Received-SPF: softfail
 receiver=logan.com; client-ip=171.71.176.71; envelope-from=echristley@nc.rr.com
Message-ID: <46657A10.9090901@nc.rr.com>
Date: Tue, 05 Jun 2007 10:58:24 -0400
From: Ernest Christley <echristley@nc.rr.com>
Reply-To: echristley@nc.rr.com
User-Agent: Thunderbird 1.5.0.10 (X11/20070403)
MIME-Version: 1.0
To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Re: Single rotor video
References: <list-2085186@logan.com>
In-Reply-To: <list-2085186@logan.com>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed
Content-Transfer-Encoding: 7bit

wrjjrs@aol.com wrote:
> Rusty,
>  You may be getting something that was studied with dirt bike singles 
> and narrow angle v-twins (Harleys). These engines worked well in 
> getting traction on dirt tracks due to their firing order. If I recall 
> it was called climb and retreat by the guys actually studying it. It 
> would apply to a prop very well. Your prop has at the very least some 
> flexibility in the blades. The power pulse hits and forces the blade 
> forward violently, followed by a lull. Think of it like driving a long 
> screw into wood. Every 1/2 turn you must re-position your grip. Any 
> twist in the shaft of the screwdriver is released until you start 
> turning again. The fore and aft movement might be caused by the large 
> power pulse of the single rotor. This may be a totally natural event. 
> OF course I've been wrong before, YMMV etc...
> Bill Jepson
>
>
Another dynamic factor will be that when you hit the prop with that 
pulse, will it make the blade twist?  That will be wholly dependent on 
the mass distribution around the blade chord at each station.  There's 
no way to make the blade 100% stiff, but I think composite blades would 
be more flexible than other types.  So, the pulse hits (hard) and 
produces a  slight twist in the blade which increases the angle of 
attack.  Producing more lift on the blade, which is already being 
stressed by the power pulse, causing it to flex forward.  The pulse goes 
away, or even negative, and the blade snaps back like a loaded spring.

You might want to consider finding a way to extend the prop further out 
away from your trailer before you try any more experiments.   Then see 
if maybe you can borrow a few different props and see what sort of 
reaction they give you.  You might find that a different prop will have 
a different mass distribution that doesn't exhibit the problem.  Or 
maybe you can make one yourself that has extra material to resist warping.