Downwash and tailplane effectiveness
Thread Starter
Joined: Jun 2019
Posts: 22
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From: Belgium
Downwash and tailplane effectiveness
Hello gentlemen,
I am studying POF for my ATPL and there is one huge thorn in my eye. The book (Bristol) states the following in the chapter on stability:
"When we deploy flaps on the wing, we increase the downwash over the tailplane, reducing its effectiveness. Larger elevator deflections will be required to create the same balancing forces".
I am having a hard time accepting this statement because I believe that this is only correct when the tailplane is creating an upforce. When it is creating a downforce the increased downwash from extending flaps will increase the already negative angle of attack and will cause an increase in effectiveness and will require a smaller elevator deflection. Am I wrong?
Effectiveness all depends on what you want... Is my bed "effective"? If I want to sleep, it's very effective. When I want a magic carpet so I can float to neverneverland, it's not very effective.... The same with the tailplane. If I want/need upforce the downwash reduces effectiveness, if I want/need downforce it increases effectiveness.
When you try to explain, thank you in advance, please remember I am a student and not a veteran pilot.
Thank you! And happy holidays!
Grtz,
Krieks
I am studying POF for my ATPL and there is one huge thorn in my eye. The book (Bristol) states the following in the chapter on stability:
"When we deploy flaps on the wing, we increase the downwash over the tailplane, reducing its effectiveness. Larger elevator deflections will be required to create the same balancing forces".
I am having a hard time accepting this statement because I believe that this is only correct when the tailplane is creating an upforce. When it is creating a downforce the increased downwash from extending flaps will increase the already negative angle of attack and will cause an increase in effectiveness and will require a smaller elevator deflection. Am I wrong?
Effectiveness all depends on what you want... Is my bed "effective"? If I want to sleep, it's very effective. When I want a magic carpet so I can float to neverneverland, it's not very effective.... The same with the tailplane. If I want/need upforce the downwash reduces effectiveness, if I want/need downforce it increases effectiveness.
When you try to explain, thank you in advance, please remember I am a student and not a veteran pilot.
Thank you! And happy holidays!
Grtz,
Krieks
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Joined: Feb 2000
Aviation Qualifications: CPL
Posts: 14,479
Likes: 178
From: UK
Reads like rubbish to me too - a downwash *angle* will alter trim, but is unlikely to alter elevator power. To alter elevator power, you'd need to alter the actual flow absolute mean velocity over the tail - now as you slow for approach and landing that does indeed happen, but it' not as has apparently been described to you.
However, my advice to you is to do what I did (I have a PhD in flight mechanics, and a CPL) - just accept that often the examiners are wrong, and its not your job to educate them. Simply understand what they *think* to be the case, and ensure that you use that flawed understanding to answer the questions.
G
However, my advice to you is to do what I did (I have a PhD in flight mechanics, and a CPL) - just accept that often the examiners are wrong, and its not your job to educate them. Simply understand what they *think* to be the case, and ensure that you use that flawed understanding to answer the questions.
G
