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Jan 10, 2017, 01:06 AM
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Mig 21 RC RG


I've wanted to try doing a full fuse RC rocket glider Mig 21 ever since I built a profile version a few years ago. I settled on using the same vertical stab/wing/tail of the profile and use a 2.6" bt-80 lightweight tubing I had. I found a used nose cone I had and chopped off the nose and trimmed about 3" off of that keeping just the tip. I then glued in a piece of 6mm depron to block the rear of the nose cone and glued the tip onto that to give the characteristic mig 21 look. A 28" long tube seemed about right for the body and I decided on a sort 12mm sandwich profile for the canopy and spine to reduce drag instead of a full turtledeck.

I used a rocketry program called openrocket to build a mass model putting in the weights of the components including electronics, tubing, depron etc, with an estimate of my required boost CG and what nose weight might be needed. It looked like one ounce of nose weight would do the trick and keep me at 12 oz rtf using a 24mm E-6 rocket motor which will boost that weight to around 500'. Wing area at just over 1 sq foot is about half of my other rc rocket gliders so glide will be steeper, but not as steep as my bomarc which weighs closer to 14 ounces rtf with a similar wing and much more draggy design.

I decided to use the tab for the vertical stab that goes through the body tube to hold the motor mount in place, and to use full flying tail surfaces since that was very easy to do by putting a simple aluminum pivot tube through the fuse just in front of the motor mount, and using a carbon torque rod. This meant I could get by with using just two servos for roll and pitch.

I sandwiched in some .04 styrene into the ventral to protect it for landing and used a wing skid at the front. Since the ventral sticks down in the way I couldn't put my launch rail guides at the bottom but I was able to offset them slightly and not have the rail interfere with the servos or pushrods.

Wing goes through a slot in the body tube.

I got it assembled and did some in the house glide tests without components installed and found the max rearward CG for boost to be about where I expected which required 1 oz of nose weight and my finished weight was right at 12 oz. My wing has a 14.5" chord at the root and my CG for boost is around 6.8" from the front at the root. This is about 21%MAC or 46% root chord, or about 5-6% static stability margin. Again, I'm using a slightly tail heavy condition for boost and glide will be nose heavy.

I chose not to paint the model to save weight and leave it in the white but use the markings from an Isreali captured Mig 21 which I think turned out nicely. Markings are hand cut trim vinyl, heated after application with a hair dryer. This plane was an Iraqi Mig 21 that defected and was flown to Isreal, done in two different color schemes with a 007 number, then given to the US for evaluation. There is a plane in the Isreali air force museum painted in one of the schemes but it is not actually the same plane since the US cannot seem to find/return the original to Isreal.

Here is a video of the 1st two flights, flew great off the board, just a bit of glide trim adjustment on the first flight...

RC Mig 21 Rocket glider flights (2 min 31 sec)
Last edited by burkefj; Jan 16, 2017 at 10:57 AM.
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Jan 16, 2017, 09:46 AM
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Very nice! Your YouTube video of this led me to this RCG thread. Any plans or kits in the works?
Jan 16, 2017, 10:55 AM
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Hi, I don't have plans done at this time, if you are interested I can do tracings of the depron surfaces and some dimensions of the tube and slot locations, and cg, the rest should be obvious from the build photos, let me know.

Frank


Quote:
Originally Posted by bartj1234
Very nice! Your YouTube video of this led me to this RCG thread. Any plans or kits in the works?
Jan 16, 2017, 06:44 PM
Registered User
Actually, your write up and pics are very detailed . With what you already provided, I think I can manage to reproduce the craft as my first scratch built effort. Thank you very much for sharing. BTW, in the first pic, is the red dot the center of pressure and is that one of the things that this openrocket software program calculates for you?

Thanks

Joe
Jan 16, 2017, 08:30 PM
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Hi, no that red dot is not the actual center of pressure, it is what that rocket program estimates as the CP, but for an airplane is conservative and I don't use that program, I'm only using that program for building the mass of the components ahead of time to figure out where to put the wing so I minimize the nose weight needed, and figure out my final weight to understand if I'm building too heavy or not for my rocket motor.

To minimize launch weight you have to be careful with components and placement ahead of time or you will come out with a model that requires too much nose or tail weight and overall weight is too high for good glide or boost. You need to be able to plan your CG/wing location ahead of time so it comes out about perfect when components are added. That's what i used that program for. In addition, I'll build out the model in this program including the vertical stab and fuse, and then remove the wings, and let it tell me where the side CP is located, to make sure the CG will be ahead of both the vertical wing related CP and lateral CP, if this program shows the lateral CP to be 1.5-2 body diameters behind the flight CG you are going to be directionally stable. Sometimes is is possible to get a good flight CG but not be laterally stable if the tail is too small or too far forward, like on an X-3 stilletto...

First I figure out the wing CG location using %mac or the bruder wing calc, here is exactly the dimensions I used and the launch weight, using a 20%MAC which is a good max rearward CG location for boost assuming some lift contribution from the forward fuse:
http://wingcgcalc.bruder.com.br/en_U...d6=0&sweep6=0&

then I confirmed with this calculator which is calculating static stability margin, I plugged in numbers for the wing (not including the fuselage) and the tail surfaces and the static margin you want. I used 8% static margin.

http://adamone.rchomepage.com/cg_calc.htm

That gave me the same result as the pure wing CG location of around 6.75" from the leading edge of the wing.

I then did some indoor glide tests before installing components to verify, the way I test, I hold the model in about a 10degree up attitude and push it forward hard into a mush, if it pitches over into a glide that is a good spot. If it stays pitched up or pitches up more then it is too tail heavy. Using this method I've not been tail heavy on launch, it's usually right about at the minimum acceptable for a not overly pitch sensitive boost. After boost it will be nose heavy and be more stable anyway. Sometimes I'm a little nose or tail heavy at first glide depending on the geometry of the fuse and how much that impacts things.

Little things add up in the weight, where the servos are, how heavy the servo extensions are etc. I've gotten it down pretty well now to get a model this size to wind up around 11-12 oz rtf which is about optimum for this rocket motor to boost to around 500' which is starting to get small for orientation, and give a decent enough size/wing loading for glide and landing.

Be very careful, tubing weight adds up quickly. I'm using BT-80-34 from Balsa machining company which is around 2.75 oz for the 28" length I used. If you use a different brand you can easily double the weight and then you are too heavy. Same with nose cones, my finished nose cone weight is 1.6 oz.

Quote:
Originally Posted by bartj1234
Actually, your write up and pics are very detailed . With what you already provided, I think I can manage to reproduce the craft as my first scratch built effort. Thank you very much for sharing. BTW, in the first pic, is the red dot the center of pressure and is that one of the things that this openrocket software program calculates for you?

Thanks

Joe
Last edited by burkefj; Jan 16, 2017 at 08:37 PM.
Jan 17, 2017, 07:33 PM
Registered User
Thanks so much for the additional information. Very generous with your input. All said, you save me a lot of trial and error as would have been the case when trying to replicate from just watching a video. I look forward to starting this build.
Feb 07, 2017, 07:05 AM
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Saxguy1000's Avatar
Another great model! I'm amazed how the red shows up in the sno. Good choice of paint scheme.


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