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  #1  
Old 01-10-2017, 01:12 AM
burkefj's Avatar
burkefj burkefj is offline
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Default 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 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 is a bit smaller at just over 1 sq foot than 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 wight was right at 12 oz.

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.
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  #2  
Old 01-10-2017, 08:40 AM
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neil_w neil_w is offline
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Spectacular, maybe the most "realistic" one yet. Love the nose and the sculpted cowl on top.
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  #3  
Old 01-10-2017, 08:43 AM
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mojo1986 mojo1986 is offline
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Wow! Your ability to finish these things is fantastic! Can't wait to see the flights! Way to go, Frank!


Joe
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  #4  
Old 01-10-2017, 12:18 PM
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burkefj burkefj is offline
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Thank you very much! Hopefully we'll get out of this rain/sleet/snow cycle in Oregon and I can fly it this weekend....

If you want to see more photos of how it went together you can look at my rcgroups thread:

https://www.rcgroups.com/forums/sho...65-Mig-21-RC-RG
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Old 01-10-2017, 11:28 PM
AstronMike AstronMike is offline
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Smile Neato

Always nice to see what you're cranking out, Frank!!

Reckon you're also still searching for a suitable Depron replacement as well. Please keep us posted on those developments!
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Old 01-11-2017, 03:49 AM
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burkefj burkefj is offline
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I've tried MPF and value xps and variation, softness and stiffness ruled them out, cellfoam 88 was nice and stiff and consistent with a nice finish, but it was very weak, snapping much too easily. Foamboard is too heavy for my kits that I've found so far, and I tried the waterproof flitetest foam board and CA won't stick to it, they recommend hot glue which is heavy and would not work for my kits either The waterproof foamboard is brown so would require added weight from paint. No good substitute at this time, but I do have enough parts cut for 60 more kits till I run out.

My kits were a pretty optimum weight/size and strength, so finding an easy substitute that does not require a complete redesign, or reduction in boost/glide performance is not as simple as you might think. MPF is probably the closest thing out there, but I would not want to sell kits from it at this point.

Frank


Quote:
Originally Posted by AstronMike
Always nice to see what you're cranking out, Frank!!

Reckon you're also still searching for a suitable Depron replacement as well. Please keep us posted on those developments!
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Old 01-11-2017, 12:58 PM
AstronMike AstronMike is offline
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Default Yes Indeed!

Quote:
Originally Posted by burkefj
My kits were a pretty optimum weight/size and strength, so finding an easy substitute that does not require a complete redesign, or reduction in boost/glide performance is not as simple as you might think. MPF is probably the closest thing out there, but I would not want to sell kits from it at this point.

Frank


You've pretty much run into the same set of problems I did way back 20-30 years ago, when I was trying to optimize sport gliders. Same for HPR, in a way, too.

Back in '00, when the ATF stuff was too much, and we'd lost our original HPR field, did I want to fly my venerable Astron SST HPR glider, just on smaller motors. Since it used two ply standard foamboard, I initially made a single ply version. Nope, warps too much, and too flimsy.

I then found something called 'artcor', which had in fact been already discontinued, but was a neat plastic based foamboard like deal. Much lighter than standard, and a two ply version of that flew darned well on the then-new G35 Econojets. This is the stuff I'd even made and sold some lower power kits from back near 20 years ago....

2004. Im in a Dollar Tree, and see something called 'Adams Readiboard', right there, a $1 a sheet. Hmm, its lighter yet, and glues WAY better, with all the properties of foamboard, both good and bad. This stuff r00ls for D to F based delta gliders, unless you're putting RC and stuff in them

I still have quite a bit of 3mm Depron, and that I've used a lot of, in C powered gliders that are pretty darned big for 18mm, and glide well. Ever see a Cosmos Mariner sized and looking bird boost well on a C6, and glide for like 45-60 secs? Yea, be a pity when that stuff's all gone....

Im sure something will show up in time. As far as your kitting goes, yea, weight optimization is HUGE.
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Old 01-16-2017, 11:00 AM
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burkefj burkefj is offline
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Well, snow was still 9" deep and 25 degrees out but my family braved the conditions which were actually great for flying, blue sky and no wind. Here are the first two flights, perfect boost trim and CG and just a few clicks of adjustment on the glide trim.

http://www.youtube.com/watch?v=TDsHNpCy2Oc
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Last edited by burkefj : 01-23-2017 at 01:50 AM.
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  #9  
Old 01-16-2017, 03:03 PM
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mojo1986 mojo1986 is offline
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Wow, Frank! Always great to drop in on your posts and to watch your flight videos! I also noticed your series of Christmas flights when I was on youtube and took them in too. Your Jayhawk seemed to be in glide for a very long time...........did you use a larger motor for those flights, or is it just aerodynamically superior?

Joe
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  #10  
Old 01-16-2017, 03:47 PM
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burkefj burkefj is offline
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The Jayhawk is pretty clean, uses same motor I always use, mostly it depends on a straight boost and good air if the wing area and weights are similar.
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Last edited by burkefj : 01-17-2017 at 11:27 AM.
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