Why does my modified Jetiger flip over shortly after launch? - Page 3 - RC Groups
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Jul 12, 2012, 12:58 AM
12th Pursuit Squadron
TheAeronut's Avatar
I understand trying to be true to the original design. What confuses me here is that the intake opening and ductwork on your model appears to me to be substantially different (larger intake and more taper) than the original. My feeling is that the shape of the volume of space between the duct and fin from the inlet to the TE of the wing is significant to the stability issues that you are experiencing. The intake on the original on the original looks to me to be similar in shape and not much larger than the area you blocked with the modification that you installed to reduce the intake area to better match the fan. To my eye, also your fuselage is somewhat taller than the original. Fidelity to the original is already long gone.

Seriously I do not intend to offend. I am offering my observations in hopes of being able to help you achieve your goal of a good flying replica of the original kit.

Best of luck with the test flights. I look forward to the flight reports.

Last edited by TheAeronut; Jul 12, 2012 at 01:03 AM.
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Jul 12, 2012, 01:33 AM
B for Bruce
BMatthews's Avatar
Djacob, by adding the side pieces to reduce the intake area you will starve the fan unit and fail to achieve the same sort of performance you had before. When the guys are talking about your intake area being bigger than the original design it was in relation to the reduction in the gap between the intake shround and the fins. NOT the actual area of the inlet. To get the most undisturbed air you can into the fan you really aught to take off those side covers along the sides of the fuselage.

I see that you're going to try flying it without skinning the wings. It will be an interesting test to see if it trips again when it reaches some amount of higher speed.

This time try to get a good classic amount of "3 mistakes high" before pushing out of the climb and into level or diving flight. If it does suddenly snap into a tumble throttle back right away to shutdown and try adding full down elevator to see if you can get it out of the tumble and into a dive. From there pull up gently to a glide and slowly add some throttle until it can fly around at a speed similar to that you used during the climb. Try to get a feel for the controls at this reduced speed and see how it responds. Then throttle up and climb back up and try the high speed mode again.

If it flies predictably and easily at moderate airspeeds and only trips and tumbles at high speed it supports my idea that the wing is flexing due to the air speed and loads on it and that is what is producing the tumble.

This flexing of the wing to produce the tumble is a form of flutter. To correct the flutter you need to move the axis of the wing's flexing forward and stiffen the flat plate you're using for the airfoil. The spar I see set into the wing is actually too far back to produce the correct sort of flexing to avoid the flutter. But adding the suggested carbon tube along the leading edge will move the flexing axis forward where it will better resist flexing in a divergent manner.
Jul 12, 2012, 02:17 AM
Zen in the art of foam
djacob7's Avatar
J.P., I should have added "to the original design + as much as possible." LOL!!
I struggled quite a bit to get the duct made to fit in the first place. Anyhow, to me it looks like a Jetiger (or maybe Jetigress?).
Bruce, both you and me have a point about duct area. J.P. wrote "The issue that I see is that the duct intake cross section is much greater than the FSA. This large duct entrance makes the area between the duct and the vertical fins open up rapidly."
IMHO, the large duct area I had is just like a brake. Adding that streamlined "baffle" helps to reduce the braking effect.
Jul 12, 2012, 01:22 PM
B for Bruce
BMatthews's Avatar
Sorry djacob but you're thinking wrongly on the duct. Efficiency of a ducted fan relies on the the freedom of air movement through the duct. By adding the side pieces you're choking down the duct and producing turbulence at the face of the rotor disc which will greatly compromise the thrust you can generate. An oversize inlet on an EDF duct is simply going to better ensure that the fan has a good supply of air to work with. The fan is more than capable of passing all the air that the oversize front entry can feed it. Only at low or shut off when in a dive will the size of the duct act like an airbrake.

Also JP's point was that the gap between the rim of the inlet and the fins is small. Your fix of adding the side pieces does nothing for changing that. What you need to do if you want to make the inlet mouth to fin gap bigger is change the first part of the duct to a smaller diameter front opening. If you swapped to such a shape then it would open up the gap between the duct and the fins and let more air flow in a more clean manner through that gap.

My own personal felling is that it's a non issue though. The model requires side area at the rear to achieve yaw stability. It really doesn't matter if that area comes from fins or from the side view of an oversize ducting for the fan unit. The only real downside of the small gap and shape of it is that more drag is created than what you would have otherwise. Mostly it's simply going to hurt the top speed.
Jul 12, 2012, 02:22 PM
Zen in the art of foam
djacob7's Avatar
Bruce, I think that the entry area of the duct should be just slightly larger than the FSA which it now is. The additional area just acts as a resistance.
JP's point also said very clearly that the duct entrance is way too large and I agree.
But I sure appreciate your input and I hope you're not offended by me stating my own opinion. Much thanks.
Jul 12, 2012, 02:25 PM
Zen in the art of foam
djacob7's Avatar
Here are some videos from this morning. It turned out that the fuselage height was a major culprit. It flew pretty well without the fuse cover.
Some wing stiffening would improve stability even more it seems. I'll fly it with 4S after stiffening the wing.
Launch #1 with cover on...
Jetiger Launch 1 (0 min 21 sec)

Launch #2 with cover on...
Jetiger Launch 2 (0 min 22 sec)

Launch #3 with cover off...
Jetiger Flying 1 (1 min 57 sec)

Launch #4 with cover off...
Jetiger Flying 2 (1 min 42 sec)
Last edited by djacob7; Jul 12, 2012 at 02:32 PM.
Jul 12, 2012, 03:52 PM
B for Bruce
BMatthews's Avatar
Well, looks like you found the trouble. It's that the cover is causing too much of a forward fin effect.

As was mentioned in an earlier post by someone they noticed that you made the fuselage deeper than the original. Apparently this produced the imbalance of forces between the fuselage shape and fin area.

If you make your fins larger, move them back more or simply do a little of both you will be able to fly with the cover in place again.

Once you get that part working I'd still suggest you try removing those side pieces. If you look around online for fan facts I'm pretty sure I've never seen anywhere that says that too BIG an inlet is any sort of problem when it comes to fan efficiency. On the other hand choking down the inlet or outlet does produce problems.
Jul 12, 2012, 05:49 PM
Zen in the art of foam
djacob7's Avatar
I will reduce the height of the fuse to that which it is without the cover.
About the duct inlet being BIG, it may not affect fan efficiency but it still is a source of drag. Just imagine an inlet twice or three times the size I had before (or 50 times larger?;-)
Jul 12, 2012, 06:14 PM
12th Pursuit Squadron
TheAeronut's Avatar
I am glad to see that it flies, and reasonably well at that. Now that you have a handle on what the biggest problems are you can experiment with what solutions work best.

I still say move the fins out and back. I would also reshape the inlet to make it narrower - kind of like the inlets on an F-4 Phantom. To my eye that would make it more interesting. It is all up to you. While you have a model as a starting reference, you can tweak it to make it look however you like. These are some of the joys and challenges of being a model designer and not just a kit (or ARF ) builder.

Once you have all the tweaks worked out so that it flies as you want and has a general shape that you like, then you can start planning the second build to look nice and fly well. More joys of being a designer...

Best of luck

Jul 12, 2012, 08:52 PM
Zen in the art of foam
djacob7's Avatar
I reshaped the inlet and made it smaller. I also reduced the fuse height by about 1.25 inches and removed the two side baffles. We'll see how it does this evening. Thanks J.P.
Jul 12, 2012, 09:11 PM
12th Pursuit Squadron
TheAeronut's Avatar
Happy to help out. I look forward to seeing pictures and video as the project progresses.

Jul 13, 2012, 04:57 PM
Zen in the art of foam
djacob7's Avatar
Here are some pics and vids after the latest fixes. The side pieces were removed, the fuse height was reduced by 1.25", and the intakes were reduced significantly. The next step is to stiffen the wings with paper and white glue. I'm a bit worried about distortion after the glue completely dries. But I want to learn how to do that, it's about time.
Thanks everyone for your suggestions and encouragement!

Jetiger Flight After Fixes 1 (2 min 3 sec)

Jetiger Flight After Fixes 2 (1 min 22 sec)
Last edited by djacob7; Jul 13, 2012 at 05:25 PM.
Jul 13, 2012, 05:20 PM
Grad student in aeronautics
So would you say you like its stability and handling?
Jul 13, 2012, 05:29 PM
Zen in the art of foam
djacob7's Avatar
DPATE, yes, but not 100%. I think that there are slight wing deflections/distortions that make the plane have a mind of its own. I want to upgrade to 4S and that will definitely need stiffer wings. I'm experimenting with paper and glue to apply that to the plane later.
Jul 13, 2012, 06:41 PM
vespa's Avatar
Bruce correctly identified the problem long ago but I'll add that the term is "aileron reversal" and is a very common phenomenon. Above a certain speed, most any plane will exhibit this behavior. When the aileron is deflected, the wing twists in the opposite direction and at some point this twist dominates and the result is a roll in the opposite direction than commanded. In your case you are not yet experiencing full reversal but rather extreme adverse yaw as you command ever-increasing aileron with little result.

Your spar serves as an "axle" of sorts that the rest of the wing twists around. It's location aft of the 1/4 chord makes every part of the wing dynamically unstable and this is major cause of your trouble. You could add a second spar to the LE, remove some material from the LE (especially at the tips), skin the wing, thicken the wing, or as a last resort, add a spar to the TE. Basically anything to improve the torsional rigidity or move the torsional axis forward, especially at the tips.

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