I’ve listened and bit my tongue for a while whilst the uninformed pontification goes on as if it were gospel and the issue continues to get further confused. I even let the shot at not following in the line of my dangerous attitude go by.
Frankly, I didn’t want to engage in this argument… you know the Special Olympics and arguing on the internet and all!
However now that Cirrus has delivered it’s Holy Blessing and set the record straight I will elaborate on the role of aileron gap seals on the Cirrus.
(I was heavily involved with my late friend Jim Bradshaw when we as Knots 2 U developed aileron and flap gap seals on the Comanche line.)
95% of a gap seal’s benefit is for increased cruise speed. The high pressure air beneath the wing literally creates a vertical curtain of high pressure air chordwise along the upper surface of the wing at the leading edge of the aileron -or flap.
Visualize one of those high velocity air curtains they use at the doors to air conditioned stores and industrial freezers. Now imagine that blowing UP from that gap on your wing. It is aerodynamically like maybe a two inch metal fence. ROUGH numbers to get an idea of the flat plate area of 72" x 2" per aileron (double that because there are two ailerons) you MIGHT have a factor of TWO square feet of additional frontal area adding drag to cruise flight!
Now despite the dire warnings… the removal of the aileron gap seals will actually IMPROVE low speed handling!
Because at high angles of attack the downward deflected aileron is actually REALLY at a high angle of attack and is pretty close to the stall if not already there. The downward deflection of the aileron at or near the WING stall speed causes that aileron to become stalled long before the wing stalls. At that point is is not much of a wing, it may as well be a sheet of plywood or your kid’s arm “flying” out the car window driving down the highway. It is only providing flat plate lift from the bottom and NOTHING from the upper aileron surface.
BUT… if you remove the aileron gap seal at low airspeeds/high angle of attack, you will have a nice flow of air between the trailing edge of the wing and the forward curve of the leading edge of the aileron! This airflow is relatively slow now so it will allow the aileron to continue to “fly” a little longer and deeper towards the wing stall.
Kind of like a crude Fowler flap or a leading edge slat on the heavy iron!
So, the aileron gap seals improve cruise performance and MAY reduce aileron effectiveness near the wing stall.
This is why FLAP gap seals aren’t generally used, The effect of the flaps in cruise is insignificant when balanced against the decrease in effectiveness in lowering stall/landing speeds!
All of this was borne out in the STC process flight testing that I was involved in many years ago.
In fact, Roy LoPresti never did offer a flap gap seal for the exact reasons I just explained.
We did because post 1961 Comanche’s had full fowler flaps and the flap gap seals really didn’t do anything detrimental to the low speed handling. Cruise performance improvements were less than 1 knot @ 150 KIAS and were not repeatable so they are pretty much ineffective. Jim figures adding an extra couple of strip of aluminium for $20 would help justify the $1000 cost.
So you’d get FOUR strips instead of two!!!
In summary, IF I had have lost a gap seal prior to the Cirrus Decaration I would not have flown (hey I’m not a test pilot!) unless it was an EMERGENCY… like a double date with Rick supplying the girls! And then I would have certainly removed the other seal because asymetrical ANYTHING that far out on the wing can’t be a good thing if I happen to find myself DEEP into an aerodynamic coner… no matter how insignificant!
Hope this helps.