Actually, Al my top speed did increased with the larger prop about 4 mph (but not what I would consider significant).

 

But, to your point -- trying to get theory to answer real performance is hard thing to get at calculations wise, but here is my take on it.   

 

OK; Ed – given your expertise on technical explanations, I’d like your take on why your increased hp resulting from the increased RPM is not giving you higher top end speed.  Is it exhaust back pressure or less than optimum prop, or other?

 

Al

First, as you suggested , there are many factors relative to aircraft performance and the prop is only one – a somewhat important one, but its performance can be also affected by these other factors.  That being said, I am going to see if I can address your point focusing on the prop factor - understanding that I am certainly not a propeller expert.

 

But I think using this equation for thrust address what happens

 

Here is an  equation for calculating thrust, you can find others, but this one is fairly straight forward.  T being the thrust and D being the diameter of the prop.  Dv being the additional acceleration given to the air by the prop’s rotation.   So all things considered Dv is principally  a product of engine RPM and propeller pitch – but pitch is fixed in this case, so does not change.  Therefore the dominate factor in accelerating the air in this case is the engine rpm.

 

 

So why if I am producing more power with the 2.85 ratio - why aren’t I going faster on the top end.

 

Well, the thrust factor  Dv as produced by the turning prop accelerating the air. But as incoming air flow V increases during flight  Dv gets smaller.  Airspeed will increase with the inflow air V increasing  until a point is reach where the propeller is producing no additional thrust or where Dv = 0.  In other words, the velocity of  air flow out of the prop disc is the same velocity as the air flowing into it and there is no further acceleration or thrust or Dv = 0.

 

As a results,  a larger diameter (but slower turning prop) reaches the point in flight where Dv = 0 sooner than a smaller diameter but faster turning prop does. Reaching Dv sooner limits the velocity the aircraft can achieve So in theory, that is why the racers want (in part) these short diameter high rpm props.  So that is why all things else being equal a larger diameter, but slower turning prop,  is not the best for top speed. 

 

But, as you know, there is more to it than just this.  For example an aircraft with less drag (and/or more power) can spin a larger prop faster than one with more drag or less power (all things else being equal).

That is one reason I believe that Tracy’s RV-4 gets a higher top speed with the same prop than I do. He may or may not have a more powerful engine (probably does), but he certainly has less drag than my hefty RV-6A.  But, in both cases, I believe the reason the top speed increased was more power was available as the torque load was loader on the engine permitting higher rpm.  Had the power being produced remained unchanged then of course my top speed would have dropped. 

 

I once had one older gent who had raced in the Cassute? class that stopped by and chatted with me a while at a fly in – told me if I wanted to go faster to trim some off the diameter of my prop.  Well, I don’t care about top speed, I want the take off performance to get me off grass strips and over tall trees off short runways.

 

So I have a larger diameter prop which helps in take off, but it turns slower so it’s not the optimum for top speed.

 

That’s about the best answer I can give you, Al.  There are numerous other factors such as advance factor, Coeffiences of Thrust and Power you find in some thrust equations and on and on.  But on the back of the envelope that’s the way it looks to me.

 

Ed

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Al Gietzen
Sent: Tuesday, February 09, 2010 1:10 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: rpm vs Power was : Throttle limits was Re: N.A. Renesis to turbo

 

Ed wrote:

The new set up gave a noticeable benefit in take off performance (and I do

mean noticeable).  However, the top end was relatively unaffected - although

I did measure an approx 4 mph increase in top aircraft speed.

 

OK; Ed – given your expertise on technical explanations, I’d like your take on why your increased hp resulting from the increased RPM is not giving you higher top end speed.  Is it exhaust back pressure or less than optimum prop, or other?

 

Al

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Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm

 

 

 

Ed Anderson

Rv-6A N494BW Rotary Powered

Matthews, NC

eanderson@carolina.rr.com

http://www.andersonee.com

http://www.dmack.net/mazda/index.html

http://www.flyrotary.com/

http://members.cox.net/rogersda/rotary/configs.htm#N494BW

http://www.rotaryaviation.com/Rotorhead%20Truth.htm