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Author Topic: Stab aspect?  (Read 1263 times)

Offline Bob Reeves

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Stab aspect?
« on: January 06, 2007, 03:39:27 AM »
For several years I have had a SIG Somethin'Extra RC kit in the rafters with the idea of building it into a stunt ship. Mainly because my company name is Somethin'Xtra and how could I not build an airplane with a name so close.
The success of the Top Flight Score and it's resemblance has renewed my intrest and thinking it would be pretty safe to use the Score numbers and actually end up with something that is competitive.

Have already decided to build (stretch) the wing to the Score aspect but one issue I was wondering about....

The stab/elevators on the Somethin'Extra have a pretty low aspect compaired to the Score. If I keep the same area and ratio as the Score but change the aspect will I turn a great flying design into a dog or will it make any difference?

See ARF's do lead to building  ;D


Offline phil c

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Re: Stab aspect?
« Reply #1 on: January 06, 2007, 07:14:41 AM »
Dig out Wild Bill's articles and calculate the control forces on the elevator and flap pushrods.  Low aspect stab/elevator results in higher pushrod forces and controls that stiffen up faster with speed(or lose effectiveness with a loss in line tension, the Netzeband "wall").  If you make the aspect ratio too high you can make the control forces so low you  lose the feel at the handle.
phil Cartier

Offline Dick Fowler

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Re: Stab aspect?
« Reply #2 on: January 06, 2007, 08:09:39 AM »
Dig out Wild Bill's articles and calculate the control forces on the elevator and flap pushrods.  Low aspect stab/elevator results in higher pushrod forces and controls that stiffen up faster with speed(or lose effectiveness with a loss in line tension, the Netzeband "wall").  If you make the aspect ratio too high you can make the control forces so low you  lose the feel at the handle.

Phil, wouldn't the control loads be more affected more by the  stab/elevator chord ratios  than aspect ration?
Dick Fowler AMA 144077
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Offline Elwyn Aud

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Re: Stab aspect?
« Reply #3 on: January 06, 2007, 01:20:18 PM »
I don't recall seeing many stunt planes with a low aspect ratio stab but I do recall seeing a few combat ships with one. Of course these weren't flapped and many didn't have a fixed stab except for maybe just enough area to support a hinge of some sort.
Elwyn Aud AMA 19413OK
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Offline Howard Rush

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Re: Stab aspect?
« Reply #4 on: January 06, 2007, 09:03:18 PM »
The Impact stab aspect ratio is lower than most.  It's a lot lower than the aspect ratio of my combat plane stabs.
The Jive Combat Team
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Offline Jim Pollock

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Re: Stab aspect?
« Reply #5 on: January 06, 2007, 09:31:29 PM »
Howard,

My understanding is that the Lazer had a lot to do with the low aspect ratio stab on the Impact.

Jim Pollock   :)

Offline Howard Rush

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Re: Stab aspect?
« Reply #6 on: January 06, 2007, 09:52:34 PM »
I don't know, but low aspect ratio is what you'd expect of a structures guy.

I just calculated the aspect ratio of Rushpuppy stabs.  It's about 6 for the AMA version and 6.4 for the F2D.  The common definition of aspect ratio, by the way, is span squared over area.  There are different rules of thumb for how to account for wing or stab area inside a fuselage. 
The Jive Combat Team
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Offline Bob Reeves

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Re: Stab aspect?
« Reply #7 on: January 07, 2007, 04:26:50 AM »
Thanks Howard for mentioning the Impact... I have a UHP Impact kit, will take a look at the stab.

Offline Crist Rigotti

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Re: Stab aspect?
« Reply #8 on: January 07, 2007, 12:52:00 PM »
Bob,
3.75 to 4 is a good ratio.
Crist
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Offline phil c

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Re: Stab aspect?
« Reply #9 on: January 07, 2007, 07:32:12 PM »
Phil, wouldn't the control loads be more affected more by the  stab/elevator chord ratios  than aspect ration?

I don't have WB's articles in front of me, but he had a graph to account for the ratio of stab to elevator chord.  I think it was taken from a 50's Navy textbook for pilots.  As I remember, the ratio between stab and elevator has little effect on the amount of lift generated.  Narrow elevators and wide elevators work nearly the same.  The big difference is wide elevators need more control force needed to move the surface. 

A second factor is the absolute width of the elevator.  From practical experience the last couple years, a 3 in bellcrank can't reliably work an elevator more than 2 in. wide, regardless of the stab/elevator ratio.

My original Giles model used a built up scale tail.  The aspect was about 4.  The elevators were 50% of the chord, and about 3.5 in. wide.  This setup caused no end to control problems.  Any speed change resulted in very wide changes in the force needed to move the elevator.  It was fine at a 5.1 lap, say, but would start to stiffen up markedly at 5.3.
phil Cartier

Offline Bob Reeves

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Re: Stab aspect?
« Reply #10 on: January 08, 2007, 07:50:48 AM »
Some good info, might be able to take everything and come up with something that looks about right but follows stunt numbers better. Have a 3 1/2 inch bellcrank in my Score but the Somethin'Xtra will have a 4 inch.

Just measured the Score elevators, they are 3 inches at the root without allot of taper and the 3 1/2 inch bellcrank doesn't have any problems moving them. Thinking I could probably go to 3 1/2 at the root with more taper and come up with something that has close to the same area but would look more like the Somethin'Extra.

Still throwing it all into the hopper and seeing what comes out the other end.. Thanks for the help.

Kim Doherty

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Re: Stab aspect?
« Reply #11 on: January 08, 2007, 12:51:41 PM »
A second factor is the absolute width of the elevator.  From practical experience the last couple years, a 3 in bellcrank can't reliably work an elevator more than 2 in. wide, regardless of the stab/elevator ratio.


Phil,

Sorry, I dissagree. A 2.75 inch bellcrank can operate a 3 inch wide elevator with great authority!

Kim.
« Last Edit: January 09, 2007, 08:23:01 AM by Kim Doherty »

Offline Dick Fowler

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Re: Stab aspect?
« Reply #12 on: January 08, 2007, 06:24:38 PM »
The thing that isn't being mentioned is the area of the elevator. Regardless of width of the elevator the control loads are a function of how much area is getting flopped into the wind.
Dick Fowler AMA 144077
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Kim Doherty

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Re: Stab aspect?
« Reply #13 on: January 09, 2007, 08:53:38 AM »
The thing that isn't being mentioned is the area of the elevator. Regardless of width of the elevator the control loads are a function of how much area is getting flopped into the wind.

Dick,

While it may not be any type of baseline, the SkyWriter's elevator is 75.06 sq/in in area and is actuated by a 2.75" in bellcrank with .79" pushrod arm length. The stab is 75.46 sq/in in area with a span of 27.5" Subjectively, the controls are very powerful!

Kim.

Offline phil c

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Re: Stab aspect?
« Reply #14 on: January 10, 2007, 12:37:15 PM »
Kim, if you read a bit more closely, I was saying that wide chord elevators and low aspect ratio stabs aren't as forgiving.  If you get the speed outside a certain range the controls stiffen up faster.  This isn't an either/or condition like a 4-2 break.  Controls don't suddenly stop working.  Restricting the elevator width is an easy way to limit the windup of control forces.
phil Cartier


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