Aerodynamics Question

ProudPilot said:
Headwind counts as airspeed.

It's all potential and kinetic energy. The total is thrust. When you add power you can either take that energy and place it into the potential category (height/Altitude) or kinetic (airspeed). Having a headwind reduces the total amount of energy you need to get to your goal speed for takeoff.

It's like this, what requires more total energy to move... a Cessna 172 or a Boeing 737? The C172 is obvious, but why? The C172 needs less total lift to takeoff, and the wing is designed as such to fly at a lower speed. So, the c172 has less drag, and less speed requirements to takeoff, resulting in lower energy needs from the engine (thrust). Where that thrust comes from doesn't really matter. However, headwinds are just free energy that they engine didn't have to put out. If you rolled an airplane down a hill it could get enough speed to takeoff. Then you would either need to lose more altitude to maintain speed (convert potential to kinetic, aka glide) or get thrust to overcome the drag (drag does occur always, even at .0001 kts). BTW, when you get to commercial, ask why winglets create thrust.
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A headwind does not count as airspeed, it only counts for runway performance. Once airborne, a headwind ceases to exist. (as far as an airframe is concerned)
 
I really appreciate all of your help, guys. Especially ProudPilot. I really appreciate you going so far as to draw me a picture. I still don't quite understand it, but I have never been good at understand math/science.
 
Oh, and headwind does count as airspeed, free airspeed. True, it doesn't help in the air, but it is AIRSPEED, call it WINDSPEED if you like, but tomato / tomato (with the elongated o).

I love the "wings don't make lift until takeoff" debate as well. Go out on a windy day and put your hand behind the wing. You can feel the downdraft, the wingtip vortices and the stagnation point on the front (use a finger otherwise your hand will block too much wind). Tgrayson is awesome as he points out the physics of this... not the airplane flying handbook.
 
ProudPilot said:
Uh... wouldn't this be the same thing
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I don't understand why you would think so. The thrust and drag curves solely reflect thrust and drag, not stalling. The low speed point you have labeled as "power on stall" is merely where we are in a slow flight configuration. Even if the aircraft never stalled, the curve would look the same.

However...if the aircraft has lots of thrust, the aircraft may never get draggy enough in the unstalled condition to produce a descent. Perhaps that's why you're identifying the stall with the low speed intersection of the thrust/drag curves?
 
tgrayson said:
I don't understand why you would think so. The thrust and drag curves solely reflect thrust and drag, not stalling. The low speed point you have labeled as "power on stall" is merely where we are in a slow flight configuration. Even if the aircraft never stalled, the curve would look the same.

However...if the aircraft has lots of thrust, the aircraft may never get draggy enough in the unstalled condition to produce a descent. Perhaps that's why you're identifying the stall with the low speed intersection of the thrust/drag curves?
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Ok, now I'm really confused. So... say we take the F18, it does a tail skid in midair where it uses the power to maintain altitude were the pitch is 70 degrees or so as it flies along the horizon. Would this not be fully stalled? Sure the nose isn't dropping, but the critical AOA has been well exceeded.

So we should relabel the low airpseed intersection of thrust and drag as the minimum level airspeed?
 
ProudPilot said:
So we should relabel the low airpseed intersection of thrust and drag as the minimum level airspeed?
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Yes...because if you go any slower, your drag will increase and you will have a thrust deficit, producing a descent. This is true even without a stall. Some aircraft may stall at a faster speed than that or they may stall at slower speeds.

As for whether the F-18 is stalled at a pitch attitude of 70 degrees, insufficient information. A great portion of the "lift" at that sort of attitude will be provided by thrust, reducing the need for conventional wing lift. For instance, if the pitch attitude were 90 degrees, the AoA needs to be about 0....clearly not stalled.
 
powerstall.png
 
I think of anyone I've ever known of, you of all people, should write a book.
 
Jeremy said:
I still don't quite understand it, but I have never been good at understand math/science.
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What is it that you don't particularly understand now? Your first question appears to be completely I am guessing you are hung up on the Lift and Thrust issues? If you would, try to be as specific as possible. The best answers are achieved by asking specific questions or explaining what you think things are so they can be corrected and allow the person responding to your question to see your thought process.


Proudpilot, we stall at an AOA. If you look at the thrust/drag curve, where it meets on the lower end may not be at or above critical AOA. All that is happening at that point is you no longer can gain altitude. Remember, and you know this, excess thrust only has to do with climbing and descending. It is completely possible to be at that intersection on the low end of the drag curve and not be above critical AOA and therefore not be stalled. That help?

Demo this in the airplane, fly the aircraft a knot above stall speed in the landing configuration and stop the descent with power. Now imagine your in a different plane with a slightly smaller engine, reduce the power a little and keep the same speed. You are still at a point beyond where the curve meets and still not stalled. I know older 150's can demonstrate this at full power, and some 152's I have flown, where you can go beyond the low point on this graph.
 
tgrayson said:
powerstall.png
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-Awesome, I see it.

tgrayson said:
Yes...because if you go any slower, your drag will increase and you will have a thrust deficit, producing a descent. This is true even without a stall. Some aircraft may stall at a faster speed than that or they may stall at slower speeds.

As for whether the F-18 is stalled at a pitch attitude of 70 degrees, insufficient information. A great portion of the "lift" at that sort of attitude will be provided by thrust, reducing the need for conventional wing lift. For instance, if the pitch attitude were 90 degrees, the AoA needs to be about 0....clearly not stalled.
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Pitch 70... flight path 0. The thrust is providing the ability to maintain altitude, but the wing has become just a sheet of metal, it's way past stalled, that's what I was getting at.

Below that point you may still have a power off stall lurking at a slower speed, but that is the point where with full power, the wing has become stalled and unless you have sufficient thrust to counteract any descent from a stall. Which, when I read that I see the folly. However, saying that you have thrust to maintain altitude is NOT a reason to say that the wing is not stalled. If the wing is stalled, the tail should be stalled as well, but we've both seen aircraft with enough induced airflow from engines to get funky around a stall, but that usually only makes a knot or two difference anyways.

I do appreciate the graph, what book is that from? I still need to find a few days to read through Aerodynamics for Naval Aviators.
 
ProudPilot said:
However, saying that you have thrust to maintain altitude is NOT a reason to say that the wing is not stalled.
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Agreed, and I didn't say it was, but few aircraft would have sufficient thrust to maintain altitude with the wings fully stalled. The drag curve departs from its parabolic shape and rises precipitously once the airflow separates. The graph was not intended to depict a universal truth, but a representative truth.

If the wing is stalled, the tail should be stalled as well,
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Not necessarily. The tail has a lower aspect ratio airfoil, which stall at higher AoAs than the main wing....this is intentional. You need tailplane authority to recover from a main wing stall.

I do appreciate the graph, what book is that from? I still need to find a few days to read through Aerodynamics for Naval Aviators.
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"Introduction to Flight", by John D. Anderson. Good intro book for someone with an engineering background.
 
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