Developing an EZ Version of a SR- 71

[L Edge]

@LitterBug decided to make a EZ version with Elevons and I decided to do it for the EZ version 2 setup with throttle and differential thrust.

Someone proposed , can it fly in a gym?, so here goes what my problems will be designing it.

My 64 mm EDF SR-71 has a long fuse and tough to do tight turns. So we need a large wing area to fly slow and stability in the turns(a gym?). I have designed a number of other EZ planes and the closest is my flying wing. So, decided to have the wing area close to that. Weight is a big factor and the electronics is 25.1 grams with all electrons loaded. One nice advantage is it doesn't make it in the gym, will it fly outside?

Okay, model needs nose protection, 2 rudders, a fuse design that prevents bending in half. Very tight for model with all features.
UGH!!!!
After many mental sketch's, decided the paper size that makes it work and still look like a SR-71 or A-12 is 21 inches long and 12 " wide.

Paper drawing:

Using Adams board,

cut out above is 24.3 grams. The weight of the 2 motors, FC and battery is 25.1 so the total 49.4 grams. I am hoping to be less than 60. No glue yet.

Need to mull this around to see what options I have or do I trash it now? Where do I put prop/motors to be most effective for the turns? etc.

 

[Mr Man]

Don’t use hot glue, try something else. At least that’s my opinion.

 

[LitterBug]

Proceed! You are way ahead of me. mine is still flying around my emptyy skull.

 

[Flying Cherokee]

for tighter turns I recommend putting the motors at the widest point on the wing.

Looking good so far!

 

[L Edge]

Like my 64mm Sr-71, I have the same problems. With the long nose, it is a cantilever and it can flutter, bend and rotate or combos. Weight is getting to be a problem as well.

So I am going to use 1/64" bass wood on the bottom that is hot glued and goes about 2" on the fuse. With the thin wood, I used an iron to reheat the wood and glue and get a good adherence. I have made the F-117 and Dark Star of black Adam's foam(very bad in flight against trees) so I used black packing tape.

I added a wood nose protector also so it doesn't crumple especially during testing outside.

A CG is set when using 1/2 throttle(add more-climb,less-descend) so tape electronics and work on it. Once I found it, placed the motors(so flow is over rudders) and now for test chucks.

In order to prevent damage to model, used my tarp in the cellar to prevent crashes. It is hung from the top, angled to slide down and be caught with a half loop at bottom.

Looking for the CG at 1/2 running throttle, I first stood 2 ft and released and nose heavy. Moved battery back, redo at 2 ft, then 4, 6,8,10, and now 12 ft as of end of testing for today.( cellar is usable for about 42 ft.) No damage to model!!!!!!!

So, day 3 have a SR-71 that is flyable and needs more work to be trimmed including checking start of turn by slow or high rates switch). Also, the outside test as well as able to fly in gym?

 

[Flying Cherokee]

Like my 64mm Sr-71, I have the same problems. With the long nose, it is a cantilever and it can flutter, bend and rotate or combos. Weight is getting to be a problem as well.
View attachment 247027

So I am going to use 1/64" bass wood on the bottom that is hot glued and goes about 2" on the fuse. With the thin wood, I used an iron to reheat the wood and glue and get a good adherence. I have made the F-117 and Dark Star of black Adam's foam(very bad in flight against trees) so I used black packing tape.

View attachment 247028

I added a wood nose protector also so it doesn't crumple especially during testing outside.

A CG is set when using 1/2 throttle(add more-climb,less-descend) so tape electronics and work on it. Once I found it, placed the motors(so flow is over rudders) and now for test chucks.

View attachment 247029

In order to prevent damage to model, used my tarp in the cellar to prevent crashes. It is hung from the top, angled to slide down and be caught with a half loop at bottom.

Looking for the CG at 1/2 running throttle, I first stood 2 ft and released and nose heavy. Moved battery back, redo at 2 ft, then 4, 6,8,10, and now 12 ft as of end of testing for today.( cellar is usable for about 42 ft.) No damage to model!!!!!!!

So, day 3 have a SR-71 that is flyable and needs more work to be trimmed including checking start of turn by slow or high rates switch). Also, the outside test as well as able to fly in gym?

Nice!

 

[L Edge]

The SR-71 is flying 30 ft from target and I have some very interesting problems.

Holy Cow!!!, It actually flies like my 64mm EDF version. The nose actually pitches up/down by itself if you exceed a large change in pitch. If it climbs too fast it starts fluttering and goes into a stall. Back off on thrust a very small amount and the negative pitch just dives it to the ground. Low rate isn't enough and high rate begins get the heart beating.

The concept of the radio where the CG is located at 1/2 throttle, to climb, you add throttle, to glide subtract throttle from 1/2.
A turn is initiated where one engine speeds up and other slows down changing thrust from each side. There are 2 rates to turn and to initiate a SR-71, high is needed due to long nose. That's the problem. I have designed 6 other flying models and the radio setup is excellent, It is the design of the SR-71 causing the problems.

So, to keep it level, a continuous smidgeon of 1/2 throttle change is needed. Initiating a roll(the thrust change is in high mode) sends the plane erratic in flight. So, Houston, I have a problem. Not giving up yet, conceded to not being able to fly in a gym. Outside, will see.
It will be interesting if @Litterburg elevator addition will work on his.

By the way, if you ever fly an 64mm EDF SR-71, here is a trick to remember due to it's design. To make a relax turn, level it first, then, add a touch of rudder followed quickly by the roll of the aileron only and then apply later the elevator. That timing is important and I also use that to my Warthog especially on that close ground turns.

 

[L Edge]

Did some more testing by moving further back and began to notice it tends to drift to the right. So this is what I did.

Took a concept of the pylon days and applied a self tapping screw behind the CG and on down left side to draw it to the left when released. So next step is to go outside and see what happens when launched. Days are cold, windy and wet (frost melting) so who know when.