CFI Pop Quiz #106

[Chris Burgess]

What statement is true regarding takeoff performance with high density altitude conditions?

A) The acceleration rate will increase since the lighter air creates less drag.
B) The acceleration rate is slower because the engine and propeller efficiency is reduced.
C) A higher-than-normal indicated airspeed is required to produce sufficient lift since the air is less dense.

[Aussie_Paul]

Got to be (b)! I think

Aussie Paul.

[Brent Drake]

B & C both make alot of sense

[automan1223]

This is like you have to know this before you go fly kind of thing. Each machine is effected at a different rate. After 90 f its too hot to get much performance out of your machine. Performance can be cut in half from 50 to 90 F.

Answer is B.

Answer C is only if you were trying to compute TAS, True Airspeed.

Jonathan

[ventana7]

B is correct.

It is extremely important for pilots flying in the mountains not to be sucked in by the concept in C.

[Rehan K.Janjua]

B sounds to be true
C is also a reasonable statement.
If I understand correctly.

Rehan.

[JByrd]

The answer is B.

A is clearly incorrect.
B is clearly right.
C is one of those answers that can easily mislead. Indicated airspeed is a measurement of the difference between total and static pressure (dynamic pressure), represented indirectly as km/hr or mi/hr motion rate at standard temperature and pressure.

It is interesting to note that wing performance is not strongly a function of gas type, temperature, density, chord length, or physical gas speed within usual bounds as long as the flow is subsonic (for the gas involved) and the boundary layer rapidly transitions to trubulent within the first 2% of the chord length. It is however fully responsive (and scalable) to dynamic pressure and angle of attack, regardless of the gas medium.

Therefore a wing will have the same performance at sea level, 10K, or 20k feet altitude on Earth or Mars-(only 1% of Earth's atmospheric pressure and cold CO^2 to boot) as long as a (i.e. 55 KT IAS) can be maintained by whatever suitable propulsion system. The development of the Mars Airplane project posed some interesting design constraints.

On the other hand the TAS (gas shear rate) will vary greatly as a function of gas type, temperature, & density. Hence the correction to IAS to compute TAS is needed to compute physical gas speed, largely for navigational purposes.

It is a perplexing question. I hope I stated it clearly.

Jim

[Chris Burgess]

Hey Jim, now that was deep, even for me!! Thanks.

One bonus questions (not for Jim). I am flying at 60 miles per hour IAS and the temperature is 32 degrees, then, I fly at 60 miles per hour IAS and the temperature is 80 degrees (calm air, all other variables the same), is my groundspeed 60 miles per hour each time also?

[automan1223]

Ok, given that data and putting our true airspeed at sea level pressure your TAS: according to my whiz wheel.

SLP @32 : 60 indicated True 58.5 mph.
SLP @80 : 60 indicated True 61.5 mph.

This does not seem like a large difference but it is.

Jonathan

[Chris Burgess]

That was the point. At 80 degrees, you cover a littler more ground at the same Indicated Airspeed!!! Longer takeoff roll etc. Don't forget dehydration when you fly, especially we open-frame guys.

[StanFoster]

Chris: This is a very interesting and educational idea for this forum. Thank you for initiating it. I am silently participating...and learning. Some of the questions I know...some I look like the RCA dog with my head tilted...but...the tilt soon leaves.


Stan