Thanks Ed.

BW

 

 

Clipping the prop in my case resulted from several considerations.

 

1^st thanks to Tracy Crooks warning – my first WOT take off (fairly short 2000 ft runway with trees at one end) with the 76x88  was only terrifying and not disastrous – thanks again for the warning Tracy.  I found that full rudder in my RV-6A was insufficient to hold it on the (somewhat narrow 35’ wide runway),  after a hairy lift off, I found myself drifting to the left despite full (and I do mean full) right rudder – that left side of the runway is the side the hangars are on(naturally).  Fortunately, I had already gained sufficient altitude to lower the right wing and adding the slip to my rudder’s yaw capability – I managed to get it back over the runway, avoid the hangars  and  make it safely into the air. Asking myself the entire time … why didn’t I take up something like Golf, fishing, parasailing, sky jumping, NASCA, chasing women, starting a fight in a biker’s bar,…you know -  something nice and safe{:>)

 

In fact, even with 2” taken off the prop diameter, I still do not have sufficient rudder authority until around 40 mph – otherwise I have to tap the right brake to keep it aligned.  As with most things (except perhaps women)  - once you become familiar with what’s happening and develop a technique to cope with it – it becomes manageable. So I generally ease in the throttle until I reach 40 mph or so and then feed the rest in during take off.

 

So I decided I had more “torque, P factor, wind whirl, et” than I needed or at least wanted {:>)

 

2^nd The 76 inch prop diameter did not leave much room between the tip of the prop and the ground – in fact less than 5” – a Tail dragger wouldn’t have that problem.  Also the dips in some grass strips are just waiting to catch that prop with the nose gear dropping into a small depression.  Not – good.  An inch more clearance doesn’t sound like much but it can make the crucial difference.  Not a strong reason perhaps, but one I considered important.

 

3^rd  As I indicated previously, I wanted 200 more rpm for static/take off moving me from 5800 – 6000 rpm –6000 is such a nice round number {:>).  I calculated that gave me around 170 HP on take off.  On a nice cold morning I can get over 6200 rpm on take off and fuel flow around 19 GPH.

 

Seems like there was one more – but, can’t recall what it was.  But all three of the above played a role in my decision.

 

Those with aircraft like the RV-7A or RV-8, tail draggers or others which stand a bit taller than the RV-6A and have a bit more vertical fin and larger effective rudder area would probably not elect to trim the prop from 76 to 74, but for me it was the thing to do.

 

Ed

 

 

Veeeerrrrrry interesting indeed.  I think the key difference is, not so much the greater HP, but the greater torque (albeit via HP) to turn that larger prop.

One thing I want to avoid is that accident you referred to with too fine pitch.  For now I have an electric MT with auto pitch control.  That should help me avoid problems like this.

I have a lot of jet time, but little prop time.  So when it comes to AOA, I think in terms of the wing producing max lift.  The AOA changes with elevator input.  A prop is not unlike a wing, but I still don’t have a grasp of what is causing AOA change in a fixed pitch prop.  But, usually I have to read things three or four times in order to “get it”.

 

What gave rise to seeing a need for clipping the larger prop?  Was the torque not there?  I’m sure you answered this a few days ago, I’ll look over the old posts.

 

And yeah, it’s possible to overthink this thing.

 

Fun nonetheless.

 

BW

 

 

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

 

 

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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