Stall Speeds and CofG

[IRRenewal]

I was doing a check flight in a light single (TB20) which performed to the book on everything apart from the stall speeds, which were a bit on the high side (both for stall warning and actual stall).

With 2 pilots and 250 litres of fuel on board, the CofG was inside the envelope but close to its forward limit at 39.1". The forward limit at the weight we were at is about 37.7", the aft limit is 47.3".

A/C manufacturers like showing their product in the best possible light, so I was wondering if the stall speeds in the book could be for an aircraft with a CofG nearer the aft limit.

Is there a way of estimating the change in stall speeds with a CofG change?

The maintenance organisation is checking the airspeed calibration for me, but given that all other tests were spot on the book numbers I don't think it's too far out.

An added complication is that the Socata manual aren't particularly clear in stating wether data is IAS or TAS. The 'stalling speeds' graphs just say 'stall speed (kt)', were other graphs in the POH state either IAS or TAS. Also, there is no usable IAS-TAS correction graph in the POH. There is a bizarre statement on page 5.3 (for those of you with access to the POH) which says:

AIRSPEED CORRECTION

Vp: Speed relative to ground with no wind
Vc: (conventional speed) = Vi (indicated airspeed) + airspeed correction

Vc=Vi

I have a feeling something got lost there in the translation from French to English.

Any thoughts greatly appreciated (before I go and fly the A/C again with my observer in the back seat, shifting the CofG backwards by about 4").

[Mad (Flt) Scientist]

Originally Posted by IRRenewal

A/C manufacturers like showing their product in the best possible light, so I was wondering if the stall speeds in the book could be for an aircraft with a CofG nearer the aft limit.

Is there a way of estimating the change in stall speeds with a CofG change?

Both Part 23 and Part 25 require stall speeds to be for the most adverse (i.e. forward) cg position. I assume that would apply to your data too.

Very roughly, you can turn the cg shift into a tail download and thus into a pseudo-weight effect.

Say the distance from 1/4 chord wing to 1/4 chord tail is 50". Say the cg shift is 5" forward. The increase in nose-down weight moment is 5" * weight. The change in downforce at the tail to balance that is (5" * weight) / 50" i.e. 10% of weight. So if the cg went forward 5", you'd have an increase in tail load equivalent to 10% of a/c weight, so you could say that the stall speed should increase as if the a/c were 10% heavier (i.e. about 5% increase in stall speed). That's pretty crude but should give you a ROM to work with.

[IRRenewal]

Cheers

Much appreciated.

It turned out that the ASI over reads by up to 5 knots at the low end of the scale, which brings my readings within the accepted tolerances.

[Daniel_11000]

Stall speed

Good evening. All stall speeds are published as IAS, as this is the only way, for the pilot, to have a single reference speed at different altitude.
Correction tables correct then IAS to CAS. Clearly IAS should be free from instrument error - in flight test this is called TIAS, for True Indicated AirSpeed, where a perfect pitot tube feeds pressure to a perfect anemometer.
Cheers
Daniel

[Genghis the Engineer]

Originally Posted by Mad (Flt) Scientist

Both Part 23 and Part 25 require stall speeds to be for the most adverse (i.e. forward) cg position. I assume that would apply to your data too.

Very roughly, you can turn the cg shift into a tail download and thus into a pseudo-weight effect.

Say the distance from 1/4 chord wing to 1/4 chord tail is 50". Say the cg shift is 5" forward. The increase in nose-down weight moment is 5" * weight. The change in downforce at the tail to balance that is (5" * weight) / 50" i.e. 10% of weight. So if the cg went forward 5", you'd have an increase in tail load equivalent to 10% of a/c weight, so you could say that the stall speed should increase as if the a/c were 10% heavier (i.e. about 5% increase in stall speed). That's pretty crude but should give you a ROM to work with.

Although, for many aeroplanes at forward CG the stall definition shifts from the classic aerodynamic stall to something more pitch authority limited (the "mush"). Once in that region, the stall speed can go up somewhat more significantly, and this can be hard to predict.

The TB20, being of conventional configuration with fully reversible controls, I'd certainly expect to fall into that category.

G

[Mad (Flt) Scientist]

Originally Posted by Genghis the Engineer

Although, for many aeroplanes at forward CG the stall definition shifts from the classic aerodynamic stall to something more pitch authority limited (the "mush"). Once in that region, the stall speed can go up somewhat more significantly, and this can be hard to predict.

The TB20, being of conventional configuration with fully reversible controls, I'd certainly expect to fall into that category.

G

Yes, of course, concur entirely, the assumed weight method only works if the stall CLmax is the same; if you can't get there for other reasons, it won't work.

Since stall warning is usually also a fixed alpha/fixed wing CL (whether artificial or natural) it should still work for that case; in the kind of cases Genghis mentions, the warning-to-stall speed margin would presumably be rather different at the forward and aft cg cases (with more margin aft than forward, I'd expect)