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Date: Sun, 27 Feb 2005 00:04:56 -0600
From: Charlie England <ceengland@bellsouth.net>
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To: Rotary motors in aircraft <flyrotary@lancaironline.net>
Subject: Re: [FlyRotary] Monster Prop Air Brake
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Ed Anderson

snipped

>  
> Monster Prop Air Brake
>  
> Well, I will add one more possibility (perhaps the most likely).  I 
> found out today by experimentation that with the big monster prop 
> (76,88)  that if I pull the power back sufficiently, the airspeed 
> decays very rapidly. Let me explain -  It appears that so long as I 
> have the prop turning over fast enough so that it is providing a "no 
> drag" to the air flow - everything is just fine.  However if I lower 
> the power such that the prop is no longer a "no drag" component air 
> speed decays much faster than with my old prop.
>  
> For instance if I am doing 89-90 mph (as if on final) with the engine 
> turning around 2800-3200 rpm everything is just fine.  However, if (as 
> I recall doing on that hard landing) I retard the throttle to around 
> 2000 rpm (which I did that day as I was slightly over shooting my 
> touch down point) , the air speed decreases rapidly and the rate of 
> descent goes up considerably.  Never had that type of thing happen 
> with the smaller prop or at least not to any noticeable extent.
>  
> I speculate that the combination of the  larger disk area (25%) of the 
> 76 vs the 68 inch prop dia and the 2.85:1 gear box offering more 
> resistance to the airstream turning the prop (compared to the 2.17).  
> So if the prop is  turning at least fast enough so that is not a drag, 
>  there is no noticeable effect, but once the engine is retarded to 
> where it becomes a drag component to the air stream - it apparently be 
> comes a "BIG" drag component.
>  
> So don't know whether this theory holds water, but the effect is there 
> and I found today that keep the engine at 3200 rpm resulted in a 
> considerably difference.  Any  ideas from you prop guys??
>  
> Ed
>  
>  
>  
> Ed Anderson
> Rv-6A N494BW Rotary Powered
> Matthews, NC
> eanderson@carolina.rr.com <mailto:eanderson@carolina.rr.com>


My non-prop-guy (but RV-experienced) opinion is that you might have had 
a bit more drag but what you really had was a lot *less thrust*.

My 1st RV-4 had a 72" dia cruise prop (would run 195 mph at 2550rpm, 
9.3gph in still air on 160hp) so it probably wasn't too far from your 
prop. If prop rpm at idle was set higher than around 700rpm (sitting 
still on the ground) you almost couldn't make the plane land because of 
the extra thrust.

Is there a chance that you got used to a flattened approach slope with 
the old gearbox/prop because it was still making a fair amount of 
thrust? With the wider ratio gearbox & pulling the engine to idle, I'll 
bet you actually got the prop slow enough to stop making thrust.

Charlie
(always trying to fly patterns at idle from abeam the numbers to touchdown)