flyrotary@lancaironline.net Mailing List Archive

Hmmm! If I understand our engines correctly, they make about 150 ft-lbs of torque. The torque curve is virtually flat from about 3-4000 rpm, while the HP curve continues to climb with rpm. Since we have the 2.85 PSRU, we are turning the prop with about 425 ft-lbs of torque thru the entire range of cruise and WOT operation. This would indicate that the engine has all the power it can use to crank the prop from a very early stage. My static rpm is just over the 5250 rpm where the torque and HP are equal. Since the torque stays level and the HP continues to climb, I don’t get why the static rpm does not continue to climb??

I apologize for continuing to slap you upside the headache about props! :>)

Bill B

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Ed Anderson
Sent: Thursday, March 04, 2010 4:49 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Vance Jaqua and Propellers

Well, Bill, can’t disagree that I’m producing more HP, but the Prop doesn’t know or care about HP, it cares about Torque. The new gear box provides the increased torque to turn the larger prop. It’s true that if the engine was not producing more power then there would be less torque and certainly less rpm for the prop. So it all counts.

Look at it this way, you can have the same engine (200HP) that can either lift a small helicopter or speed a fixed wing to speeds of several hundred miles per hour.

The major difference is the diameter or the prop and the prop’s rpm (gear box ratio selection) – the HP is the same for both. That is why I say HP itself is not seen by the prop.

I agree, what you are calling for is a controllable pitch prop – no question that would be the last major development that would really make our rotary installations sing. But, despite one serious attempt – nothing has yet come of it – unless you can and want to spin some big $$ as some of our rich friends have done as you mentioned {:>).

Since I don’t care about top speed (never fly there except to see what it is), I knew I wanted my performance tailored for take off out of grass strips over tall trees. My philosophy is “…. Top speed doesn’t matter if you don’t clear the trees!!! …” . But fortunately, it actually helped my top end a few mph as well.

I certainly don’t claim nor believe I understand props – I just go to a reliable prop manufacture and tell told him what I had and what I wanted. When I originally got my 76x88 prop, I decided I wanted 200 more engine rpm and that is all I told Clark Lydic and he cut off an 2” off the diameter and I got my 200 rpm increase. Could have been just lucky, but a good prop guy can give you the right prop – IF you give him the right information.

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 Bill Bradburry
Sent: Thursday, March 04, 2010 4:26 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Vance Jaqua and Propellers

Ed,

I think a major cause of the increased performance after you changed the gearbox and prop was due to the fact that you were now able to get more rpm out of the engine on takeoff and therefore were generating more horsepower.

It still seems to me that to get the most performance out of the engine, it would have to be capable of 7500 rpm at what ever flight regime you wanted the most performance (takeoff or cruise). I don’t think a fixed pitch prop will allow both areas to be maximized. You have to pick one or the other. What I assume that most of us have done is pick somewhere in the middle that does not allow a maximum performance at either end of the spectrum. (With the exception of some of the moneybags among us like Mr. Steitle who has a con$tant $peed MT prop on his ES.. :>) )

I am considering(actually have decided to do it) cutting my prop down to 74” as well. What I have been wondering about is: We lose some performance from the prop by cutting it off, but we gain a lot in HP of the engine due to the higher rpm and also gain from the prop due to the fact that it is rotating much faster than it was at the longer length. Cutting 2” off the prop gives a loss of 5.2% of the cylinder of air that it is moving. If this results in a gain of ~800 rpm, the loss is recovered and the HP of the engine is increased considerably. I am not talking here of static numbers, but at, say, takeoff, climb out speed and power.

I am hoping some of the folks flying with these props will chime in with some info on what actually happens in these scenarios.

Bill B

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Ed Anderson
Sent: Thursday, March 04, 2010 12:40 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Vance Jaqua and Propellers

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.