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Design => Engineering board => Topic started by: Jim Pollock on September 26, 2009, 05:09:38 AM

Title: Tail Power formula
Post by: Jim Pollock on September 26, 2009, 05:09:38 AM

Gentlemen,

Could someone post Brett Buck's tail section power formula here please?

Thanks in advance.....

Jim Pollock   ;D

Title: Re: Tail Power formula
Post by: phil c on September 29, 2009, 08:18:36 AM

don't know if this is what Brett uses, but the standard tail volume calculation is:

tail moment * tail area/avg chord*wing area

tail moment is the distance from the CG to the MAC of the stab, but since this is kind of an estimate(it changes when the plane is maneuvering) you can use the distance from the wing MAC to the stab MAC and still be close enough.
 

the units cancel out and you get a number which is roughly indicative of the effectiveness of the tail- bigger is more stable, smaller is less stable.  Typical numbers range from .1 for a combat plane to around .5  for many modern stunters and scale planes.

Title: Re: Tail Power formula
Post by: Jim Pollock on September 29, 2009, 12:13:35 PM

That's the one Phil.

Hope this is posted to a formula board for all modelers to use on this
forum.   Ahhhah, well this is that board, hopefully we'll keep it near the
top for use during designing airplanes!

Jim Pollock   ~>   n~

Title: Re: Tail Power formula
Post by: Howard Rush on October 13, 2009, 05:09:22 PM

Dean Pappas amplifies this in a nice article in the latest (I think) Model Aviation.  Pitching moment due to pitch rate is still missing, but he may cover it next month. 

Title: Re: Tail Power formula
Post by: phil c on November 23, 2009, 03:09:41 PM

Quote from: Howard Rush on October 13, 2009, 05:09:22 PM

Dean Pappas amplifies this in a nice article in the latest (I think) Model Aviation.  Pitching moment due to pitch rate is still missing, but he may cover it next month. 

Good point.  As the plane turns the downwash from the wing increases, reducing the effectiveness of the tail.  So the harder the plane is turning the less stable it  becomes.  One reason why planes with marginal static stability seem to get "stuck" in a hard corner and are hard to bring out of the turn on heading.  That is in addition to the pilot getting locked up in the turn too.  It can be hard to pull a hard turn and then back out of it at the right time.

Title: Re: Tail Power formula
Post by: Dean Pappas on November 23, 2009, 06:33:23 PM

Thanks Howard,
but I think I had most of the audience on the theoretical ride for a little too long.
The next month I switched over to doing a how-to on crankshaft bearing replacement.
I was promted to do so after being horrified by a club-mate who sent an engine back to Illinois because the bearings made noise!
Some day, yes but I'll have to talk about the destabilizing effects of the prop first.
Dean