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Climbing at -6 degrees vs 0 degrees?

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So...When you are still you have zero relative wind, and Thus no way to increase or decrease AoA, and thus no way to accelerate.  So...how do you?  But once you start moving even 1mph, you have relative wind and NOW you can accelerate?  How do you get from no AoA and no way to accelerate to accelerating down the runway?  

There must be some mechanism other than AoA that allows acceleration.  Newton's third law of motion comes to mind:  "For every action there is an equal and opposite reaction".  In this case, the action is that air is accelerated rearward by the propeller, causing the reaction of the airplane moving forward.

Now the question becomes:  Is there something magical that happens that causes Newton's law to get suspended once an airplane has air flowing over it's wings?  Or is there at least some component of airplane speed change dependent on Newtonian forces in the air, just as on the ground?  After all, even an airplane *without* wings will accelerate to a pretty good clip (more than takeoff speed, for sure) just using the prop on the ground, so the amount of force generated by prop thrust alone and not AoA is not insignificant.

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Andy, Our discussion has been about flying aircraft. I agree that you have to have acceleration to get moving, but the thrust alone does not increase or decrease the speed of the airplane in flight.  while on the ground if you have enough thrust you can gain enough speed to lift off with out changing the angle of attack, but that speed is going to be very high. When you rotate on takeoff you are increasing the angle of attack allowing the wing to produce the required lift at a lower speed.

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2 hours ago, Tom Baker said:

Andy, Our discussion has been about flying aircraft. I agree that you have to have acceleration to get moving, but the thrust alone does not increase or decrease the speed of the airplane in flight.  while on the ground if you have enough thrust you can gain enough speed to lift off with out changing the angle of attack, but that speed is going to be very high. When you rotate on takeoff you are increasing the angle of attack allowing the wing to produce the required lift at a lower speed.

 

This kind of defies reality in my head.  Propeller thrust provides enough power to move an airplane along the ground at 80mph or more, against the rolling friction resistance of the wheels, but as soon as the wheels leave the ground, that thrust force has no effect?  I agree that once you increase AoA at flying speed, the increase in lift allows the airplane to leave the ground.  But the ability to raise the AoA is a result of speed, which is a result of the thrust of the prop.  

Saying that AoA generates speed and thrust has nothing to do with it seems backwards.  To my way of thinking, thrust is what accelerates the airplane, and then that kinetic energy can be either left as kinetic by way of speed, or converted to potential energy in the form of altitude by increasing AoA (or if speed is low, to maintain altitude or minimize sink).

I think AoA is what allows an airplane to change attitude and maneuver, thrust is what gives the airplane the ability to change AoA (at least to increase it).  You will certainly speed up if you are cruising at 3000ft drop the nose 10°.  But is that because AoA was lowered, or because the lower AoA gave you access to additional thrust in the form of a gravity assist?  

When the engine is out, why does speed decay in level flight?  AoA sets the speed, but only based on what energy state the airplane is at and what additional energy it has access to.  You can lower the AoA all day long, but if you don't have something providing thrust (i.e. an engine, or gravity), that speed will run out.  I think I am correct in saying AoA (or pitch, if you want to be more crude) sets the speed you have access to, thrust is what enables you to "bank" more energy for use now as speed, or later as altitude.

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38 minutes ago, FlyingMonkey said:

 

This kind of defies reality in my head.  Propeller thrust provides enough power to move an airplane along the ground at 80mph or more, against the rolling friction resistance of the wheels, but as soon as the wheels leave the ground, that thrust force has no effect?  I agree that once you increase AoA at flying speed, the increase in lift allows the airplane to leave the ground.  But the ability to raise the AoA is a result of speed, which is a result of the thrust of the prop.  

Saying that AoA generates speed and thrust has nothing to do with it seems backwards.  To my way of thinking, thrust is what accelerates the airplane, and then that kinetic energy can be either left as kinetic by way of speed, or converted to potential energy in the form of altitude by increasing AoA (or if speed is low, to maintain altitude or minimize sink).

I think AoA is what allows an airplane to change attitude and maneuver, thrust is what gives the airplane the ability to change AoA (at least to increase it).  You will certainly speed up if you are cruising at 3000ft drop the nose 10°.  But is that because AoA was lowered, or because the lower AoA gave you access to additional thrust in the form of a gravity assist?  

When the engine is out, why does speed decay in level flight?  AoA sets the speed, but only based on what energy state the airplane is at and what additional energy it has access to.  You can lower the AoA all day long, but if you don't have something providing thrust (i.e. an engine, or gravity), that speed will run out.  I think I am correct in saying AoA (or pitch, if you want to be more crude) sets the speed you have access to, thrust is what enables you to "bank" more energy for use now as speed, or later as altitude.

I never said thrust has no effect once the wheels leave the ground. Thrust certainly has an effect. It is what allows you to descend, maintain level flight, or climb at a given speed.

I didn't say AoA generates speed. I said angle of attack controls speed. On any given day for the conditions and weight of the airplane for each fraction of a degree of angle of attack there is only one speed at which the airplane will fly. You change the angle of attack you change the speed. Where thrust comes into play is when you change the angle of attack unless you change the available thrust you will either be climbing or descending at the new speed.

Don't confuse attitude with angle of attack. When you do a loop the AoA will remain lees than the critical angle of attack if done properly, even though the pitch attitude changes 360°.

With the engineout speed decays because the angle of attack is changing. As soon as you lose thrust the airplane will start to descend. because of this the angle of attack increases. This leads to the slower speed. If you lower your nose to keep the angle of attack your speed will stay the same.

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15 hours ago, Tom Baker said:

I never said thrust has no effect once the wheels leave the ground. Thrust certainly has an effect. It is what allows you to descend, maintain level flight, or climb at a given speed.

I didn't say AoA generates speed. I said angle of attack controls speed. On any given day for the conditions and weight of the airplane for each fraction of a degree of angle of attack there is only one speed at which the airplane will fly. You change the angle of attack you change the speed. Where thrust comes into play is when you change the angle of attack unless you change the available thrust you will either be climbing or descending at the new speed.

Don't confuse attitude with angle of attack. When you do a loop the AoA will remain lees than the critical angle of attack if done properly, even though the pitch attitude changes 360°.

With the engineout speed decays because the angle of attack is changing. As soon as you lose thrust the airplane will start to descend. because of this the angle of attack increases. This leads to the slower speed. If you lower your nose to keep the angle of attack your speed will stay the same.

Thanks Tom, that clears things up a bit.  I think I mostly agree with what you are saying.  I definitely get the difference between AoA and attitude, if that was not clear from something I said.  

If you agree that AoA that can be maintained is a function of available thrust in combination with drag, then I think we're on the same page even if some of our terms are different.

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17 minutes ago, FlyingMonkey said:

Thanks Tom, that clears things up a bit.  I think I mostly agree with what you are saying.  I definitely get the difference between AoA and attitude, if that was not clear from something I said.  

If you agree that AoA that can be maintained is a function of available thrust in combination with drag, then I think we're on the same page even if some of our terms are different.

Angle of attack can be maintained regardless of thrust. If you remove the thrust you simply have to lower the nose to maintain the same speed, and the angle of attack remains the same.

Now to maintain altitude at any speed or angle of attack you must have thrust equal to the drag.

 

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33 minutes ago, Tom Baker said:

Angle of attack can be maintained regardless of thrust. If you remove the thrust you simply have to lower the nose to maintain the same speed, and the angle of attack remains the same.

Now to maintain altitude at any speed or angle of attack you must have thrust equal to the drag.

 

I think we're saying the same thing.  By "maintained" I meant without losing either airspeed or altitude.  In the absence of sufficient thrust one or the other must be sacrificed if you want to maintain AoA.

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28 minutes ago, FlyingMonkey said:

I think we're saying the same thing.  By "maintained" I meant without losing either airspeed or altitude.  In the absence of sufficient thrust one or the other must be sacrificed if you want to maintain AoA.

 If you lose power altitude is the only one that can be sacrificed to maintain angle of attack. If you lose airspeed then angle of attack has not been maintained.

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18 minutes ago, Tom Baker said:

 If you lose power altitude is the only one that can be sacrificed to maintain angle of attack. If you lose airspeed then angle of attack has not been maintained.

We agree.  It all depends on the energy state of the airplane.  If it has positional energy (altitude), that can be traded for maintain AoA and airspeed.  If it has airspeed that can be traded for altitude by increasing AoA and climbing.  As airspeed decays AoA goes up.  

It does seem there must be a limit on how low AoA can go, and at some point you just can't get any less AoA at the zero bound.  But it could also be the case where AoA decreases asymptotically toward zero, but can never quite reach it.  In that case going from 100kt to 200kt might decrease AoA by 3°, 200kt to 400kt by 1.5°, 400ft to 800kt by 0.6°, and so on.  This would make sense if you consider  that AoA must be positive to generate lift, and at an AoA of zero there might be zero lift.  Not stating that this is true, just speculating on what might be the case to make all this work out.

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Andy, you can go negative on the angle of attack. That is how aerobatic airplanes do outside or negative maneuvers. As far as upright positive G flight it really depends on the airfoil I think. Some airfoils produce a lot of lift due to their shape.

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32 minutes ago, FlyingMonkey said:

I meant in level flight.  If going faster lowers AoA, it seems there has to be a lower limit.  Not sure if that is 0 or slightly above that.

I think it might be dependent on the shape of the airfoil. The limit is going to be power to overcome drag. It takes something like 4 times the power to double the speed. 

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On 9/26/2017 at 7:38 AM, Ed Cesnalis said:

 

I agree. Its an odd duck to fly and this is the way to become good at it.  It's very maneuverable and tolerates steeper banks then one might think. 

Hi Ed! Yes I fly about everyday just to get my fix!??

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