Ok, thanks, Bill – memory not what it once was {:>)

Here is my take on your situation.  With the same prop I produced 5800 rpm from my old 13B.  This indicates to me that you might be premature deciding to take any length off of your prop.  I (if I were in your situation) would  feel more comfortable if  the engine was turning at least 5600-5800 rpm with that prop.  I suspect that your induction/exhaust system may not be as effective as they could be.  Therefore, IF you did decide at this point to trim the prop and later on found more power you might wish you had not.   I presume when you get that RPM you have the throttle wide open and your mixture rich for best power (around 12.5 :1 air fuel ratio) and not running on the stiochiometric  or lean side. 

Now in the scheme of things, given that I trimmed 2” and picked up 200 rpm, the same might result for you putting you up around 5450 whereas the 2” trim put me up to 6000 rpm.  I think that my engine set up is pretty good and produces a good amount of power for the older 13B.  If I had a Renesis I would be expecting around 6200 rpm with my current prop.  So probably trimming 2” wouldn’t render your prop unsuitable for higher power if that happened later, but I think I would first look again at my induction/exhaust set up.  There might be some more power there.

Ed   

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Ed Anderson
Sent: Friday, March 05, 2010 6:17 AM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Vance Jaqua and Propellers

Well, Bill – that may be the answer – shorten the prop.

But, since it is difficult to add the length back on later {:>), consider carefully whether there may be anything else you could do to ensure you’re prop load is really what is causing the limit.   5250 rpm with a 2.17 and the correct prop would not be at all bad.  When I first started the best I could do with the 2.17 was around 5000 – later increased it up to the 5300-5400 rpm range.  However, 5250 with a 2.85 is on the low side.  But, make certain it is not some limitation in say the induction or exhaust system.

More than one individual have cut down their prop only later to find with changes in other areas they had increased their power and could use the extra length.

Again, remind me of the size of your prop??

Ed

From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On Behalf Of Ed Anderson
Sent: Thursday, March 04, 2010 8:53 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: Vance Jaqua and Propellers

Ok, Bill, taking your  statement “ 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’m not certain I understand your question  but I think the one word answer is “Equilibrium” . Basically increased HP implies increased RPM (all things like air/fuel ratio, Volumetric efficiency, etc remaining constant).  More RPM at WOT generally means more air + more fuel = More Power = more RPM until you reach equilibrium between load on the engine and power it is producing.  The engine is a volumetric pump meaning it always displaces the same volume per revolution.  So since that volumetric displacement is a constant for any give engine, the only easy way you change power is to change the density of the air in the combustion chamber and therefore the fuel it can burn. That is exactly what your are doing as you open the throttle plate.  Again the chicken and the egg thing.

In other words, I know of only three  ways to get more power at full throttle there are 1) increase the air density in the intake manifold (turbo, super), 2)add an oxidizer (Nitrous Oxide) or 3). increase the engine RPM (by reducing load).  Its easy to add more fuel – what’s tough is getting more air to burn the fuel.

Basically once you hit your “Static rpm” you have reached the point were the load of the prop and the power it is consuming is equal to what your engine is producing.  IF (I repeat IF) you could increase your engine rpm at this point – you would produce more power, but  the only way you could do it is either to reduce the load on the engine (variable pitch prop) or increase the density of the air (kick in some boost) or other forced induction technique .  Otherwise you have reached equilibrium.

Perhaps I did not understand what you are asking?

Ed

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

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.

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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