Relation between AoA, climb/dive and airspeed

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viperhaf

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Unread post20 Apr 2019, 10:05

Currently learning to fly a jet (buckeye) and i wanted to ask you something. If anyone could help me i would be really glad.

Here are the facts. Flying constant AoA. 15 units AoA let's say. Thrust required is 90%. Airspeed will be about 110 KIAS. Then i add 5% rpm(95%). I normally raise the nose, stabilise at 15 units AoA once more, at a higher pitch attitude(irrelevant how much the attitude is). Climbing unaccelerated now at approximately 100-105 KIAS.
Level-off, 90% once more, always 15 units AoA airspeed will be once more 110.
Then i reduce 5% rpm(85%). Establish a nose low attitude, stabilise at 15 units AoA, diving unaccelerated at approximately 115-120 KIAS.

So here is my question. What is the explanation for the different airpeed between climb or dive while on the same angle of attack?

Thank you in advance for your help.
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sprstdlyscottsmn

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Unread post20 Apr 2019, 14:28

Well I am going to start by assuming that 15 is a trimmed, hands off, 1G, AoA at your starting speed for the sake of this discussion.

The lift coefficient is equal to the AoA multiplied by the slope of the lift curve. So constant AoA means constant lift coefficient.

Since lift force is equal to the lift coefficient multiplied by the dynamic pressure (determined by density and velocity, .5*d*V^2) and the reference wing area, and the lift coefficient and wing area are constants, in this scenario lift is controlled by dynamic pressure.

In steady, level, unaccelerated flight Thrust is Equal to Drag. You talk about increasing the thrust. Now that thrust is greater than drag you begin to accelerate. As you accelerate the you are increasing your dynamic pressure, and thus your lift, raising the nose. How far does the nose raise?

steady state climb rate is found to be the following (thrust-drag)*true airspeed/weight. So you will keep pitching up until your climb rate lines up with that equation. The airspeed that comes out of that equation will be your climb or dive speed.

Another way to look at it is that at 15 AoA your thrust is adding to your lift (not really, but it is helping to oppose weight) at a value of Thrust * SIN(AoA). So if you remember the previous lift equation of L = CL * q(dynamic pressure) * S (wing area) you can add T*SIN(AoA) to it.

L = CL*q*S+T*SIN(AoA)

As we are moving between different trimmed 1G conditions we are moving between points where L is equal to the aircraft weight. When you increase T something else has to reduce to balance the equation since AoA, CL, and S are constant. That leaves q. As such you will see a decrease in airspeed until you have climbed to a point where the air density is lower. Same thing with reducing thrust.
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viperhaf

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Unread post20 Apr 2019, 17:14

Yes. starting point is straight and level, hands out trimmed, 15 units, landing configuration. When i add or reduce power, i dont let the nose drop or rise. I establish the attitude i want, at the same time i set the throttle rpm. And i establish a steady climb/dive with 15 units. But the airspeed is different when climbing and when diving. Why is that so?

I cant say i fully understand your point. I dont think what you said explains what i asked.
While flying the same AoA, the lift of the wing will be greater as airspeed increases. So when climbing, when i have lower airspeed (appr. 105KIAS), the lift produced will be lower. So the added thrust compensated for the loss of lift and there is some excess so the a/c is climbing. Vice versa when diving (higher lift created, lower power setting). At both cases the AoA is 15 units.
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vilters

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Unread post21 Apr 2019, 10:24

You are making your life more difficult then required.

Ok, set aircraft as follows at a safe altitude: Exactly as you started out :
Landing config, 110Kts and 15AOA exactly as you did before and trim to hands off. Let it stabilise for at least a minute and DO NOT TOUCH the stick. => You NEVER touch the stick for this test.

Increase power by 5% slowly as you did before BUT DO NOT TOUCH THE STICK.
Re-Trim to maintain 110kts if required and let the trimmed aircraft do its thing and if required ONLY adjust the flight path using the rudder pedals. Let it stabilise for 1 minute. (Again : DO NOT TOUCH the stick.)
What do you see?


Set power back to original: Re-Trim to maintain 110kts if required. Let the aircraft do its thing and let it stabilise for 1 minute.

Now reduce power by 5%: Re-Trim to maintain 110kts if required. Let the aircraft do its thing and let it stabilise for 1 minute.

And go back to your starting position by adding 5% and re-Trim to maintain 110kts if required.

What did you see during this test?

Remember, you NEVER touched the stick.

The only difference you will see is that Climb rate and Sink rate are different. And that difference is gravity.

There might be a slight variation due to fuel being burned off and less weight to carry around.
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viperhaf

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Unread post21 Apr 2019, 11:39

It is not that i am making my life harder. This excercise has the objective to familiarise the student with the slow flight characteristics.
The steps are:
1)Establish straight and level, 90% RPM, 15 units AoA, hands out trimmed. (No specific airspeed, it just happens to be appr. 110 KIAS)
2) Add 5% RPM (so i set it at 95%) and establish a climb with 15 units AoA for 500 ft climb. (Again no specific airspeed, it just happens to be appr. 100-105 KIAS)
3) Reduce 5% RPM so you fly straight and level at the new altitude (Same parameters as step 1)
4) Reduce 5 % RPM (so i set it at 85%) and establish a dive with 15 units AoA for return to the initial altitude. (No specific aispeed during the dive, it just happens to be appr. 115-120 KIAS).

My question is simple. Why is the airspeed different? The answer might be simple. I simply can't figure it out.
Hope i clarified it as much as possible.
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guy@rdaf.dk

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Unread post21 Apr 2019, 14:20

It is due to the fact that some of your lift is generated by the downward component of your thrust!

Lets say that in level unaccelerated flight with 15AOA, 10% of your lift is provided by the downward pointed (15 degrees) component of your thrust. The remaining 90% is generated by the wing flying with 110kts and 15AOA. When you increase power to 95%, you also increase the amount of lift comming from your thrust. Lets say in this case that with 15AOA and 95% power, 15% of your lift now comes from your thrust. If you maintain 15AOA and 110kts, you will now have 105% lift (15+90). So to reduce the lift to 100%, you only have one option, and that is to slow down.

The same happens when you pull the power back. 85% power will in this case only give you 5% of your lift from you thrust. As you have elected to maintain your AOA at 15, you now need to increase your speed to achive a total of 100% lift (5+95).

Hope it makes sense.
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Unread post22 Apr 2019, 10:07

Answer to your question is IMHO the following :your attidude AoA( 15 degrees) must be subtract / or added from / to the glide (or the climb) slope. So the real AoA is higher /lower than the cockpit AoA gauge will show to you (15 AOA). It is 15AOA+dive AoA or - climb AoA. Also flaperons angles of incidence to the air under the wing is not the same from local AoA changed by climbing or diving and create a different drag.
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saberrider

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Unread post22 Apr 2019, 10:26

What is the explanation for the different airpeed between climb or dive while on the same angle of attack? Weight of the jet is changing by fuel burning and air intake pressure is also not ideal and air density at the moment is not constant.
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viperhaf

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Unread post22 Apr 2019, 18:48

Weight can't be making any notable difference. Airspeeds are noted one immediately after the other. Like 30 sec appart, maximum. I think i have narrowed it down to being differnet because of the different thrust. The thrust line is not really parallel to the fuselage. There is a 6° angle between the nozzles and the fuselage. The difference in thrust creates a pitching moment. When equilibrium is once more reached, there is a difference in the control's deflections between climbing and diving. So the Cd (coeeficient of drag) in each case is different resulting id different characteristics. I think this is as close as i can get to an explanation.
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viperhaf

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Unread post22 Apr 2019, 18:55

Actually just got another opinion on the matter. Going to post it as well.

"Consider three airplanes: a normal airplane, one that is heavier than normal and one that is lighter than normal. All of them are travelling at 15 units AoA. You will agree that the lighter one will be the slowest (it needs to go very slow as not to have a smaller AoA) and the heaviest one will be the fastest (it needs to be very fast in order not to have a greater AoA).

In the above example we vary the weight of the airplane, but ultimately the change is due to wing loading. In your example, you do not vary the weight, but the thrust vector. In climb, the thrust vector is pointing upwards. Part of the weight of the airplane is now carried by the propeller instead of the wings. As far as the wings are concerned, this is the same as carrying a lighter airplane. In order to maintain a high AoA, you pull up more and slow down.

In descent, the thrust vector is pointing down. The wings have to carry more weight, which, at constant AoA, means a greater speed.

The effect here is independent of the airplane. Your particular airframe may have additional characteristics that may cause such effects. For example, if increased thrust causes a pitch-up moment on your particular airframe, the tail will have increased lift, reducing the wing loading. Again, to maintain a high AoA, the speed will have to be reduced."
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huggy

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Unread post13 Jun 2019, 20:36

Where are you learning to fly a Buckeye? Idaho?

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