Winter / Focus / Prop Drag

[Adam Glatt]

Larry Diamond wrote:

>This has been a very interesting thread on Prop Drag Vs Idle..
>
>Couple of thoughts...
>
>1) From a non-engineering mind or non-aerodynamics mind, Stop Vs Idle seems
>to be about the same as a motor vehicle. The vehicle slows better when the
>tires are not in a skid.
>  
>

In the car situation it is because the phenomena of static friction can 
create more force than kinetic friction.

If it phenomena we are trying to explain was caused by the drag from the 
prop itself, we would witness 0 RPM having the most drag, with drag 
decreases at some rate until it hits zero drag when the pitch of the 
prop times the distance it is travelling equals the air speed, which is 
not quite true, as highest drag seems to be somewhere in between, with 
the second situation being zero and the 0 RPM situation giving maybe 70% 
of the drag that a prop at 1800RPM gives (calculated by: air velocity 
squared * cross sectional area * drag coefficient).

My theory is that the rotation of the prop is causing the air inside 
(and to some extent, outside) of the prop's two blades to also rotate, 
as well as being pushed backwards (but the added push is quite small at 
1800RPM, and if the plane is in flight, smaller than the cross-sectional 
drag caused by the blades). This rotation of the air is only significant 
near the blades themselves, but has the effect of making the blades 
wider, which increases their cross-sectional drag.

So, at 0 RPM prop blades gives most drag but no added drag, at, say, 
1600RPM prop gives decent drag and quite a bit drag from added area that 
resists moving with the airflow.  Going to 4000RPM on the downline makes 
for almost no prop drag and even more added area drag.  In order for our 
props to work at 8500RPM, when the plane is at a terminal velocity 
flying horizontally, this added drag (the side-effect of producing 
thrust by rotating an angled blade), must be small relative to the 
thrust.  This is backed up by my experience that a stopped prop gives 
almost the same amount of drag as an 1800RPM prop.  I honestly have a 
hard time telling the difference on a landing approach (I only keep it 
running so I can better control how close to myself I land).

A graph of RPM vs overall force the prop produces, with constant air 
velocity of 80mph, and RPMs going from -8500 to 8500 would be great 
evidence when trying to explain this.
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