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From: scw@cepu.UUCP (Stephen C. Woods)
Newsgroups: net.aviation
Subject: Re: High speed snap rolls
Message-ID: <439@cepu.UUCP>
Date: Wed, 30-Jan-85 13:59:16 EST
Article-I.D.: cepu.439
Posted: Wed Jan 30 13:59:16 1985
Date-Received: Sat, 2-Feb-85 14:49:53 EST
References: <1053@opus.UUCP>
Reply-To: scw@cepu.UUCP (Stephen C. Woods)
Distribution: net
Organization: VA Wadsworth Med. Center; LA CA
Lines: 59
Summary: 

In article <1053@opus.UUCP> scott@opus.UUCP (Scott Wiesner) writes:
>To prevent anyone from freaking about about this article, let
>me say up front that while everything described applies to any
>aircraft, the incident occurred with a radio controlled MODEL.
>
>I flew a new plane this weekend.  I had bought it at an auction
>for what seemed like a good price.  As models go, it was a little
>overweight for it's size, and grossly overpowered.  (The big engine
>was the reason for it's being overweight).
>
>The plane had a tendency to snap roll at high speed.  Whenever I
>pulled a little extra [...] started slowing down.  The plane started
>heading straight down, but was NOT in a spin.  Just flying vertically 
>down, with the engine at an idle.   Whenever I tried to give a little
>up elevator, it would start to pull out, then do another snap.  I
>probably went through this snap/recovery sequence 4 times before it
>went in.  
>
>Now my question.  Can anyone out there give me a good explaination
>of the aerodynamics that were going on here?  It would seem that 
>when at full throttle, quickly changing the angle of atack would 
>cause the wing to quickly stall out, resulting in the engine torque
>taking the plane for a ride. 

You didn't discribe the wing section, but it sounds to me like there was a
litle rigging problem.  I'd suspect that you had at least 2 of the following
problems:

(1) CG too far aft.
   This sounds unlikley given the big engine but not impossable given that the
   model was overweight.

(2) A poorly chosen airfoil, airfoils with sharp leading edges have undesireable
stall charisterics (like WHAMO ), there are also airfoils that have VERY poor
lift moments (as you increase the angle of attack the center of effort moves
forward) in this case if your CG was near the aft limit, all of a sudden it's
behind the center of effort and you get an increasing angle of attack, which
moves the center of effort forward (divergence so to say).

(3) The wing had a lot of wash-in (the wing tip has a higher angle of attack
that the rest of the wing), this is very bad as the wingtip stalls before the
wing root (Vola snap roll).

(4) Elevator had too much throw (very easy to overcontrol).

(5) Elevator push rod was too flexable (assuming that it pushed for up
    elevator).
       This produces a very wierd effect, as the aircraft slows down the
   elevator increases in deflection.  Usually this is not a very pronounced
   effect, but I had a glider once that was very difficuly to fly because of
   this, when I replaced the push rod with a stiffer one it flew a whole lot
   better.

(6) An asymetrical wing (did it always snap the same direction?) different
airfoils on right and left wing will produce wierd effects.
-- 
Stephen C. Woods (VA Wadsworth Med Ctr./UCLA Dept. of Neurology)
uucp:	{ {ihnp4, uiucdcs}!bradley, hao, trwrb}!cepu!scw
ARPA: cepu!scw@ucla-cs location: N 34 3' 9.1" W 118 27' 4.3"