The scratch-built twin-EDF CH-F14
- Thread starter Chuppster
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- 64mm edf edf f-14 scratch build
I can say from experience (tho I can’t go into detail just yet) that elevon-only control does indeed work for jets like this. But you do in fact need a larger than usual movable elevator surface. Your roll rate won’t be quite as snappy, but that’s not necessarily a bad thing on a big scale jet.
Stay tuned next week!
Stay tuned next week!
Chuppster
Well-known member
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.
I am an avid 3d printer, but I have been wanting to keep this model as easy to reproduce as possible. I too have been thinking, what about 2 pieces of plywood, one on the outside of the nacelle and one on the inside, with concentric holes that just allow a rod to slide through? Then pass the rod through both pieces of ply and put a stopper on the inside, and do like Mid7Night on his Phantom with the tail. Rudimentary cross-section attached.
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Chuppster
Well-known member
I can say from experience (tho I can’t go into detail just yet) that elevon-only control does indeed work for jets like this. But you do in fact need a larger than usual movable elevator surface. Your roll rate won’t be quite as snappy, but that’s not necessarily a bad thing on a big scale jet.
Stay tuned next week!
You're such a tease! Looking forward to this mystery airplane that you always talk about but has always been off-frame.
Namactual
Elite member
It looks good from here bud, just be mindful of the extra weight that far aft of the CG and you will be golden.
A lot of the EDF kits I have use stabilators with elevon mixing and they work great. They really give you a lot of leverage especially at lower speeds. The only thing to watch is control rod/horn slop. That gets really amplified with those massive surfaces at high speeds.
The elevon mixing on the ailerons will cause a little... mushing maybe? when you pitch, but it really does help at lower speeds in reducing tipstall. I use it on most of my sporty jets with aggressive wing sweeps.
With a little surface tuning and mixing you make make that fly like a trainer. It will just take a little trial and error.
A lot of the EDF kits I have use stabilators with elevon mixing and they work great. They really give you a lot of leverage especially at lower speeds. The only thing to watch is control rod/horn slop. That gets really amplified with those massive surfaces at high speeds.
The elevon mixing on the ailerons will cause a little... mushing maybe? when you pitch, but it really does help at lower speeds in reducing tipstall. I use it on most of my sporty jets with aggressive wing sweeps.
With a little surface tuning and mixing you make make that fly like a trainer. It will just take a little trial and error.
Chuppster
Well-known member
Update:
Yesterday I was able to cut a slot for the spar in the fuselage, so now the wings are not quite so high. I also mixed the elevator with the ailerons. It flew really well (no video, sorry) and finally it has plenty of roll rate. I do think the pusher prop is putting too much strain on the back of the F-14. The vstabs tend to pull towards each other as if the nacelles are rotating into each other. I hope this will be less prominent once I put the top plate on.
Yesterday I was able to cut a slot for the spar in the fuselage, so now the wings are not quite so high. I also mixed the elevator with the ailerons. It flew really well (no video, sorry) and finally it has plenty of roll rate. I do think the pusher prop is putting too much strain on the back of the F-14. The vstabs tend to pull towards each other as if the nacelles are rotating into each other. I hope this will be less prominent once I put the top plate on.
mrjdstewart
Legendary member
trust me, it will be worth the wait. absolutely amazing work on his part.
not that i have see one, already have one in hand, did a mock up for skins, and a BIG motor just waiting in the wings to build it, or anything like that.
what can i say, i know a guy...
i think i'm almost as excited as Ben. this thing is cool and i can't believe i already have my hands on it. 3 days, 15 hours and counting until the cat is out of the bag. swore to Ben i would not say a word until he did the big unveil @ FF. probably shouldn't even be saying this much...
laters,
me
Chuppster
Well-known member
I finally put the top piece on the fuselage. It has a few fitment issues but hey, that's what Alpha plans are for, right?
Then I took it out to fly it with my new receiver and realized right after I let go of it that I had forgotten to reverse the ailerons. That was a rough crash, but it didn't take long to glue it back into shape. The issue is that I broke the only servo horn I had for the wing mechanism. It looks like the F-14 will have to hang here while a new (and hopefully more powerful) servo makes its way here from China. Unless anyone knows how to build a new servo horn...
Then I took it out to fly it with my new receiver and realized right after I let go of it that I had forgotten to reverse the ailerons. That was a rough crash, but it didn't take long to glue it back into shape. The issue is that I broke the only servo horn I had for the wing mechanism. It looks like the F-14 will have to hang here while a new (and hopefully more powerful) servo makes its way here from China. Unless anyone knows how to build a new servo horn...
Chuppster
Well-known member
The new servo came in, and it is far better than the old one! Moves the wings with no issue. So I got her all flight worthy and took her up. Takoff was fine, but it wasn't long before I found myself in a state of questionable controllability. It was very strange, I would give it aileron input and it wouldn't do anything, then it would roll like mad. It also went high alpha at one point and just kind of floated in mid air. I was doing all this at a relatively low altitude, as I could't really get the airplane to do what I wanted. Eventually, I just cut the throttle and let go of the sticks. I found it in the corn eventually with a bashed up nose and the wing spar broken clean. Repairs shouldn't be terrible, but I think I'm going to wait until after harvest to continue development.
I have two theories about where issues could arise. The first one is the big servo. When I installed it, after it reached the limits of the sweep (I have endstops) it would stall and the control surfaces went crazy. I'm assuming this is because of too high of current draw from the servo. I dialed back the throw so it wouldn't stall, but that may have triggered mid-flight and caused issues. I plan to give the servo its own BEC.
Another thing is the pusher prop. 700 watts on the backside of this thing may be causing issues. I'm not sure how the torque changes things in a stall.
Any suggestions would be quite welcome in this circumstance. I wish I would have taken a video. I plan to buy the EDFs before long, as if this doesn't work out I could use them in a nnA-10.
I have two theories about where issues could arise. The first one is the big servo. When I installed it, after it reached the limits of the sweep (I have endstops) it would stall and the control surfaces went crazy. I'm assuming this is because of too high of current draw from the servo. I dialed back the throw so it wouldn't stall, but that may have triggered mid-flight and caused issues. I plan to give the servo its own BEC.
Another thing is the pusher prop. 700 watts on the backside of this thing may be causing issues. I'm not sure how the torque changes things in a stall.
Any suggestions would be quite welcome in this circumstance. I wish I would have taken a video. I plan to buy the EDFs before long, as if this doesn't work out I could use them in a nnA-10.
Namactual
Elite member
The prop torque at low speeds is a huge factor on roll behavior. If you are trying to roll against the prop it will barely roll at all, then you roll with it and it turns into a centrifuge.
What is the amp rating on your current BEC? Over drawing the BEC would definitely explain the odd behavior as well.
What is the amp rating on your current BEC? Over drawing the BEC would definitely explain the odd behavior as well.
Chuppster
Well-known member
It's a Red Brick, so 5 amps.
Do you think the EDFs would do better with the roll behavior? My 70mm EDFs have a crazy roll rate both ways (I love 'em!).
Namactual
Elite member
The torque roll on EDF's is almost non existent.
Here is an example of two aileron rolls, one against the prop and one with.
Thresher Vid 1:02
The first roll at 1:02 is against, the following roll is with. The same plane, roughly the same speed, both using full deflection.
You will not have to fight the torque of a big prop like that with the edf's. I still say you would need more of a control surface in full sweep though. Especially st low speeds.
Here is an example of two aileron rolls, one against the prop and one with.
Thresher Vid 1:02
The first roll at 1:02 is against, the following roll is with. The same plane, roughly the same speed, both using full deflection.
You will not have to fight the torque of a big prop like that with the edf's. I still say you would need more of a control surface in full sweep though. Especially st low speeds.
Chuppster
Well-known member
Yes, I have a good understanding of torque roll on tractors, but I didn't know if it was different on a pusher (no propwash on control surfaces) and the lack of control that I experienced was rather extreme.
Zephyr1
Elite member
"I found myself in a state of questionable controllability. It was very strange, I would give it aileron input and it wouldn't do anything, then it would roll like mad. It also went high alpha at one point and just kind of floated in mid air."
Ok, so this is pretty much exactly the scenario that I found myself in with the maiden on my F-14. It happened again on the third flight. The first crash resulted in a tree landing and the other one lawn darted into my driveway. But i know exactly what you mean in terms of controllability. It didn't matter what you did with the aileron, it just wouldn't move. Same goes for the high alpha, it just kinda floats there.
I didn't come up with anything to fix it, it only happened with the wings swept forward (second flight they were back and it was fine). I haven't come up with any fixes yet, but I put my project on hold for a little while (summer school's the worst
Ok, so this is pretty much exactly the scenario that I found myself in with the maiden on my F-14. It happened again on the third flight. The first crash resulted in a tree landing and the other one lawn darted into my driveway. But i know exactly what you mean in terms of controllability. It didn't matter what you did with the aileron, it just wouldn't move. Same goes for the high alpha, it just kinda floats there.
I didn't come up with anything to fix it, it only happened with the wings swept forward (second flight they were back and it was fine). I haven't come up with any fixes yet, but I put my project on hold for a little while (summer school's the worst
Chuppster
Well-known member
Well friend, we shall hold one another accountable to finish these birds, despite them having some crazy evil tenancies. I find it very interesting that we both had the same issue, and you are using EDFs while I have a prop. I'm wondering if full sized stabilators will help... next time I try and fly the bird will feature them, if I can sort out the logistics.
If anyone has an possible theory as to why these would stall this way in the wings forward position we would love to hear it.
Chuppster
Well-known member
It's been a few months since I've had a chance to work on this, and I managed to pick up a pair of 70mm EDF's for it. However, I can't seem to bring myself to finish it. Our friend @Splinter189 has given us a marvelous example of a twin 70mm Tomcat and he's clearly put a lot of time into making it a beautiful flying model. I look forward to him releasing the plans, and I'm laying this project to rest. I've learned a lot and I thank you all for following. Cheers!
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