Stall speed
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Stall speed
I would like to collect some ideas on why two airplanes of the same model, which look the same, at the same weight & CG can have almost 10% difference in stall speed (power off, full flaps)? I have tested a number of things, but have not found an explanation. Any inputs are appreciated, has anyone come across this before?
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The airplane with the higher stall speed has been used as a test airplane. The stall is aft stick limited. I have eliminated the horizontal tail and elevator authority, it is sufficient to get it stalled properly. I tried different angles of stall strips, that did not change stall speed. It is a single engine, piston. Adding a small amount of manifold pressure as opposed to throttle fully closed decreases stall speed by several knots. The wing leading edge contour was slightly different from the other airplane, we tried to rework it to match, that reduced stall speed slightly, but not enough.
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1. How sure are you of the aircraft weights?
2. What is the history of the annual stall speed checks (if these are CAA aircraft)
3. Have you been applying mods to just the one aircraft, or to both. Are you modding the faster or the slower staller, if just the one.
Im a bit suspicious of your statement that moving the stall strips is doing nothing. Does it USUALLY on this type. If so, then it sounds very much like something else is trigerring a stall earlier than the range of stall strip positions, and that LE contour issue you mention is a likely culprit Id have thought.
have you tried stall checks in other configs, such as clean. perhaps its a flap rigging issue?
2. What is the history of the annual stall speed checks (if these are CAA aircraft)
3. Have you been applying mods to just the one aircraft, or to both. Are you modding the faster or the slower staller, if just the one.
Im a bit suspicious of your statement that moving the stall strips is doing nothing. Does it USUALLY on this type. If so, then it sounds very much like something else is trigerring a stall earlier than the range of stall strip positions, and that LE contour issue you mention is a likely culprit Id have thought.
have you tried stall checks in other configs, such as clean. perhaps its a flap rigging issue?
Tabs please !
Originally Posted by SCaro
The wing leading edge contour was slightly different from the other airplane, we tried to rework it to match, that reduced stall speed slightly, but not enough.
Is there still a difference in profile ?
Do they weigh the same ?
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I should have posted more details, sorry. I checked the airspeed indications on both airplanes with GPS, they are right on. Both are new airplanes (this is in the US). The one with the higher stall speed was weighed several times, the weight seems to be accurate. We have only modified the higher stall speed airplane. The stall speed flaps up and take off flaps is also higher than normal, but not as much as with full flaps (slotted flaps). I measured the angle of attack at which they stall, the one with the higher stall speed also stalls at a lower angle of attack. We checked the flap rigging, and set it to the largest flap deflection within the tolerance, no change in stall speed. I tufted the flap upper surface (low wing), the flow on the flaps was attached at full flaps before the wing leading edge modification, afterwards it was not, even though the stall speed was lower (another mystery, normally this would be a loss of lift). The wing contour is now as close to type design as we could get it, although on one side it semms to be slightly thinner than it should be. Stalls are straight. I'm out of ideas.
Tabs please !
You did not check the airspeed with a GPS, you checked the groundspeed.
Originally Posted by SCaro
I should have posted more details, sorry. I checked the airspeed indications on both airplanes with GPS, they are right on.
<snip> I measured the angle of attack at which they stall, the one with the higher stall speed also stalls at a lower angle of attack.
The stall is aft stick limited. I have eliminated the horizontal tail and elevator authority, it is sufficient to get it stalled properly
We checked the flap rigging, and set it to the largest flap deflection within the tolerance, no change in stall speed.
I tufted the flap upper surface (low wing), the flow on the flaps was attached at full flaps before the wing leading edge modification,
afterwards it was not, even though the stall speed was lower (another mystery, normally this would be a loss of lift). The wing contour is now as close to type design as we could get it, although on one side it semms to be slightly thinner than it should be. Stalls are straight. I'm out of ideas.
In the meantime, could you give us a couple of clues...
- Composite / metal / other?
- High/low/mid wing?
- High/low/mid tailplane?
- What sort of values are we looking at in the weights and stall speeds?
G
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It looks like I will not get anywhere with this without telling you everything. I'm an aeronautical engineer and DER test pilot.
The airspeed readings were verified with the GPS triangle method and were within +-1 kt. I measured angle of attack with a string attached to a boom on the wingtip and a scale next to it. The accuracy was verified by measuring the angle of the fuselage at different airspeeds in level flight. It correlates well up to about 8 deg alpha, at higher angles the string on the wingtip indicates too high, but this was mainly to get a comparison between the two airplanes. The difference in stall alpha was at least 3 deg on the wing tip gage.
The stall can be either defined by the airplane pitching down or by the stick reaching the aft stop. In this case the stick reaches the aft stop and can be held there, but the lowest speed is reached right at aft stop and then increases together with the sink rate. The nose stays up. I had increased the elevator up deflection until I got the airplane to pitch gently, but this had no effect on stall speed.
The wings are not swept and the aileron rigging was checked (within type design tolerance).
Engine speed at idle is about the same, I have to add about 3-4 inHg of manifold pressure before the stall speed decreases, but it does that for all of our airplanes.
I have run some airfoil variations through XFOIL, but the analysis showed no difference. At stall angles of attack any analysis is more of a guess anyway. We took "splashes" of the wing leading edges at various stations (wood templates bonded to the wing with filler, then removed, to get the exact airfoil shape) and compared them to plots of what the shape should be, that's how I know about the "thinner" issue. It is not apparent to the "naked eye".
The airspeed readings were verified with the GPS triangle method and were within +-1 kt. I measured angle of attack with a string attached to a boom on the wingtip and a scale next to it. The accuracy was verified by measuring the angle of the fuselage at different airspeeds in level flight. It correlates well up to about 8 deg alpha, at higher angles the string on the wingtip indicates too high, but this was mainly to get a comparison between the two airplanes. The difference in stall alpha was at least 3 deg on the wing tip gage.
The stall can be either defined by the airplane pitching down or by the stick reaching the aft stop. In this case the stick reaches the aft stop and can be held there, but the lowest speed is reached right at aft stop and then increases together with the sink rate. The nose stays up. I had increased the elevator up deflection until I got the airplane to pitch gently, but this had no effect on stall speed.
The wings are not swept and the aileron rigging was checked (within type design tolerance).
Engine speed at idle is about the same, I have to add about 3-4 inHg of manifold pressure before the stall speed decreases, but it does that for all of our airplanes.
I have run some airfoil variations through XFOIL, but the analysis showed no difference. At stall angles of attack any analysis is more of a guess anyway. We took "splashes" of the wing leading edges at various stations (wood templates bonded to the wing with filler, then removed, to get the exact airfoil shape) and compared them to plots of what the shape should be, that's how I know about the "thinner" issue. It is not apparent to the "naked eye".
Last edited by SCaro; 1st May 2006 at 18:07.
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Do the two ships retain aileron effectiveness similarly through the stall? --
Which is maybe another way of asking if the stall originates at the same spanwise position. I like to see the stall begin at the root & progress outboard.
Which is maybe another way of asking if the stall originates at the same spanwise position. I like to see the stall begin at the root & progress outboard.
A belated welcome aboard SCaro, and an interesting problem you have there. Sorry if I sounded a little patronising, but it's not always possible when somebody appears in the FT forum to gauge how much of a "grown up" they necessarily are.
A couple of thoughts...
- I used to own a little composite canard single seater, and have flown a couple of others of the same type. A common problem on that was differing washout between wings/aircraft which could alter both stall characteristics and trim. Any risk this might be a player?
- Presumably the CG values are similar? I've previously seen two otherwise identical aeroplanes (single engine, high wing) show quite different stalling speeds as a result of differing CG values. (Fwd CG leading to higher Vs).
I'll admit those are longshots. I shall go away and think....
G
A couple of thoughts...
- I used to own a little composite canard single seater, and have flown a couple of others of the same type. A common problem on that was differing washout between wings/aircraft which could alter both stall characteristics and trim. Any risk this might be a player?
- Presumably the CG values are similar? I've previously seen two otherwise identical aeroplanes (single engine, high wing) show quite different stalling speeds as a result of differing CG values. (Fwd CG leading to higher Vs).
I'll admit those are longshots. I shall go away and think....
G
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" I measured the angle of attack at which they stall, the one with the higher stall speed also stalls at a lower angle of attack."
Commonly, aft C of G as mentioned by Gengis.
Long shot. Wing/mainplane angle if incidence?
Hope this may be of some help.
Commonly, aft C of G as mentioned by Gengis.
Long shot. Wing/mainplane angle if incidence?
Hope this may be of some help.
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The stall originates at the stall strips, which are about 30 inches outboard from the fuselage. I tested stall speed at forward and aft CG, in this case no difference, although usually there is about a one knot difference (slower at aft CG). This airplane has very benign stall characteristics, less than 5 deg of roll, and very little aileron input is needed to keep the wings level.
We measured incidence of wing and tail and everything is within type design tolerance.
We measured incidence of wing and tail and everything is within type design tolerance.
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Originally Posted by SCaro
The stall originates at the stall strips, which are about 30 inches outboard from the fuselage.
.....
I tried different angles of stall strips, that did not change stall speed.
.....
I tried different angles of stall strips, that did not change stall speed.
If I understand you, the stall on this model is supposed to originate from the stall strips, YET on this specific tail number the stall speed did not react to the position of the stall strips. This implies that the stall is actually being triggered by a mechanism independent of the stall strips for this aircraft.
How far did you move the stall strips, and in which sense? If you moved them further down (under) the leading edge, which would correspond to a higher stall AoA than normal, hoping to delay the stall and get a lower speed, and nothing actually happened, this would be consistent with the theory that something else is tripping the flow.
That something is, I'm afraid, quite possibly your deformed LE profile.
If you wanted to try a test to confirm the theory (albeit not really solve anything...) you could calculate the delta stall strip position consistent with a 3 degree ADVANCED AoA, and place the strip in front of this position. This should then cause a change in stall AoA as the strip finally gets a chance to work before the other unidentified trip mechanism takes effect.
That would confirm the behaviour, it won't FIX anything. I fear you may be looking at further work on the LE - perhaps even reskinning, although I know thats an ugly word.
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We are planning to do something along what Mad Flt S. suggests, starting with more extensive tufting of various parts of the airplane. We know that without the stall strips the stall speed would be several kts lower, but that is not an option. This airplane is certified as spin resistant, and needs them for that purpose.