clark Y, zero lift angle?
01-08-2009 | 11:50 AM
#26
RE: clark Y, zero lift angle?
pimmnz about downwash,
ORIGINAL: pimmnz
I wouldn't worry about any 'downwash' off the main wing either. I have somewhere a publication where the chief aerodynamicist for BAC (who have the time, money, requirement, tools etc to find out) states that the local change in the freestream direction (downwash) due to the lift produced by the wing can extend as far back as 1.25 chords behind the trailing edge of the wing. If a 100 ton airplane, operating at speeds, AOA's and lift coefficients unapproachable by any model airplane, and influencing cubic acres of air can get the downwash to extend as far as 1.25 chords behind the wing, I would go so far as to say that no normal (wing at front, tail at back) model airplane will have any downwash effect that could possibly influence any tailplane. He also stated that this wash actually reduces the lift generated by the wing (like the wing was 'climbing a hill' all the time). I think you can forget any 'downwash' calculations with wing/tail setting angles.
Evan, WB #12.
I wouldn't worry about any 'downwash' off the main wing either. I have somewhere a publication where the chief aerodynamicist for BAC (who have the time, money, requirement, tools etc to find out) states that the local change in the freestream direction (downwash) due to the lift produced by the wing can extend as far back as 1.25 chords behind the trailing edge of the wing. If a 100 ton airplane, operating at speeds, AOA's and lift coefficients unapproachable by any model airplane, and influencing cubic acres of air can get the downwash to extend as far as 1.25 chords behind the wing, I would go so far as to say that no normal (wing at front, tail at back) model airplane will have any downwash effect that could possibly influence any tailplane. He also stated that this wash actually reduces the lift generated by the wing (like the wing was 'climbing a hill' all the time). I think you can forget any 'downwash' calculations with wing/tail setting angles.
Evan, WB #12.
I agree with Joe Counter.
I think you have to speak with your chief aerodynamicist again and be careful with writing down important information without knowing the facts.
For the design of gliders we use the factor αwα to calculate the effect of the downwash on the stab, depending on dimensions, distances and so on. Not different from our engine models of course, same air.
For high speed design it is important because of the calculating of wing incidences and drag of the fuselage.
Second point is we use the factor (1 – αwα) for the calculating of ineffectiveness of the elevator depending of the position of the wing and the downwash from the wing on the stab.
With the Canard type the downwash of the stab has less influence but still we do not forget about it, downwash from the (relative high loaded) stab on the centre section of the wing.
For that last point I show you a picture of a high speed low resistance Canard glider designed by me for 2m (80”) thermal contest and for slope gliding in the mountains.
Cees
01-08-2009 | 02:45 PM
#27
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From: Lawrenceville,
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RE: clark Y, zero lift angle?
Actually, after you made the incidence changes, your wings had the same "lift", but the trim angle of the horizontal stabilizer/elevator was different. People tend to confuse the fact that a model that needs down elevator for level flight does not mean it has "too much lift". No such thing.
[quote]
Bax,
I think that in trying to keep readers from being confused you took exception with my use of the term "too much lift" a little too hastily. Of course lift always equals aircraft weight in stabilized flight[sm=49_49.gif]. However, with excessive positive wing incidence and elevator trim set to anything near neutral an unstable condition exists: more lift than gravity, the very state that allows an aircraft to pitch upwards and climb. I do not consider it technically incorrect to describe this as a state of "too much lift". It creates an uncommanded and unintended pitch up from an undesired excess of...lift!
You are absolutely correct to point out that it is a matter of trim, and that (stabilized) flight could be maintained in such a situation by trimming the elevators such that wing incidence relative to the airflow (not the fuse datum) is reduced, returning a "stabilized" state of level flight with no pitch-up. However, the tail would be following the front of the fuse at a considerably higher altitude. Works, but looks pretty goofy and creates lots of drag.
Hope this is not too esoteric. I think we both are on the same page but saying it in a different way.
[quote]
Bax,
I think that in trying to keep readers from being confused you took exception with my use of the term "too much lift" a little too hastily. Of course lift always equals aircraft weight in stabilized flight[sm=49_49.gif]. However, with excessive positive wing incidence and elevator trim set to anything near neutral an unstable condition exists: more lift than gravity, the very state that allows an aircraft to pitch upwards and climb. I do not consider it technically incorrect to describe this as a state of "too much lift". It creates an uncommanded and unintended pitch up from an undesired excess of...lift!
You are absolutely correct to point out that it is a matter of trim, and that (stabilized) flight could be maintained in such a situation by trimming the elevators such that wing incidence relative to the airflow (not the fuse datum) is reduced, returning a "stabilized" state of level flight with no pitch-up. However, the tail would be following the front of the fuse at a considerably higher altitude. Works, but looks pretty goofy and creates lots of drag.
Hope this is not too esoteric. I think we both are on the same page but saying it in a different way.
01-08-2009 | 02:54 PM
#28
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From: Lawrenceville,
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RE: clark Y, zero lift angle?
ORIGINAL: Bax
Actually, after you made the incidence changes, your wings had the same "lift", but the trim angle of the horizontal stabilizer/elevator was different. People tend to confuse the fact that a model that needs down elevator for level flight does not mean it has "too much lift". No such thing. You had a situation where you needed to alter the trim of the model to make the control positions as you desired them. You could have gotten the same resolution by changing the linkage to give you the down elevator you needed to achieve level flight. Your solution, though, would look better. Also, because you altered the relationship between the top and bottom wings, you likely got a little-bit better tracking of the model. Originally, you had the top wing at -1/4 degree in relation to the bottom wing, and you ended-up with the top wing at -1 degree to the bottom wing. Model biplanes with the top wing at a lightly-lower angle of incidence that the lower wing usually track a bit better.
Actually, after you made the incidence changes, your wings had the same "lift", but the trim angle of the horizontal stabilizer/elevator was different. People tend to confuse the fact that a model that needs down elevator for level flight does not mean it has "too much lift". No such thing. You had a situation where you needed to alter the trim of the model to make the control positions as you desired them. You could have gotten the same resolution by changing the linkage to give you the down elevator you needed to achieve level flight. Your solution, though, would look better. Also, because you altered the relationship between the top and bottom wings, you likely got a little-bit better tracking of the model. Originally, you had the top wing at -1/4 degree in relation to the bottom wing, and you ended-up with the top wing at -1 degree to the bottom wing. Model biplanes with the top wing at a lightly-lower angle of incidence that the lower wing usually track a bit better.
Bax,
I think that in trying to keep readers from being confused you took exception with my use of the term "too much lift" a little too hastily. Of course lift always equals aircraft weight in stabilized flight. However, with excessive positive wing incidence, and elevator trim set to anything near neutral, an unstable condition exists: more lift than gravity, the very state that allows an aircraft to pitch upwards and climb. I do not consider it technically incorrect to describe this as a state of "too much lift". It creates an uncommanded and unintended pitch up from an undesired excess of...lift!
You are absolutely correct to point out that it is a matter of trim, and that (stabilized) flight could be maintained in such a situation by trimming the elevators such that wing incidence relative to the airflow (not the fuse datum) is reduced, returning a "stabilized" state of level flight with no pitch-up. However, the tail would be following the front of the fuse at a considerably higher altitude. Works, but looks pretty goofy and creates lots of drag.
Hope this is not too esoteric. I think we both are on the same page but saying it in a different way. Thanks for your comments about "tracking" with various relative wing incidence settings, one to another, in bi-planes.
(sorry, previous post "quote" was goofed up due to my lack of familiarity with online posting. I'm learning.)
