I was asked this in my ground school session today and couldn't reply.

Through my A&P schooling and flight training thus far I'm not sure that I've ever known there to be a standard relative humidity. I guess a higher humidity would make for less dense air, so it would make a difference.

All I've ever known is temperature 59F (15C) and 29.92Hg.

Anyone know the answer?

Another question he asked me is when in an erect spin, applying full rudder in the direction of rotation will cause what to happen? I said I was unsure, but answered that a flat spin would occur.

He said it was either a flat spin, or an inverted spin. My textbook doesn't go into that kind of detail. It just mentions how to correctly recover from the erect spin, explains what a flat spin is, and says that an inverted spin has roll and yaw in opposite directions. Leading me to believe that my answer was correct.

Why you'll never see an ISA day, it's a dry gas based on 0% RH.

Screw with your instructor, do all your work in Kelvin, 288.16 at sea level. Or ask him what a cubic meter of fog weighs - just under 13 grams.

Flying a C-150 so I'm not really sure why these topics were my "take home projects" for my last flight? I pretty well nailed the rest of the oral so maybe he just doesn't want me getting complacent.

He probably just wants you to be aware of what certain control inputs can cause the aircraft to do in a spin. If you know that adding ruder in the direction of a spin causes it to spin more, then you will understand why you should never do it.

I am confused why he asked a question he himself didn't know they answer to and then wants you to figure it out.

This leads me to believe he has no clue what he is talking about.

2StgTurbine explained it already so no need to repeat. But what he said on flat/inverted spin is right.

I do have few things to clarify:

1) Ailerons into or out of the spin does nothing for the stage of the spin. What I mean is if you are not in the developed stage, applying ailerons won't get you there. They merely tighten or flatten the spin, regardless of stage.

2) A 172, loaded in the utility category in the most aft CG position, will not develop in my experience. I have 100 or so spins in this aircraft, most of which were spent trying to get the spin to develop. That jagged, for lack of a better word, feeling you get while spinning is the mark of an incipient spin. The wing is stalling, snapping over for about a half turn, and then the stall breaks to stall again and repeat the cycle. A fully developed spin gives you the feeling of 1G with some, constant, lateral load based on the speed of rotation.

3) A 172 can get into an inverted spin. You have to swap rudder direction though. IE spinning to the right, flipped inverted requires left rudder.

Edit: This is why an inverted spin can be so confusing to a pilot. You apply opposite rudder, but before letting the spin stop/slow rotation you apply full forward elevator. The aircraft flips inverted and you have the proper rudder to excite that inverted spin. You don't like the feeling of the spin speeding up so you do opposite rudder again and yank back, now your upright again with rudder in the direction of the spin, whoops. Rinse and repeat till you slam into the ground.


As for your 150, that is a far squirlier aircraft than the 172. It can get into inverted spin but, I suspect, lacks the power and control throw to enter a flat spin.

For humidity, I suspect a trick question. Your instructor is giving me a bad taste in my mouth. To my knowledge, and I am not positive here, RH is 0 for the measurements of standard temp/pressure.

Na, I wouldn't say that. He's a nice guy and a very experienced pilot. It wasn't like he was grilling me on these questions it was more like a normal converstation ending with "jot that down, see if you can find an answer and we'll discuss it next time."

Also, as a private pilot I don't have to be proficient with, or even demonstrate for that matter, secondary, accelerated maneuver stalls, cross-controlled stalls, elevator control stalls, etc. Is a good understanding of them important? Absolutely. But for now we're just doing power on/off stalls, slow flight, steep turns so that topic gets a little ahead of where I'm at anyways.

I appreciate the answers though. Better detail than my Jeppesen!