At What Point is Stall Speed Captured?

[macrafic]

I have had this question for as long as I have been flying (1969) but have never pursued the answer because I have only ever flown certified aircraft with a manufacturer POH. However, I am flying our own-built RV-7A and the question is now front and center.

When determining the stall speeds of a homebuilt aircraft, when is the actual speed captured and what speed is used?

Is the speed captured at the pending stall (buffeting, etc.) or is it captured at the moment the wing falls? Is IAS or TAS generally used?

During testing, I captured the IAS at the moment the wing fell but, since I am calibrating an AFS AOA, which is dependent on the identified stall speeds, I want to make sure I am as accurate as possible.

Appreciate input!

[Ironflight]

Ahhhh...stall speed. While we are taught that it is simple, it really isn’t, and every airplane type has different characteristics.

Part of the problem is that the entire wing doesn’t always stall at the same time - or the same way twice. Plus - if you look at a plot of AoA vs. Lift Coefficient for an airfoil, the peak is a curve, not an abrupt angle. So you are still generating a little lift at some point after Max Cl (until you’re not, which is just a little later....).

Then of course, you have airframes that simply don’t have a stall break - they just sit there nose high and start to sink. Short wing RV’s generally don’t do that, so if we’re talking RV’s, take the stall when the one nose drops at the break. And, of course, that is IAS becasue that is traditionally what a pilot is going to see.

Paul

[RV3bpilot]

I agree with Paul, when one or both of wings quit giving lift and the nose drops that is the stall speed to write down.

[macrafic]

Terrific! Thanks all!

[Saville]

Originally Posted by macrafic

I have had this question for as long as I have been flying (1969) but have never pursued the answer because I have only ever flown certified aircraft with a manufacturer POH. However, I am flying our own-built RV-7A and the question is now front and center.

When determining the stall speeds of a homebuilt aircraft, when is the actual speed captured and what speed is used?

Is the speed captured at the pending stall (buffeting, etc.) or is it captured at the moment the wing falls? Is IAS or TAS generally used?

During testing, I captured the IAS at the moment the wing fell but, since I am calibrating an AFS AOA, which is dependent on the identified stall speeds, I want to make sure I am as accurate as possible.

Appreciate input!

Just this week I went up to establish the power off stalls - flaps up and flaps down - as a prelude to working on short field landings.

I took buffet onset as the stall speed.

[penguin]

When measuring your stall speed it is important to maintain 1g flight, that means slowing down slowly. All the books quote 1kt per second, which is much more slowly than a typical 'CFI stall'. But there are 2 aspects, first ensure the airplane does nothing too sporting when it stops flying, and then figure out what the stall speed is. Sounds like you are already passed the first part :-).

Start at about 75kt and trim, ensure you have plenty of height. Close the throttle and let the aircraft slow down, don't try to maintain altitude and expect the aircraft to descend. Concentrate on slowing down gradually, assuming you will stall at 55kt indicated, it will take 20 seconds to reach the stall. The stall is the minimum speed reached before the nose pitches down, a wing drops or you cannot raise the nose any further. Once either of those happens recover and try again, but from 10kt above the stall to save time. Ensure you are not continually losing height. It will take a few attempts to be able to note the minimum speed, and to get repeatable results over 2 or 3 tries. Try again with full flap.

Will be necessary to climb after a few attempts, warms the engine also. If the stall is approached too quickly the wing will be supporting more than 1g and the stall speed will be higher than the minimum attainable. try to fly in reasonably smooth air.

Pete

[rocketman1988]

I?m not sure that using the buffet as the stall speed is a good approach. If you trying to be accurate, that buffet speed will typically be a bit faster than the actual stall. If you are using the stall speed to determine other speeds, like approach speed, the error will compound and you will always end up fast. Not especially good in a short field scenario. Each additional knot will require additional runway, and it adds up fast...

[BobTurner]

Definitely IAS, since it ( stall speed in IAS) is independent of density altitude.

[Saville]

Originally Posted by rocketman1988

I?m not sure that using the buffet as the stall speed is a good approach. If you trying to be accurate, that buffet speed will typically be a bit faster than the actual stall. If you are using the stall speed to determine other speeds, like approach speed, the error will compound and you will always end up fast. Not especially good in a short field scenario. Each additional knot will require additional runway, and it adds up fast...

I noted buffet speed and then the speed at which the stall broke.

I wanted to know both. But to be conservative I will think buffet speed.

From Vans "Stall Testing":

"Allow the speed to bleed off until you feel a slight buffet. Note the airspeed and recover......

...the buffet which does occur does so within just an mph or two of the fully developed stall. "

He does not say note the airspeed at which the stall fully develops.

If 1 knot is going to make the difference between landing and not landing I won't be trying to land there unless I have to.

[Vac]

If you have data logging capability, you can plot IAS, G and AOA (if you have one installed). Pretty easy to pick the stall IAS out from the data. If you fly a series of full power-off stalls, you'll see fairly consistent data for each flap configuration that you fly at a given gross weight.

The peak AOA/lowest IAS on the data plot correlate with a clean nose drop (i.e., loss of elevator authority).

Cheers,

Vac