IAS and Altitude Question.

[vheissu]

The other day while practicing Chandelle's, I was noticing that toward the end of the maneuver, the IAS would be below specified stall speeds (48 kts). I could get it below 48kts with no stall. Why is this? I've always been taught that IAS is the same regardless of altitude.. so should'nt the airplane stall at the same IAS, no matter what? I have alot to learn... Thanks in advance.

[meritflyer]

Originally Posted by vheissu

I've always been taught that IAS is the same regardless of altitude.. so should'nt the airplane stall at the same IAS, no matter what?

Nope. The airplane will stall at the critical AoA regardless of weight, attitude, or airspeed, not KIAS. Stall speeds can vary from what the aircraft manufacturer publish. Things that may alter stall speed are weight, load factor, CG, wing contamination, and flap position.

The reason your airplane didnt stall at the published speed was because your critical angle of attack, or angle the aircraft will stall at regardless of weight, attitude, or airspeed, was not exceeded.

[desertdog71]

Originally Posted by meritflyer

Nope. The airplane will stall at the critical AoA regardless of weight, attitude, or airspeed, not KIAS. Stall speeds can vary from what the aircraft manufacturer publish. Things that may change a affect stall speed are weight, load factor, CG, wing contamination, and flap position.

The reason your airplane didnt stall at the published speed was because your critical angle of attack, or angle the aircraft will stall at regardless of weight, attitude, or airspeed, was not exceeded.

That and also if you have ever done a slip, you will notice that the IAS will be quite a bit lower slipping one direction as opposed to the other. The IAS may not reflect the actual airspeed due to interference of the airflow into the pitot tube.

[meritflyer]

Originally Posted by desertdog71

That and also if you have ever done a slip, you will notice that the IAS will be quite a bit lower slipping one direction as opposed to the other. The IAS may not reflect the actual airspeed due to interference of the airflow into the pitot tube.

Yup. True.

[vheissu]

Thanks for the quick replies. I guess what got me thinking was that, for example, on an approach to a high altitude airport, you use the same IAS no matter what.. so I thought it had to have some sort of relationship to stall speeds as well.

[tgrayson]

Originally Posted by vheissu

The other day while practicing Chandelle's, I was noticing that toward the end of the maneuver, the IAS would be below specified stall speeds (48 kts). I could get it below 48kts with no stall. Why is this? I've always been taught that IAS is the same regardless of altitude.. so should'nt the airplane stall at the same IAS, no matter what? I have alot to learn... Thanks in advance.

Those are power-off stall speeds. Power-on stall speeds are lower (and unpublished) due to

1) Propeller slipstream over the wings
2) Thrust supporting some of the weight of the airplane.

[seagull]

Heck, wouldn't just being below max togw be enough on a part 23 marked ASI?

[flyer8]

Originally Posted by vheissu

Thanks for the quick replies. I guess what got me thinking was that, for example, on an approach to a high altitude airport, you use the same IAS no matter what.. so I thought it had to have some sort of relationship to stall speeds as well.

You are correct, the indicated stall airspeed at higher altitudes will be the close to the same, some errors do creep in at the higher altitudes, but they are not larger than a knot or two. However as stated above your stall speed was probably lower than specified because of weight, load factor, CG, wing contamination, and flap position.

-Nik