Series 5 Tailplane stall

[n85ae]

Hi -

That sounds very similar to what I had happen. Not the kind of thing you
expect. Just one second your flying along, and the next it's "Whoa, what
heck ..."

Where do you fly from? I learned to fly out at KBJC and get out there a
couple times a year.

Regards,
Jeff

[rogerh12]

Is this Flap induced Elevator Stall caused mostly by the inboard section of the flaps?

[jtpitkin06]

Thank you for sharing your incident with us. I hope others can learn from your experience.

I believe you experienced flap overspeed.

All airfoils have a maximum speed limit. The reason for the limit can be structural, where flutter can destroy the airframe in seconds; or, aerodynamic, where controlability becomes an issue. The aircraft might become super sensitive or non-responsive.

Let’s see if we can clear up what probably happened to you.

Extending flaps or flaperons dramatically increases the camber and extends the chord. Both changes move the center of lift aft. The center of lift shifting aft pitches the aircraft forward.

The pilot normally corrects for the forward pitch by trimming nose up. Some aircraft are unable to put in sufficient nose up trim requiring the pilot to hold a small amount of back pressure. Because the aircraft is not in trim it is easy for the speed to increase when the pilot’s attention is diverted.

When a wing produces lift, some air is deflected downward in the form of a wake. The wake or downwash aids the down force normally required by the stab/elevator to maintain pitch.

At higher speeds, with flaps/flaperons extended, pitch control becomes very sensitive. The amount of downwash can change very quickly, up or down, with small inputs from the elevator. The pilot can get into pilot induced oscillations with almost imperceptible inputs.

Elevator stick pressure may become very light due to rapid changes in downwash forces. It is possible for the aircraft to pitch up rapidly when exceeding the maximum flap speed resulting in excessive G forces. The aircraft may also pitch down rapidly leading a pilot to think the elevator has stalled. A rapid pitch down lowers the amount of downwash on the elevator. The pitch control may suddenly feel “mushy” even though it is still functional.

This phenomenon is not a blanking of the tail or a stalling of the stab. It is a case of flap overspeed where pitch control becomes extremely sensitive. It is not something peculiar to the Kitfox. It can happen in any aircraft equipped with flaps or flaperons.

To recover from flap overspeed the pilot must slow the aircraft or reduce the flap extension. Immediate speed reduction with steep negative pitch is almost impossible, so flap reduction is the action of choice.

You were very sharp to dump the flaps. Doing so put the aircraft back into a controllable state with partial flaps. Atta Boy!!!

The rapid pitch down slammed you upward with negative G’s. The sensation was the same as you might get from an accelerated stall. Your instinctive reaction from all your previous flight training was to lower the nose. I can’t fault you for that as you were in your own corner of uncharted territory.

I suspect your aircraft accelerated beyond the maximum flap speed with just a moment of inattention while you were looking at your descent path. It can occur very easily when near the maximum if you are out of trim. If your target speed was 80 you could easily pop up to 90, 100 or more in a blink.
We tend to think of the red line as the only maximum speed limit. We see that the maximum speed for flaps at the top of the white arc is just as important as the ultimate red line.

I ‘m pretty confident a flap overspeed is what happened to you. We’re happy you are here to share your story.

Regards,
John Pitkin

[n85ae]

That makes sense. I initially thought I'd done some kind of high speed stall,
then I started thinking tailplane stall. I'm still not sure, it was not particularly
pitch sensitive. I'd say it felt positive in control until the moment there was
no control.

I was very aware of the flaperons as I had dumped them in intentionally, so
it wasn't requiring much thought when it happened that they had caused it.

John at Kitfox said he was going to see if he could reproduce it based on my
description, if he does I'm sure he'll let me know. If so I'll post what he finds.

Regards, Jeff Hays

[SkyPirate]

well put JP,..we might need to check your lineage lol there might be some bernelli back there somewhere

[Dorsal]

Or perhaps some Bernoulli (sorry Chase, just busting your chops)

Jeff, Let me also say thanks for posting this, I rarely use the 2nd notch of flaps but I will be paying closer attention now.

[n85ae]

Less than the speed at which I extended them. I'm not saying I did the right
thing, I'm simply saying what I did and what happened.

Regards,
Jeff

[kmach]

Is there no placarded speeds for flap extension in a V?

Hi,

in my series 5 the maximum flap extension speed is listed and marked as 80 mph, I usually only apply flaps on final and below 80 mph.

I am very glad to hear that this story has a live ending

[n85ae]

I don't recall the exact speed, BUT it was definetly less than 100 I'd
say in the 80-90 range. I didn't just yank the handle at cruise speed,
this I can say for sure.

I honestly did not think I was going that fast, as I had already pulled
in 11 degrees, and trimmed the plane. I pushed over steeper, speed
started to build so I pulled in 22 held some back pressure with the
stick I honestly don't remember if I continued trimming, but I do recall
the stick went slack, and all hell broke lose at the same time.

Regards,
Jeff

[SkyPirate]

What matters Jeff ,..is that you knew how to fix it and your hear to tell us about it