flyrotary@lancaironline.net Mailing List Archive

You got it, Bryan, the larger prop spins at a lower rpm – however, because it is geared to the engine with a 2:85 ratio there is less load on the engine and therefore the engine revs to a higher rpm producing more power (and compensating some by the increase in engine rpm). But, the prop knows nothing about the HP of the engine – all it knows is the torque the gear box is providing it to turn. The 2.85 provides more torque for the same engine rpm so can turn a large prop for the same power. But, because the engine is not loaded as heavily with the 2.85, I can get a bit more rpm –so this compensates a small bit for the slower turning prop – because its turning a bit faster than it would had the engine rpm remained the same as with the old prop and gear box.

The prop rpm decreased for the same old engine rpm (but, I got a bit of that back due to the higher engine rpm with the new gear box)– however, the larger diameter of the prop pushes more air mass so therefore I accelerate faster on take off roll. To determine the prop rpm simply divide engine rpm by gear box ratio – to get engine rpm simply multiply prop rpm by gear box ratio.

With my old 68x72 I would turn around 5200-5400 on a normal day. So prop rpm was 5300 / 2.17 = 2442 rpm. With the 74x88 and 2.85 gear box, the engine would turn 6000 rpm So new prop rpm is 6000 / 2.85 = 2105 rpm or almost 300 rpm slower. But the old prop disc area was P(68/2)^2 = 3632 sq inches where as the new one is P(74/2)^2 = 4300 sq inches. That gives me a 15.5% increase in disc area with the new prop compared to a 11.6% decrease in RPM. So if everything else were equal it would appear I would have an net 4% increase in thrust (not counting the supposedly greater efficiency of the slower turning prop – which might be a couple percent. But, remember this is 4% is constantly applied during every revolution of the prop on take off. So its similar to applying a constant acceleration to an object. Even using the very small thrust of Ion rocket engines – they get the space craft up to tremendous speeds because of the continuous application of that small acceleration.

So as Ernest says – there are many, many factors involved in how a prop functions – our math calculations are very simplified models of the real world – so don’t be surprised if the don’t always provide the expect answer – nature really doesn’t care about conforming to our simply models {:>)

Enough – makes my head hurt.

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 Bryan Winberry
Sent: Thursday, March 04, 2010 1:49 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Vance Jaqua and Propellers

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

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.