I don't care for as much, the 30 minute gives you about 5 before it starts to thicken.

Z-Poxy is a 50/50 epoxy which means that the hardener has some sort of filler in it. This is done to make it easier to measure quantities. The problem with hobby hardeners is that they will harden over time so there is "freshness" aspect of it. I recently had to throw away a whole bunch of epoxy that has hardened up. Z-Poxy finishing resin uses a different kind of liquid filler which does not thicken it should last a couple of years. Thickness-wise, a bottle of fresh Z-poxy resin is pretty close to West System in terms of thickness and should suffice.

Do you have a feel for how West Systems compares? Do they have an equivalent finishing resin and a slow cure epoxy glue? Every time I've thought about ordering some West Systems epoxy to try from places like aircraft spruce, I see all the huge gallon containers, too many part numbers to choose from and give up. If you have a part number for small quantities of the stuff you use, could you post it?

The system is a little convoluted but I figure it out. Here it goes:

1) All WS products use the same resin called "#105 Resin".
2) Hardeners come in three varieties: Fast, regular and slow. Mumbo jumbo aside, for our needs, we need the "fast" hardener called "#205 Hardener". It has 1 hour work time and will cure in 10 hours.
3) All WS products mix at the same ratio of 5:1. 5 parts of resin to 1 part hardener.
4) You will need to buy special measurement pump kit called "#300 pump kit". You only need to buy it once.
5) Dispensing is easy: one pump of harderner and one pump of resin. The pumps will dispense proper amounts.
6) Some shops sell "kits" which are basically hardener and resin combined.
7) You will need 32oz #105 resin, 7oz #205 hardener and #300 pump kit.

Aircraft Spruce has good price on the "kit". The kit you want is what they call A-1 kit:
http://www.aircraftspruce.com/catalog/cmpages/westepoxy.php

The pump kit:
http://www.aircraftspruce.com/catalog/cmpages/301PUMP.php

This should last you a looong time.