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Final exam 2 Q28 (Aero text book)

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Simathawk77 created the topic: Final exam 2 Q28 (Aero text book)

Hi,

The answer for this question has me baffled. The Q asks which factor will alter the indicated stall speed of the aircraft. The answer given is 'power setting'. The explanation basically is that at high AoA thrust is acting above the flightpath, thus relieving the wings of some of the lift requirement, so smaller AoA can be used at any given speed (that's the first aspect I don't understand). It goes on to say that the stall speed decreases if power is applied (the second point of confusion).

Could someone assist me with this pls?

Thanks

Simon
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  • John.Heddles
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John.Heddles replied the topic: Final exam 2 Q28 (Aero text book)

If you are flying along normally you will have some angle of attack which results in the wing lift's providing almost all the upwards force value necessary to balance the weight of the aircraft. The propeller thrust force is pointing somewhere out to the front but, generally, with a small inclination upwards. This inclination results in our being able to figure that the major part (horizontal component) of the thrust is sorting out the drag, while a little bit (vertical component) is attending to providing a small part of the total vertical force needed to balance that last bit of the weight force.

Now you can juggle the situation in various ways but, at the end of the day, you need to have the sum (total) of the upwards acting (vertical component of the) wing lift and the upwards acting (vertical component of the) propeller thrust force being equal to (and balancing) the downward weight force.

You can increase/decrease the vertical thrust component by increasing/decreasing the power setting. If you want to keep flying straight and level, though, if you increase/decrease the vertical component of the thrust force, you will (necessarily) have to decrease/increase the vertical component of the wing lift force so that the total of the thrust and lift vertical force components continue to add up to the weight force. If you don't do this, the aeroplane is either going to climb or descend according to whether the total upwards force (lift and thrust components) exceeds, or is less than, the downwards acting weight force.

You have no choice there, if things are not balanced, there will be an acceleration. The same sort of argument applies horizontally - if the forces don't balance out, then you will necessarily have an acceleration which either increases or decreases the aeroplane's speed.

Now, we said that we can increase/decrease the thrust force by increasing/decreasing the power setting. We can change the wing lift bit by changing the angle of attack (think back to the graph of lift coefficient drawn against angle of attack). So, presuming we want to fly level, if we do something to increase the vertical thrust component, then we must do something to reduce the vertical wing lift force component. So we pitch down a bit, the angle of attack reduces and the vertical component of wing lift force reduces. Providing the total of the wing lift upwards force component and thrust upwards force component continue to add up to the weight force value, we will continue to fly level.

But this means that we are now further away from (below) the stalling angle of attack. If we want to, we can pitch back up to increase the angle of attack, which will decrease the speed. If we keep pitching up, we eventually get to the stall (but now at a lower speed than with idle thrust). We can continue this argument until we are at full power (maximum thrust) with a quite measurable reduction in the stall speed at the maximum thrust setting when compared to the idle thrust situation.

Consider an extreme situation - you have an engine/propeller combination which has enough grunt available to produce a thrust equal to the weight of the aircraft. Providing that you can address any stability considerations, you would be able to pitch to the vertical and hang there on the thrust force with the wings having a total holiday. Stall becomes irrelevant in this situation.


 

Engineering specialist in aircraft performance and weight control.
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Simathawk77 replied the topic: Final exam 2 Q28 (Aero text book)

Thanks John I appreciate that.
What I'm still stuck on is the idea that the stall speed is significantly reduced with more power (vs idle pwr). Eg practising stalls at 4000ft, a power off stall occurs at say 50kt, a stall with some pwr on happens at 50kt also it just takes more back pressure to induce it. I'm obviously missing something...
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  • John.Heddles
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John.Heddles replied the topic: Final exam 2 Q28 (Aero text book)

First, be careful playing with power on stalls unless you have information regarding the stall and post stall characteristics of the particular aeroplane. I can't comment specifically on your experience as I would need to have a look at the sequence in the specific aeroplane.

One consideration to keep in mind is that the PEC may vary and influence what you are seeing on the ASI with variations in power.
Another is the rate at which you permit the speed to reduce during the approach to the stall. This would need to be similar to be able to make any sensible comparisons. Typically, flight test work is done with a rate of not more than 1 kt/sec reduction.

With the above in mind, one would have a look at the stall with idle power and then with progressively increasing power settings so that you sort of sneak up on the higher power stalls. Never any point taking a big bite and then getting a big surprise. If you have been comparing stalls, say with idle and then a trickle of power, you are not going to see any real effect. You will need to be looking at a reasonable variation in power.

What you will observe, though, is that the speed at stall will reduce.

You need to be aware though, say if you are in your typical light trainer, that the body angle is going to be VERY different at a high power setting so it might be a bit eyebrow raising first time around - hence the need to sneak up on the variation. Just be careful and make sure that you get the nose down promptly once you get to the stall and that you don't put any significant yaw inputs into the equation. For a conventional aircraft at the stall, the pitching moment should change quite noticeably with a resulting uncontrollable nose down pitching motion.

Engineering specialist in aircraft performance and weight control.
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Simathawk77 replied the topic: Final exam 2 Q28 (Aero text book)

Thanks John
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bobtait replied the topic: Final exam 2 Q28 (Aero text book)

There is no doubt that a stall with power on will occur at a lower IAS than a stall with no power applied. I've demonstrated that to students hundreds of times.

To see the effect, you need to be careful to ensure that when you do the stall with power on, you have maintained level flight throughout. I note that you said 'you need more back pressure to induce it'.

I suggest it is likely that when you did the stall with power on, you have actually begun an involuntary climb. If you use the VSI to ensure that the aircraft is maintaining level flight throughout, you will find that the stall occurs at a lower IAS with power on.
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Simathawk77 replied the topic: Final exam 2 Q28 (Aero text book)

Thanks Bob
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