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Angles of Attack and Variation in Weight

  • weirdguy99
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weirdguy99 created the topic: Angles of Attack and Variation in Weight

Hey guys,

I understand that best glide and best range must be achieved at the best lift/drag ratio, and as such, should be conducted at a constant AoA (4-degrees). Just wondering if that 'rule' applies to other manoeuvres.

Consider an aeroplane weighing 1000kgs in a best rate (or angle) of climb (in ISA conditions) where the AoA is approximately 9-degrees or so.

If I load that aeroplane with 1300kgs and climb at best rate (or angle) in ISA conditions, will the AoA still be 9-degrees for that type of climb, albeit at a higher speed?

If so, could you say the same thing about flying at best endurance and approach speeds?

Cheers!
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bobtait replied the topic: Angles of Attack and Variation in Weight

If we assume that things such as wing area and air density remain the same, the lift generated is simply a product of angle of attack and indicated air speed. You are correct in saying that the best lift/drag ratio is dependent on angle of attack. So, if angle of attack is to remain constant, a greater weight will require a greater speed. That's all very well when we are considering best lift/drag ratio for a glide or for range. However when it comes to climbing, another factor comes into play - the power and thrust output of the engine/propeller at various speeds. To achieve the best rate of climb you must climb at the speed where surplus power is at its greatest. That may not necessarily be the same speed as best glide since the engine power output is controlled by other factors. Likewise, the speed that produces maximum angle of climb is the speed at which maximum surplus thrust is available. That depends on the engine/propeller combination and will not necessarily be the same as best lift/drag ratio.
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weirdguy99 replied the topic: Angles of Attack and Variation in Weight

Thanks for the reply Bob.

If we climb at the speed for best rate (max. excess power) at two different weights (everything else constant), will the AoA be the same in both cases?

What I'm asking is do we try to climb (or even make an approach-to-land) at a given airspeed to achieve a constant AoA (like we do for a glide)?
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bobtait replied the topic: Angles of Attack and Variation in Weight

In any unaccelerated flight condition where airspeed is kept constant, the angle of attack would also be constant. This is because, for a given wing area, lift is a function of IAS and angle of attack only so, if you fly at a constant IAS, you must be flying at a constant angle of attack as well.

However, in a climb the airframe is not the only consideration, the engine power output must also be considered. The best lift/drag ratio angle of attack is considering only the airframe, so it wont necessarily be the angle of attack required for best rate or angle of climb. However, it remains true that, if you climb or descend at a constant IAS, you must also be climbing or descending at a constant angle of attack.

If weight is increased, you would need to make more lift to compensate so you would need a higher IAS at any given angle of attack, however that angle of attack would not necessarily be the best lift/drag ratio angle of attack. It would be the angle of attack associated with the IAS for maximum surplus power or thrust. Once again, the IAS to use is the IAS where the maximum surplus thrust/power is available and that depends on BOTH the airframe and the engine/propeller.
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weirdguy99 replied the topic: Angles of Attack and Variation in Weight

"If weight is increased, you would need to make more lift to compensate so you would need a higher IAS at any given angle of attack, however that angle of attack would not necessarily be the best lift/drag ratio angle of attack. It would be the angle of attack associated with the IAS for maximum surplus power or thrust."

I understand that it's not the best life/drag ratio AoA, but is it still the same AoA regardless of weight?

If I climb at 10-degrees AoA for best rate of climb at 1000kg, will I still climb at 10-degrees at 1300kg?
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bobtait replied the topic: Angles of Attack and Variation in Weight

I can't see any reason why the angle of attack would have to be the same. This is really a compromise between the the power required to fly level and the power available for climbing. There would always be a particular IAS that would correspond to maximum surplus power at any particular weight and there would always be an angle of attack corresponding to that IAS, but it would not necessarily be the same angle of attack for different weights.

It's good to see you asking such in-depth questions - you are obviously a thinker. Please remember that what I have said is my opinion based on my understanding of the issue. If you come across different opinions let me know. All the best with your study.

Bob
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