Question about airworthiness

[cncpc]

An airplane has a Robertson STOL kit installed. It flies flaps up straight and level. When 10 flaps are added, it begins to roll and will roll inverted if the pilot does not intervene.

Apparently Robertson maintains that this is typical of the MOD. It seems to me that this airplane no longer demonstrates inherent stability, which is a requirement of the initial certification.

Any input?

[oldtimer]

You did not say what type of airplane is involved.
I flew a Cessna 337 a bit before and a whole lot after the Roberson STOL. One thing I did notice was aileron control was much more sensative in all configurations and not quite as stable in roll as the unmodified airplane but nothing at all scarey. I also flew a Cessna 206 RSTOL a bit and a whole bunch of time in an unmodified airplane and noticed very little difference. Now, an RSTOL Piper Twin Comanche, there was a piece of shit airplane. Always got airborne below Vmc and stability was something other airplanes had.
Maybe it is just like the de-ice boots and hot props or alkey slingers of old where all that was required as far as certification is concerned was to demonstrate that the installation did not have any adverse effect on the airplane or systems. Did not have to prove the equipment would allow safe flight in icing conditions.
It is my understanding that STC certification only deals with the installation of the equipment, readjusts the performance section of the Airplane Flight Manual if required and verifies that the original certified standards of stability and flight characteristics are maintained.
Maybe whoever is doing your maintenance is using the unmodified airplanes maintenance manual and the flaps or ailerons are slightly missrigged. Just a wee bit will make quite a change.
The airplane should not roll enough to cause an unsafe condition.
This is just a guess on my part as I am not a mechanic, owner or designer, just an oldtimer who flew RSTOL airplanes a bit.

[cgzro]

Sounds like the flaps are not deploying symmetrically. Were they removed as part of the installation? Id check the throws on both sides and also see if there is any difference in play when lightly loaded by hand.

[PilotDAR]

Depending upon the type of aircraft, it is likely that the deployment of 10 flap is also deploying some aileron, as with the Robertson kits I know (100 and 200 Cessnas) extending flaps droops ailerons in proportion.

I suggest very carefully confirming the travel of the ailerons and flaps, and confirming with the applicable maintenance manuals, both original airframe (Cessna?) and Robertson.

If that does not show up a deficiency, I would next look (again, thinking Cessna) at the wing angle of incidence. It could be that the angle of incidence of one of the wings has been adjusted to make it fly straight, and now the drooping aileron is counteracting that adjustment.

Is it a STOL kit with a leading edge cuff? Are they installed correctly?

Beyond that, were the wings rebuilt? Is their washout the same? Cessna gives data to confirm that, which is fairly easy to use, even wings on the plane.

STOL kits can reduce stability on roll, particularly if it is a STOL kit with a cuff on the leading edge. But, the plane should not do what you are describing...

[cncpc]

It's a 210. Robertson STOL. No VGs.

When the first notch goes on under 140, there is a very pronounced roll to the left, which is controllable by the pilot, but you certainly know its there. So you end up holding the left wing up while you're flying.

We've done a test and not stopped the roll. It was still rolling at 80 degrees bank when the pilot stopped the roll.

If you're not ready for it and have just neutral aileron on takeoff and 10 flaps, it rolls left as soon as the left wheel lifts.

I'm told all the tests and rigging and all that have been done ad nauseum and it still does it, and Robertson says that's just the way they are. My point is that to be certified, and aircraft must demonstrate inherent stability. This one doesn't in roll.

[iflyforpie]

Sounds like one of the flap bell cranks is out of rig. You can set flaps up and down to limits with the push pull rods, but if the bell cranks aren't synced, the flaps will split between the up and down limits.

Another thing is wing incidences. They are usually set for cruise flight and no flaps.

That being said, I don't notice any problems with my 337 that has an R/STOL.... other than the lack of roll control on approach that requires liberal use of the rudders to check.

[PilotDAR]

My experience is not with 210's, and they do have a different wing to the other Cessnas.

However, in general you are right, the plane should be at least neutral, if not positively stable, it should not roll off. In certification testing, there was probably a little "give" for some roll off at the stall, provided it could be easily controlled with normal use of the controls. But it should not be such that it is a distraction in cruising flight, and certainly not for a plane likely to be flown IFR.

If this is new, take back to whomever last had it apart, and tell them to put it right. Otherwise, you may have a lot of experimentation ahead of you. The problem will be that adjusting one control at one speed and flap setting will probably cause an effect at a different speed or configuration.

[GyvAir]

I recall some hangar talk regarding a similar issue on a 180 or 185. No recollection of any resolution being found though.

This discussion on another forum talks about installing aileron gap seals to mitigate the roll control issues:
http://www.backcountrypilot.org/forum/robertson-stol-16

[hoptwoit]

There is an answer to this and its all in the rigging. But I wouldn't want to turn this thread into weeee look how much I know about Robertson STOL kits...

[photofly]

I think the roll-stability issue is a red herring. If there's an aileron position (somewhere) that stops the roll then it's clearly stable. The fact that that aileron position changes depending on the degree of flap deployment is not strictly a stability consideration.

If it were unstable in roll then at best you'd be jockeying the ailerons left-right all the time trying to keep the aircraft right side up, fighing instability. At worst you'd be unable to control the roll at all and it would inevitably end up inverted. Clearly that's not the outcome.

[Heliian]

There is an answer to this and its all in the rigging. But I wouldn't want to turn this thread into weeee look how much I know about Robertson STOL kits...

There are people who would like to hear your experiences with it, hence the question and answer model of the forum.

As far as airworthiness is concerned, the installation of a Supplemental Type Certificate (STC) is a change to the type certificate and meets the standards of airworthiness. Flying is one big compromise, you wanted better stol performace but had to sacrifice some inherent stability. Like hoptwoit said, some rigging might get you back on the straight and level.

[Taco Joe]

I used to fly a 414 with the Robertson STOL kit, never noticed any type of roll when deploying flap. As many other people have said, it should be able to be resolved with an adjustment of the rigging.

I'm very surprised that Robertson would just say "Meh, that's just the way it is".

[cncpc]

I think the roll-stability issue is a red herring. If there's an aileron position (somewhere) that stops the roll then it's clearly stable. The fact that that aileron position changes depending on the degree of flap deployment is not strictly a stability consideration.

If it were unstable in roll then at best you'd be jockeying the ailerons left-right all the time trying to keep the aircraft right side up, fighing instability. At worst you'd be unable to control the roll at all and it would inevitably end up inverted. Clearly that's not the outcome.

I was thinking that inherent stability means, to a certain extent, the aircraft, if flying wings level were to disturbed, would eventually after a series of corrections built into the design, return to a more or less wings level attitude.

Thanks for all the suggestions. An engineer who has had a look at this says its in the rigging.

[LV2FLY2]

An engineer who has had a look at this says its in the rigging.[/quote]

I would agree with the AME. Was the STC recently installed?