Ed Cregger
Posts: 8078
Joined: 1/31/2002
From: Ringgold,
GA, USA
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quote:
ORIGINAL: AllTheGoodNamesAreTaken
quote:
ORIGINAL: Ed Cregger
You won't need the downthrust on the DB if you don't jack the wing's leading edge into positive incidence.
Ed Cregger
Ed,
Why did Joe design it that way with 2 deg down thrust and ~1 deg pos on the wing?
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My theory is that the old guys from that era were still designing models with reed flying setups. Old habits die hard. Another thing that is detrimental to model designing and thinking is flying full size planes. Many general aviation pilots can't shake their old habits when thinking of models, or designing them. That is one of the things that ruined many of Sig's earlier model offerings.
We fly models in an almost continuous circle, or stretched lines with quick turns. Full size aircraft (an older reed sets or escapement models) were designed to be stable and fly for extended lines. As a result, the models did not fly all that well. This is just me analyzing the situation and nothing more. No one has to agree in the slightest manner.
Downthrust is mostly used to defeat zooming when power is applied. This assumes that the model will climb because of an assymetrical airfoil, which is biased to provide lots of lift with increased airspeed, or a wing that has been canted with the leading edge going up (positive incidence). Again, this is a holdover from the days of yore and is totally unnecessary, in fact is detrimental to a model supposed to perform equally well whether upright or inverted.
It is also possible that downthrust was used to offset the drag of the fuselage above the wing, which will make the model move toward the canopy side of the model when in knife edge flight. Cancelling this move with downthrust is also accompanied by unintended consequences in other flight modes.
It all depends on which way you feel comfortable in compensating for idiosyncrasies that pop up after we make our "best compromise choices". If you don't mind holding a bit of down elevator in knife edge flight, then the downthrust isn't necessary. But be sure that you are going to be holding a correction some place else in another flight mode.
Ed Cregger
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RE: Dirty Birdy Build Thread - 
