Design > Stunt design

Side area distribution

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Curare:
Hey guys, just another off-the-cuff question, hopefully this one should be reasonably straightforward, although in stunt circles it seems nothing ever is.

I've been toying around with a new ship design, and it seems that a lot of (not all)stunt ships seem to have a lot more side area behind the CG rather than forward of it.

I was under the impression that having more area aft of the CG would make the aircraft weathervane, and sort of "tip in" to overhead maneuvres.

Am I wrong? If so why?

Is there a decent starting point for side area in terms of percentage? My current design area centroid is about 55% of the fuselage length.

john e. holliday:
I beleive there is a balance on side area behind the CG.  I have built several of Dave Tribles designs and if you look at the Nobler there is more side area behind the CG.  I think the reason is so the plane stays out on up wind side of circle and doesn't pull so hatd on down wind side.  My thoughts. D>K

Dave Hull:
Hmmm. Ok, let me try a few thoughts in a few sentences..... (Hah!)

Doc's onto it. What you want is aerodynamic balance that is unique to a stunt plane--which is different from a full-size plane. Because it is tethered to the ground and on each trip around the circle it sees lateral winds in both directions you would like the design to aid the pilot with stable flight and not have the side area fight the pilot with loss of line tension upwind.


--Design the airframe to minimize loss of line tension flying straight and level in high winds on the upwind side

--Fins are used to stabilize things. Think of the fletching on an arrow and the fixed fins on an unguided rocket. The lateral area needs to be aft-biased.

--If there was no wind or drift then the center of lateral area would matter much less to us. On a combat plane there is very little lateral area. They generate line tension via airspeed (radial acceleration). The slower you want to go, the better your aero balancing needs to be to keep the handle forces within a limited range.

--Trying to generate lots of line tension overhead by yawing the whole fuselage is probably not optimal. I suppose the more horsepower you have to spend, the more viable this gets, but it seems like it would cause lots of other trim issues in places you care about.

--The higher the wind (and the lighter the plane) you plan to fly, the more "right" you need to get this.

--Always think of the plane as rotating in 3-space around its CG. Calculate areas ahead of the CG and behind. What is a good ratio? I don't know. I would look at a plane with similar trim techniques: with or without rudder offset; with or without engine offset; forward or aft CG; etc. [more goes here]  But don't cherry pick one feature from one plane and scab it onto a new design without considering all of the related features that contribute to yaw position and stability. Or else make everything adjustable and go "in search of...."

--Mathematically, sum the lateral moments due to side loading (body drag) fore and aft about the CG to see where neutral is.

--It isn't just projected lateral area, although that is a good start. The cross-sectional shape matters, too. I suspect that it is a fairly small effect until the wind speed becomes a significant percentage of the airspeed.

Ken Culbertson:
Let me add twin tails to the discussion.  What is the actual side area of a twin?  One rudder or both?  I don't know but I can attest that they fly very well in the wind unless you miss the wind in a maneuver.  Then not so much.   Excellent balancing of line tension and great overhead which is what high aft area is supposed to provide.

Ken

Air Ministry .:
Cording to the Aero Nauticists  %^@ The TWIN Tails ( from studying for the P #* , or is that P-38 , Have Side Gust Responce of a single tail & dirctional stabilty  of the two , as its got DOUBLE the projected side view area .
I found the yaw stabilty of this thing very good to excellent , at the ' test ' stage , which was something like wot'd be the ragged edge , with only two old OS 35-S's . It'd even do wingovers & the lower rounds on the OUTER , with a bit of leaning on the handle and a bit of a slow dive to assist . As in Go over & inverted and back , on the outer . With the 10x4s on it . 9x6s were useless .

SIDE AREA .  >:(

My blue spitfire with the invasion stripes , did the shedule on the 2nd or third flight , or both , with a weak ST 60 , and flew well .
From then on it was a [pig .. Yawinging , rocking , snaking , bobbling and kniveing . With a G 51 TOO , so I put the A ST 60 backin .

One Day I whittled the spare cowl ( the Deeper P.R. version , goes up ( or Down  ;D ) to the Head ! . LIKE IT WAS AT FIRST !
the Std ish cowls curved from the spinner back - AND has a big ugly eggish opening ( like windy's )

Thus : Fwd Side Area - effecive  -  Massive trim Change . Mutter Fume . Particularly with C G near aft limit .

Whammo , back to well behaved .  %^@    ???    :-\  Tho it did rock and skid htting its slipstream the other day . i should be doping
the replaced outer covering over the Std &age ribs in the wing ends , like wot It shoulda Had , straight off rather than the THINNED ONES .
we'll see ! .

P R type Cowl , pitcher .


NOW , cording to our friend Windy , or is that Jose ; Big Jim Favoured deep noses ( on Aeroplanes ) to balance theside area , in the overheads & wingovers ,
as the air sitting on the inner nose , holds it up . So stops it falling in .

Further , a rounded tapered un plank fuse. would slightly weave & yaw ( both lateral ) flying to slow on the near stall ecge ( Plank wing spit. ) .
As it ' Hit the Cam ' , you felt the Aerodynamic Surfaces ( 1/4 Hard New Gunea Balsa Sheet ) ' Kick In ' and GROOVE it , both laterally & in the vertical .
As if you had NO Fuselage , it was ! .

So , There you go . You did ask ! .

This Thing DOSNT really have ANY side area , in a sense . Its ALL ROUNDED . As what someone was talking off somewhere , regarding WIND . ( Ref: P-38 Comments )

Ref ' Twin Tail Q ' . its gottem .

A NOTHER ASPECT of the LONG WING ( wot wasnt asked for ) is the Leadout Guide way out there , it seems way more conspicuous , shall we say . or definative . or THERE !
thats 2 metre / 78 inch . Too Big . ( redrawn 7/8th to or something to LESS and 580 Sq in , from 680 for two FSR 25s  :P )The 6 foot / 72 Inch Mew Gull exhibited that factor also .
The CONCERN is the Tip weight , too - has a L  O  N  G   moment , out there . So I dont put it there !  ;D

And YEA , The Phantom weathervanes with to big flying surface - FIN  , as against a similar but lower / smaller less outrageous one .. Bob Hunt took a dislike to fins , with His Genisis  S?P .

Perhaps VERTICAL FLYING SURFACE  ( fin / rudder ) are a seperate if pertanant factor . The ' Big Jim '  class, seems to minimalise that . Re: previous FUSE AREA B. J. Info . :P

So , Theres Fuse side Area , AND Flying Surface - Vertical .  It would seem .

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