[NSRCA-discussion] Aero design question

[Bob Richards]

Fred,
   
  Actually, Nat has it right. To compare a helical pitch propellor to a wing, you need to start with a prop that is feathered, not the other way around. For true helical pitch, the cordline of the prop will be parallel to the prop shaft at the center (just like a wing) and gradually "twist" as you move away from the prop shaft.
   
  Bob R.
  

Fred Huber <fhhuber at clearwire.com> wrote:
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  Helical pitch is "steeper" at the root than the tip of the prop. Since you want the whole prop to go forward 1 inch.. and the root essentially has little movement it has to be steep to keep up with the tip's large movement.
   
  If you want to simulate the same thing with an aileron.. you would have more aileron chord near the root than the tip.  This would inhibit tip-stalls if the wing is constant chord (See the way the Four Star series does it) 
   
  For a tapered wing, as is common in Pattern, to get the "helical" effect you'd need the ailerons to make up 50% or more of the difference in chord from root to tip.  
   
  Depending on aileron deflection angle... a different % chord ratio from root to tip would be required to achieve the "helical" incidence(tapered or straight chord wing)  For any given tapered aileron, there will be one deflection angle that comes closest to producing helical incidence.  
   
  If Helical incidence is the goal.. you'll have to choose at what percentage of full aileron throw you want to hit  helical incidence... cunch some numbers and then cut the ailerons.
   
  Constant chord ailerons in effect gives more AOA change at the tip than the root with a tapered wing. (encouraging tip-stalls... this is why its inadvisable to use flaperon function with a Pattern design when landing... but spoilerons will help prevent dropping a wing.)
   
  But.... it may be a good thing to get the higher AOA change near the tip...  
   
  As pointed out... the Sig Four Star uses the tapered ailerons in a manner which can approach a helical incidence, so its been tried at least in one design series.  Its probably been tried in a Pattern design at some time... (if not... someone should try it)
   
   
   
    ----- Original Message ----- 
  From: Nat Penton 
  To: NSRCA Mailing List 
  Sent: Sunday, September 03, 2006 9:31 PM
  Subject: Re: [NSRCA-discussion] Aero design question
  

  Hey Tom
  I'm not providing solutions - just putt'in more wood on the fire. In steady state roll the least drag position of the ailerons would be constant AOA root to tip. This would call for the percentage of chord to progress linearly so that it would be double at the tip vs the mid span position - no different than helical pitch. A ( severe ?) negative would occur, though, when the ailerons are initiated, potentially causing a tip stall.
   
  Martin Simmons provides a curve showing Cl vs % of chord for the aileron. Going over 20% doesen't get you much for the dollar - just much more servo wear and tear ( especially with the 160 <G> ).
   
  IMO it is not worth the extra work to stop the ailerons short of the tip.                            Nat
   
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