Power Required / Power Available

Sidious

Well-Known Member
So for indicated airspeed Vy decreases as we go up in altitude and Vx increases. Got it.

However the graphs from the PHAK and in an Aerodynamic book that I'm reading show that Vy increases if you are looking at True airspeed.

My question is first, why is this... and second, why talk about indicated airspeed, Vx and Vy meeting and then show graphs that only show True airspeed?

What am I missing?

-Thanks in advance as always
 

tgrayson

New Member
Vy increases if you are looking at True airspeed.
My question is first why is this
Because the power required curve rises and rotates slightly to the right with altitude. This moves the points of lowest power required to the right. The basic reason is that at a particular TAS, the higher you go, the greater AOA you need to get the lift to sustain the aircraft (more induced drag) and 2) the less parasite drag you have

... and second, why talk about indicated airspeed, Vx and Vy meeting and then show graphs that only show True airspeed?
Because they couldn't find any analysis in any aerodynamics book that shows the Vy trends with respect to IAS. Because I haven't. ;) But here is my attempt:

Why Vy Varies with Altitude
 

Sidious

Well-Known Member
Hmmm.... Okay that was in line with what I was thinking, thanks for the graphs also.

Now this leads to another question that someone else has already ask you but if you could explain the answer a little more it would help me a great deal.
Why does TAS increase and IAS decrease?

You can't have both. What you have is, for a given IAS, TAS increases. Or, for a given TAS, IAS decreases. The reason is the air density decreases with altitude, so you have to go faster in TAS in order to get the same impact pressure in the pitot tube that corresponds to a particular IAS.​


Because the way I am seeing TAS is increasing but we are saying that IAS should be decreasing. I don't know why Im seeing it like that or how to look at like your saying. To me it appears they are moving in two opposite directions which isn't right.
 

tgrayson

New Member
Because the way I am seeing TAS is increasing but we are saying that IAS should be decreasing. I don't know why Im seeing it like that or how to look at like your saying. To me it appears they are moving in two opposite directions which isn't right.
You're talking about Vy? I may have misunderstood the other poster's question.

Vy increases with respect in TAS, but not enough to compensate for the increasing gap between TAS and IAS. As the altitude increases, the air density decreases and the impact pressure in the pitot tube is less, resulting in a lower airspeed indication for a given TAS.
 

Sidious

Well-Known Member
Alright got it...

I assume that the Thrust Available/Required graphs behave in similar fashion? Thrust available would decrease because of the decrease in power and density and thrust required I would think would change as does the Power graphs because of the changes in drag. Sound good? Or close?

Now a related question and some thoughts I had

For most graphs showing Thrust Available/Required it shows that Thrust Available DROPS off rather dramatically as higher regimes of flight are encountered. This makes sense considering a fixed pitch prop but doesn't seem to hold true for a constant speed prop where you should get max efficiency and thrust through all regimes of flight. Yes? No?

Thanks
 

tgrayson

New Member
I assume that the Thrust Available/Required graphs behave in similar fashion?

To some degree. If you plot drag in INDICATED airspeeds, it's the same at every altitude, whereas the power curve shift upward (no tilt with IAS). Thrust Available will reduce with altitude. I haven't come up with an analytical demonstration of why Vx would increase with altitude, though, even though I know it has to do with the propeller/engine, rather than the aerodynamics of the airplane itself.

For most graphs showing Thrust Available/Required it shows that Thrust Available DROPS off rather dramatically as higher regimes of flight are encountered. This makes sense considering a fixed pitch prop but doesn't seem to hold true for a constant speed prop where you should get max efficiency and thrust through all regimes of flight. Yes? No?
Unfortunately, no. Thrust drops off sharply for any propeller airplane, even for turbo fans. The only time you might see constant thrust with airspeed is a pure turbojet.

Remember that Power = Thrust X Velocity, so you if you have a relatively straight POWER available curve, such as with a CS prop, you can't have a straight thrust available curve. The math just doesn't work.

The way I think about it is that a propeller can only accelerate the airflow to a certain speed; as the aircraft flight speed increases, the increment in velocity the prop gives to the air in excess of the flight speed gets smaller and smaller, meaning less thrust. At full cruise speed, the prop may be imparting maybe 10-15 knots of velocity over what you're flying.
 
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