Ed,

You’re losing me here.  If I understand you correctly, you are taking off with lower prop RPM now compared to the original prop?  But the RD-1C allows the engine to produce more HP(via more rpm).  Isn’t the goal to turn the prop just short of its’ max RPM for T/O?  Come to think of it, maybe that’s your point.  Do you know the prop rpm’s for the two scenario’s?

Bryan

 

 

Hi Bob, Lynn

 

I agree with your assessments - doesn't make any sense otherwise.

 

There was a very smart individual I knew by name of Vance Jaqua.  I had several exchanges with him about propellers that made more sense to me than what I read in Prop theory.  I mean he really made sense about props as it relates to the real and theoretical world.  One of things he pointed out regarding static thrust - was how crucial prop loading was to thrust produced/ per horse power at/near static conditions.  He took on some of the basic tenants of current prop theory and pointed out some thinking that just didn’t seem to make sense in the real world.   Here is an extract out of his “thinking paper” on propellers he shared with me (my comments in blue)

 

The mass that we are going to accelerate for a classic airplane propeller, is roughly a sort of cylinder of air the size of which is related to the diameter of the prop.  (in other words, whatever we do and however efficient it is or is not – it’s all relative to this cylinder of air which is related to the diameter of the prop – so the diameter of the prop appears to sort of set a  foundation element for all else)

 

The size of that cylinder of air controls the amount of weight flow through the prop disk (That all important M(Mass) in our thrust and power calculations that we discussed above). Now using that MV (momentum = mass * velocity)formula, we can get 100 pounds force of thrust by either accelerating 1 unit of mass by 100 feet per second, or by accelerating 100 units of mass by 1 foot per second, (or any other combination for which the answer is 100).

 

 However, remember the expression for power - M times velocity squared, divided by two. This puts a big difference on how much power we need to make the force. The 100 feet per second case computes to 5,000 ft pounds of energy, where the 1 foot per second case is only 50 foot pounds. This is for the static thrust case, and explains why helicopters and STOL type airplanes use those big, rather slow turning propellers to get off the ground. (this also helps explains why my going from a faster turning (68x72) prop using the 2.17 gear box to a slower turning larger diameter (74x88) prop using the 2.85 gear box increased my static thrust and  take off performance – it gave lighter disc loading)

 

 As the airplane flies faster, the need for those large diameters becomes less important, because now your disk is "running into" lots of pounds per second just because of your forward velocity. 

 

I thought these few words of Vance explained more to me than an entire book of theory on props.

 

Here is a chart that Vance  provided that illustrates his point about low disc loading contributed to more static thrust.  Particularly note the difference it makes at zero airspeed (our static condition).  A very lightly loaded prop of 1 Hp /sq ft produces around 9 lbs of thrust per HP at static whereas the higher loaded (more HP per sq ft of prop disc area)  the prop is - the less static thrust produced.

 

This might seem counter intuitive but Vance points out a helicopter might have a disc loading as light as 0.25 Hp/sq ft. A helicopter can clearly produce great amounts of static thrust as it lifts its own weight vertically off the ground at “zero” airspeed.  It would appear that greater power and smaller prop produces less thrust due to the fact that a smaller diameter blade is likely highly pitched to absorb the greater power and likely has a large portion of its blade stalled or otherwise turbulent, distorted air flow as the powerful engine churns the air.  Even the book theory indicates a slower turning prop is more efficient in using power to move air.

 

 

 

 

 

Unfortunately, to the best of my knowledge, Vance never turned his notes into a finished paper.  He certainly had a viewpoint that I found understandable.

 

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

 

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