Aerotrike Cobra high speed roll instability

[LeighFly]

After having owned a Windlass trike, I am finding some significant differences in behavior with my Cobra.

In particular, if I pull the bar in to increase speed (above 65 mph), she tends to want to turn one way, then if I correct the turn, she swings to the other side. It's as if the pod is also Flying and causing a conflict between wing and pod.

Is this behavior normal for a trike with a Pod (closed cockpit), or is it a wing characteristic or wing tuning issue? I have read a bit about Pilot induced oscillation as well, which also describes this kind of behaviour, but it's as if there is no way you can fly straight at higher speeds. The only remedy seems to be to let the bar go and slow down to trim speed.

Any suggestions would be much appreciated. (I have a Spirit 15 wing, incase that is significant too)

Regards, L

[extra300s]

Sounds to me like the wing needs some tuning. Derig wing and take all the battens out. Ensure the left and right side's battern curves are exactely the same. My 2c

[nicow]

Thats not normal-check your wing!

[Conrad]

Remember that you have more surface from the side than a windlass, so yes that will happen, but in the same breath Id say that you get the wing tuned and checked as well. My Aeros fly completely different from the Windlass, I have more power on the tail (100hp)and with fast acceleration I can feel how the trike pulls to the one side and the wing tries to keep it straight. This only happens at 80 mph and up, thus, I let go, and decelerate fairly slow as then is the time when one gets the pilot induced oscillation.

[Mc Guyver]

Remember that you have more surface from the side than a windlass, so yes that will happen, but in the same breath Id say that you get the wing tuned and checked as well. My Aeros fly completely different from the Windlass, I have more power on the tail (100hp)and with fast acceleration I can feel how the trike pulls to the one side and the wing tries to keep it straight. This only happens at 80 mph and up, thus, I let go, and decelerate fairly slow as then is the time when one gets the pilot induced oscillation.

Why not get an instructor to do some flying with you to check it out?
A few suggestions:
Try using your front wheel as a rudder at high speeds. (pulling right, turn front wheel right)
(Puzzled as to why she turns the other way when corrected)
Also remember to increase speed gently.
With the Airborne trikes, it works well.
I have tried getting the factory to offset the motor to counter this, but with no joy. (It is the thrust of the motor inducing the turn on the Airborne to the right above 80 Kts)

[bobthebuilder]

........ The Cobra bite!

Before you start to tinker with wings etc, give the boys at Rainbow a call.
I suspect that this has little to do with the wing, and everything to do with the pod. The pod will start to fly at a given speed. Hek ...even a brick can fly if it is moved through the air fast enough.

There was another post a few weeks ago, whereby a larger windscreen was fitted to a pod, and pilot described the same ill flight charaterisics.

Have you noticed how the GT450 has a fin on each wheel spat. I believe that these are to combat the symptoms that you have described above.

[Conrad]

The symptoms you talk about... roll / steep turn / side wind - instability.
The screen issue is solved now, made very smaller, and one cannot believe the effects this had.

[Triker]

HI there

I also have a Cobra with Spirit 15 wing and Ive pulled it to about 73Mph without any oscillation. Dont think it has got to do with your POD (I've same fairing) and engine (582 ) and same prop (NC) and have'nt found that problem yet. Though forum member CHARLIE WHISKEY may help you on this one as oscillation do occur on high speeds with the Cobra

[LeighFly]

Gee, thanks for all these responses. All sound good to me.

Okay, to narrow it down a little and be more specific. This happens at a constant power setting, but with bar pulled in, so thrust is probably not the cause here.

Wing tuning: the wing was pulling right at trim speed. I added a little reflex at the right wing end( last 4 bat tons). This corrected the right turn at trim speed and gave a gentle left turn, but when I pull the bar in to increase speed, it again turns right. The left-right oscillation seems to be independent of the tuning, as it happens any time I increase speed using the bar.

I have seen some very extreme wheel spat fins fitted to a "racing" Cobra, which was supposedly to improve stability.
Conrad, with the Aeros, did you fit the long fin spats to make the improvement?

Who would be the best test pilot in Cape Town to check this out for me, someone who is fully versed in all things "flex wing" etc.?

Thanks again for the interest shown here. This is a great forum

[Wargames]

Sent you a PM.

Regards.

[Conrad]

This is the second set of spats, as the first set was too small for higher speeds. Yes,
it makes the trike more stable.

[LeighFly]

I went flying again this morning in very calm conditions and found the following:

The undercarriage is not aligned straight with the wing. It aims to the left relative to the wing by quite a few degrees. Is this normal? This would explain why the pod flies to one side when the airspeed is increased. The wing is trimmed to fly straight at trim speed with the pod aiming to the left. If I force the Pod to fly straighter (using the bar to twist the wing relative to the undercarriage), the trike flies to one side, which one can expect.

So the question is, what is considered a normal amount of off-set from straight for the undercarriage relative to the wing? Is this adjustable at all and if so, where?

Thanks again
Leigh

[Conrad]

No, its not normal - at low speeds.
I suggest you contact Jenya at Petit.

[Splinter]

Also check the hangblock assembly for wear. Might be more play than normal. Bushes in the hangblock perhaps.

Regards
RED

[AEROYOTA]

Hi There,
Interesting. we have two Cobras in the hanger. One with full fairing and visor with Profy wing. It is fitted with a Rattax 582 and a Warpdrive prop. The owner flies regularly at 80mph and even up to VNE 95 mph without ever having any of the above mentioned problems. The second trike (TOYOBRA) is fitted with a Spirit 15 wing and "Pod" and cruises at 75mph wthout any problems. We have not tried going faster yet. Busy with hangpoint actuator.

Hope you can sort it out.