The scratch-built twin-EDF CH-F14

[FOGeologist]

Truly amazing.

 

[Chuppster]

Congrats, it looks like it flies great.
I can not wait to see it with the EDF's.

Truly amazing.

Thank you for the support! I hope to put a bigger motor on it today and see how it behaves with a higher wing loading. Then I'll grab some EDF's and start working on another design for the spar.

 

[Chuppster]

Put the big motor on it today (roughly 700 watts) and it flew a lot better. With a 5000mah 4s its AUW is 1.7kg, and that will only be heavier with twin EDF's. The roll rate is anemic at best, and with the wings swept back its almost doesn't roll. Also some strange behavior when I pull up. I need to mix the elevator with the ailerons in hopes of getting a better roll rate. I may try fitting the top of the fuselage this week to give it a more finished look.

When I landed it the motor had broken loose the somewhat flimsy piece of the fuselage I had slapped the motor on. I'll need to reinforce that for this pusher. I think I'm going to keep tinkering with this iteration of the bird before ordering EDF's. It can look as cool as it wants, but if it flies poorly I don't see much point in investing in more electronics for it. What do you guys think? Anything I can try to make it fly better?

Here's the video of the flight. My cheap phone camera is doing its best (which isn't much), and I interrupted my poor wifey juicing a lemon to come film this, so she had a bit of trouble seeing. But she's a champ for sticking it out! I found the landing to be amusing, so if you don't want to sit through a few minutes of blurry F-14 footage I recommend you check out the end.

 

[Namactual]

It looked like it handled well when you launched it. It's really hard to get a sense of anything after that though. I did see you had a high speed stall there about 1:38. If that is what you mean by weird behavior when you pull up I would say its a mix of the wing sweep and high AoA.

A few things you could try:
Elevon mixing. The ailerons moving up with your elevators will help reduce the stall.

The ailerons are not going to be very effective with the wings in the back position. The airflow running spanwise down the wing is jsut going to roll right past them. I would make your horizontal stabilizers full blown stabilators to compensate.

Make the end of the wing have a larger under camber. The under camber you have is pretty much non-existent.

Again, I can not really see or feel what the aircraft is doing so these are just educated guesses.

 

[FOGeologist]

It looked like it handled well when you launched it. It's really hard to get a sense of anything after that though. I did see you had a high speed stall there about 1:38. If that is what you mean by weird behavior when you pull up I would say its a mix of the wing sweep and high AoA.

A few things you could try:
Elevon mixing. The ailerons moving up with your elevators will help reduce the stall.

The ailerons are not going to be very effective with the wings in the back position. The airflow running spanwise down the wing is jsut going to roll right past them. I would make your horizontal stabilizers full blown stabilators to compensate.

Make the end of the wing have a larger under camber. The under camber you have is pretty much non-existent.

Again, I can not really see or feel what the aircraft is doing so these are just educated guesses.

Namactual has a great handle on this deal; stabilators were the ticket for the full-scale aircraft when it was designed in the late 1960s. They are effective for delta planforms, which is what you've got with an F-14 in full sweep.

Many EDF scaly jets use both elevon/stabilators AND ailerons!

 

[Chuppster]

Thank you for the input! I'll definitely use stabilators, but due to the fact that my cheap radio only gives me three mixes I think I'll wait for the elevons. I've never heard of programming elevons on an airplane with an elevator before!

I guess I could run each elevator and aileron off the same channel. But I'm not sure if I installed the servos the right way for that.

More testing is definitely in order!

 

[Chuppster]

Make the end of the wing have a larger under camber. The under camber you have is pretty much non-existent.

How much under camber should I add? Do you think it will do anything while the wings are swept? It doesn't seem to want to drop a wing tip.

 

[Namactual]

Without mixing the ailerons with the elevators I would prioritize the stabilators as elevons only and bypass the main wing ailerons altogether. You might lose a little roll authority with the wings fully extended, but as large as the stabilators are on that bird you should be ok.

The under camber is only going to help with the wings fully extended. In the rearward position they are not going to do a thing. I was just thinking that once you load this thing up with two heavy EDF's and batteries to drive them you are going to have slow speed stall issues during a landing. It is certainly not required and again, just an educated guess on my part.

 

[Chuppster]

I'm guilty of not using the rudder a ton when I fly. How do you think adding the rudders will effect the flight characteristics (given I use the rudder)?

 

[FOGeologist]

I'm guilty of not using the rudder a ton when I fly. How do you think adding the rudders will effect the flight characteristics (given I use the rudder)?

When you fly a long, sweeping curve under moderate power, is the tail dragging? I mean, is it hanging towards the ground? A little rudder input will make the ship track straighter.

 

[Chuppster]

When you fly a long, sweeping curve under moderate power, is the tail dragging? I mean, is it hanging towards the ground? A little rudder input will make the ship track straighter.

Ah, that makes sense. I will have to look out for that next time I fly it. For now I've been so focused on trying to keep it in the air. Thanks!

 

[Namactual]

Like FOG stated, if you have coupling or yawing issues during a turn it might help. If you had a radio capable of advanced mixing you could even enhance your pitch authority with the rudders.


That said, unless you think you need them I would abstain from installing them. I think the extra weight would not be worth the gain.

 

[Chuppster]

Like FOG stated, if you have coupling or yawing issues during a turn it might help. If you had a radio capable of advanced mixing you could even enhance your pitch authority with the rudders.
View attachment 109844

That said, unless you think you need them I would abstain from installing them. I think the extra weight would not be worth the gain.

If this was an F-18 then yes, but the F-14 has vertical stabilizers.

 

[Namactual]

It's not so much the angle of the VertStabs at play as the drag induced by in-turned rudders above the cg is causing the pitching moment. Also the extra drag well aft of the CG also has a stabilizing effect.

You are right though, the fullscale Tomcat did not use this feature.

 

[Chuppster]

It's not so much the angle of the VertStabs at play as the drag induced by in-turned rudders above the cg is causing the pitching moment. Also the extra drag well aft of the CG also has a stabilizing effect.

You are right though, the fullscale Tomcat did not use this feature.

Wow, I'm learning all kinds of stuff today! That's a neat idea, but I don't believe I'll implement it because I would prefer to use a single servo for the tails in order to reduce weight/cost. Thanks for the info though!

 

[Chuppster]

Question:
I didn't realize until today that the actual F-14 has a flying tail. I think this would be really cool to implement, but in order to do so I believe I would need to put the hinge for the tail clear through the nacelle. Does anyone know if such a rod would spoil the air and have an actual effect on the thrust output of the EDFs?

 

[Namactual]

It would not help, that's for sure. How much of an impact it would have though is really hard to say. As long as you don't constrict the airflow with a massive dowel that takes up 50% of the tube it should not be that bad. You will take a little performance hit.

You do not have to put a pivot all the way through the fuse though. You could use some kind of surface mount hinge/pivot that would distribute the load to a large part of the outboard nacelle wall.

I will sleep on this, (been a long day), and I will post any ideas I can think of that will not add a ton of weight.

 

[Wingman04]

I have a knob on my transmitter, but for now I plan to use the 3-position switch, so they will jolt.

If you set the sweep channel as flaps, and setup as a standard flap wing, you should be able to set a delay so it doesn't jolt (as least on Spektrum Tx's).

What are you doing about the CG change when the wings sweep? This may help better your performance.

 

[Chuppster]

I flew it again tonight, this time with the tail set as elevons. It was a very short flight. I got it up in the air and flew it down the field at about 20 feet off the ground, just barley keeping it under control. Managed to turn it around and had a good landing, but man, that was intense! I've decided that without the flying tail, elevons aren't enough and ailerons will be necessary.

It would not help, that's for sure. How much of an impact it would have though is really hard to say. As long as you don't constrict the airflow with a massive dowel that takes up 50% of the tube it should not be that bad. You will take a little performance hit.

You do not have to put a pivot all the way through the fuse though. You could use some kind of surface mount hinge/pivot that would distribute the load to a large part of the outboard nacelle wall.

I will sleep on this, (been a long day), and I will post any ideas I can think of that will not add a ton of weight.

I appreciate the help! It is a big motivator to know that I'm not in this alone, and there are so many who so freely share their experience. I look forward to what you'll think of, and I'll be brainstorming too as I sleep.

If you set the sweep channel as flaps, and setup as a standard flap wing, you should be able to set a delay so it doesn't jolt (as least on Spektrum Tx's).

What are you doing about the CG change when the wings sweep? This may help better your performance.

I have a cheap FlySky radio. But on Thursday my QX7 should arrive - I'm so excited! The jolt isn't that bad on the 3 position switch, actually. The servo has to work for it, which means it can take time. Believe it or not when the wings sweep back the nose just drops a bit and it flies a little nose heavy, which isn't the end of the world. I have a feeling that this has something to do with the fact that the wide fuselage actually provides about 40-60 percent of the lift (according to Wikipedia) so the wings don't have as huge of an effect on the COL, and the wings moving back changes the CG aft-wards. The roll rate is the kicker when the wings are back. Once I get my QX7 I'll try and use both elevons and ailerons for moar rolling power!

 

[Namactual]

I flew it again tonight, this time with the tail set as elevons. It was a very short flight. I got it up in the air and flew it down the field at about 20 feet off the ground, just barley keeping it under control. Managed to turn it around and had a good landing, but man, that was intense! I've decided that without the flying tail, elevons aren't enough and ailerons will be necessary.

Aye, those elevators are way too small for that.

I have been looking online for something that would work for a pivot hinge and I have had no luck yet. Not anything reasonable anyway. You could always buy parts from an existing EDF kit, but they are pricey. I could design one if you had access to a 3d printer?

In the meantime if you wanted to test the elevons only, you could cut the hinge point on what you have farther forward. I would say about half the length of the stabilizer. That should give you some more leverage. Not as much as the full stabilator, but it should be enough to give you some roll authority with the wings back. Or you could just do like you originally planned and run a rod through the fuse. It would work for testing and would have no effect on the pusher.

I will keep investigating, I have to hit a few hardware stores sometime this week anyway. I need to figure this out myself as I plan to use stabilators on my Viper.