Stall Speed Reduction

[flyguy]

I decided to re-adjust the the way my ailerons take up the play when they faire in flight. Before they were set so they would have to rise up to a streamlined position to take out the play. I set them today where they would either be down and streamlined or would trail in a higher position as the play was now on top. That is they are now streamlined sitting on the ground with the weight pulling them down against the what play there is.
Test flight results: Climb same. Cruise same.
Stall--indicated stall is 4 mph slower. Whoa there, lets check that two more times. Was always what I thought was high right on 44 mph. Now it is 40, checked three times. No other changes made. Plane flies straight and level as before although I may have to loosen up the cables some, seem a little tight.
I didn't expect that. Every time I think that I am starting to know how airplanes work, I get a surprise from our favorite.

I wasn't thinking about it, but considering later I realized that the landing seemed awfully docile for the conditions. I'm keeping my fingers crossed until I can check it again tomorrow. Sure hope it is real.
DC

[Howard Wilson]

Re: Stall Speed Reduction

Go up 'HIGH' and try some steep slips, as to landing. See if things work ok with the tight ailerons. Had an Aeronca Chief 30 years ago (sheeeesh). My partner was an A.R. kind of guy that figured if tight was good, tighter was better. So got those trailing edges all nice and even. The next time I went flying with my instructor, who had been a German fighter pilot in WWII, we scared the crap out of ourselves when the ailerons kind of reversed themselves in a slip to landing. (With the right wing down in the slip we were holding left yoke input)! I know it's a different airfoil. Hope I'm wrong. Go up high.

[flyguy]

Re: Stall Speed Reduction

Yeah Howard I saw a beautifully restored BC12D at the Merced CA fly in a few years ago. The ailerons cables were so tight that it was difficult to move the controls. You just know the ailerons on that one would not completely straighten out by themselves in flight.
Mine are problably pretty much standard now, but I am use to keeping them perhaps a little on the loose side, so it feels tight to me. Flies fine, just doesn't have that light touch as before. Likely I will loosen it up a bit, but going to fly some first to assure what I saw is real. Hope it doesn't go away with less tension.
Thanks for the words of caution
Darryl

[VictorBravo]

Re: Stall Speed Reduction

would trail in a higher position as the play was now on top.

Stall--indicated stall is 4 mph slower.

DC

Remember we have "slotted ailerons" AND "friese type" ailerons. At high AOA, the slots accelerate air thru the slot and across the top of the wing behind the slot, energizing the boundary layer and keeping the flow attached. Some airplanes have the slot at the aileron, some have it on the leading edge (Stinson, Swift), but both designs are trying to do the same general thing.

Something is happening with the air flowing through the slots (due to the slightly different "trail" angle of ailerons) that is delaying the stall out at the tips, which is being seen as delaying the stall of the entire wing.

Test fly again with your high-trail ailerons, and be on the lookout for a bigger or sooner "stall buffet", or a "softer" stall. This will tell you that the "new" aileron slots are somehow delaying the stall at the outer wings, but since the inner wing is trying to stall first (just like the textbook) you feel the buffet from the inner wing stall while the outer wings are still hanging in there keeping the airplane in the air.

If tis is the case, then allow the ailerons to trail another 1/4 inch higher on the next test and try again. See if this delays the stall any longer. The limiting factor will be that of course you will eventually lose some rate of climb because of decreased camber and lift.

All that being said, I am guessing that your slotted ailerons are somehow not functioning properly, since the slot effect should be just as valid with the ailerons in perfect trail. Look at your slots and see if there is anything bowed in or out, bent, if the slots are completely closed with the ailerons in trail and open with the ailerons raised, etc. etc.

By all rights, you should have to lower the aileron trailing edges to get a slower stall, at the cost of a sharp and potentially dangerous "tip stall". Something's fishy

I always wanted to try tiny VG's inside the slots on the top radius of the aileron.... if you're feeling frisky DC try to put a small spanwise VG strip on the aileron, about halfway between the furthermost forward aileron leading edge and the top of the aileron. Take some pinking shears and cut a one inch wide strip thick soft plastic in half. As thick of a plastic as you can cut with the shears... or cut two and double them. you want about 1/16" thickness. Use double stick tape to put it on for testing so you can peel it off.

[pete]

Re: Stall Speed Reduction

flyguy,
I agree with the "whoa there" part. Dropping the ailerons a 1/4" might lower the stall speed a half a needle. I've got flaps on the experimental and I get about a
3mph reduction for each notch, about 10, 25, & 35. Checked with the gps at zero flaps & full flaps-looked like a 6 maybe 7mph difference. Just for fun I might try out one of those flying pitots sometime.
pete

[flyguy]

Re: Stall Speed Reduction

I guess from what you are saying Bill I misled you by omission. In flight now the ailerons seem to be pretty much in streamlined trail as before. I think one may have been a little higher than the other.

Though your analysis seems reasonable, I'm not sure it is valid, because as I remember when before I did a "precision" spin entry with one aileron down and one up against rudder and torque the give up and quit speed was always very near 44 indicated, about the same as straight and level. It is possible I am in error on that as typically I am thinking about other things at that point.

That being said, I haven't specifically checked the ailerons' positions at the exact time of the stall. They could be flopping up into the play position for all I know. I will have to check next flight if maybe they are deflecting upward when the flow starts to break up on top of the wing. I will also check for buffet and such as you suggest.

The truth is I was expecting to see a slightly higher cruise or top speed if the ailerons were positioning themselves in a more up position. I had thought the low pressure on top and high on bottom might do that. I couldn't absolutely tell if there may have been some small change in the top or cruise as it was a bit bumpy. As the ailerons appeared in proper trail I was not surprised though.

I only checked stall speed as a safety thing before I headed back to land, as I had no expectations of any change. It was quite a surprise when the second try gave the same results.

We had a gusty, cloudy day today with multiple mild thunderstorms (is there such a thing?) in the area so I didn't go up.
Looking forward to checking it out more. Will update.
Darryl
P.S. As for horizontal vortex generators I think I would be rather leery of such things as I can picture an unexpected roll maneuver with aileron deflection at or about initial flare altitude, LOL, but there wouldn't be any LOL.

[flyguy]

Re: Stall Speed Reduction

Considerably quieter and cool this AM. Checked about 5 or 6 times at altitude. Airspeed goes down to within a pointer-width of 40 consistently. Works out to about 55 VSO with a little fudge factor left over. Sure landed short with a little wind down the runway today. I'll have to check later, with a little more weight, see how much it changes.

It was maybe in the 60's this morning and I was showing 900 ft/min initial steady moving towards 1000 sometimes.

I was doing 1/2 turn spins to get down from altitude and I didn't get everything quite right and the she did a falling leaf for me. Weird, but kinda fun. Never seen that before.
Anyway everthing seems to be working well and I'm pretty happy with it. Never look a gift horse in the mouth.
DC