Cold Fusion, Mountain Waves and partially open throttles.

[Ed Cesnalis]

Roger,

 

You want to support your position with your experience and I want to support mine with a discussion about throttles and we are at an impasse.

 

I usually end up going away at some point but before I do let me point out that you crossed a line that no longer permits you to rest on your experience alone.  To explain lets pretend you were asserting that you have achieved cold fusion in your hangar. Obviously in that case you would have to provide proof or at least begin with an explanation on how it works you couldn't simply insist that your are right due to your credentials.

 

If Burt Rutan stood in front of me and told me I could make my CT go faster by retarding the throttle I would listen to him but he would have to explain how it works not tell me about his experience and require me to take it on faith.  We are at that point when you contend that a throttled air/fuel mixture produces more power than a wide open throttle.

 

You have criticized the chart I posted and said I need to develop my own?  As I said already its published by CPS and its a dyno run on a 912ULS.  They are done at standard conditions with a WOT and a variable load demonstrating power graphed throughout the 912's operating range. If memory serves this graph shows 3hp more at 5,500RPM than earlier ones I've seen.  I don't agree that I need to do my own run to have credibility.  The graph demonstrates that throughout the 912ULS operating range power always increases with RPM. WOT is used on the dyno run because it produces the highest manifold pressure for current conditions throughout the operating range. We can see at no point does power diminish with additional RPM/manifold pressure leading to the conclusion that a partially closed throttle and the resulting manifold pressure reduction will only cause a reduction in RPM and a reduction in power.  You at least confirm there is a reduction from 5650 to 5500 but you are not in agreement with the dyno results that indicate such a reduction in RPM is a lower power setting.

 

I want to challenge you on 2 more points.  first your claim that you have done extensive performance testing on CTs up to 18,000' and this testing has led to your knowledge of where best performance lies.  I have a big problem with the claim that you have done this testing to 18,000'.  That is well above a CTs absolute ceiling and the only thing a CT can do at 18,000' under its own power is descend.  How can performance testing be done above the absolute ceiling?  The fact that someone once got to the flight levels in a CT had to do with a mountain wave and not performance testing.

 

2ND POINT: Over the years you always refuse to discuss manifold pressure and your statement yesterday suggests you must be confused on the subject.

 

Your physics are too general and don't speak specifically to YOUR specific situation. And we still don't use MAP to fly by or you wouldn't keep talking about 5500 rpm. It would only be MAP numbers. Map is okay when it's pertinent, 

 

 

You would be correct if you said "And we still don't use MAP to fly by or you wouldn't keep talking about 5500 rpm. It would only be MAP numbers. Map is okay when it's pertinent, but if we did you would need to consider both manifold pressure and RPM because it takes both to determine the power setting. You might also point out that we always use a WOT number when optimizing because we don't display manifold pressure but at WOT we at least know that we have maximum manifold pressure available for current conditions.

 

Why do RPM increase when you advance the throttle?  When the throttle plate opens manifold pressure increases and more fuel/air mixture (mass) flows into the combustion chambers producing bigger explosions and resulting in more power and more RPM.  

 

You wouldn't contend that best performance can be found with a partially clogged air cleaner and yet the partially clogged air cleaner and a partially closed throttle give exactly the same result.

 

You wouldn't contend that best performance at 12K is less than best performance at 14K yet the additional 2K is equivalent to a partially closed throttle.

 

This is worth repeating and would be a good point for you to explain if I have it wrong.  With a WOT only the air cleaner, ducting and edge of throttle plate restrict manifold pressure, approximately 1 inch.  This 1 inch reduction is generally offset by a 1 inch gain due to ram effect so a WOT produces manifold pressure equivalent to ambient pressure.  To do any better than WOT you would need to boost with a turbo.  To reduce power from WOT just partially close the throttle and present the throttle plate to the air/fuel mixture flow reducing manifold pressure resulting in less fuel entering the combustion chambers and less RPM and power being produced.

 

There is no best performance with a partially closed throttle.

 

You speak so much of testing yet I haven't seen any test results at all.  You say I need to do testing.  I have thousands of flights at altitude flying with WOT.  I guess I have more experience than any other CT pilot. I used to use 2 different pitches, 1 for summer and 1 for winter.  Each flight I am striving for maximum performance.  My years of testing show that max continuous speed at altitude is found at 5500/WOT.  A finer pitch and retarded throttle result in a slower speed as expected.

 

[FlyingMonkey]

I'm not taking sides on this, and I agree with you Ed on your physics.  Max MAP + Max continuous rpm would certainly mean maximum cruise power from the engine perspective.  

 

What in my mind is not as clear is the engine/prop interaction.  In other words, is it necessarily true that a prop tuned to get the above parameters is necessarily providing optimal thrust?  Being an airfoil, I'd think prop shape and size would play a part here.  For example, at any reasonable engine RPM there is a prop size large enough that the tips would go past mach 1, leading to a large loss of thrust through transonic drag.  Likewise there is a prop so short that it produces inadequate thrust to keep the airplane aloft.  There are probably other aspects of airfoil shape that would significantly affect best cruise rpm/power setting.  

 

Again, not arguing, just stating that I'm unclear on all of the variables at play.

 

As far as the "debate" with Roger, I think you might be talking around each other.  Roger seems to be talking about general best practices for setting up Rotax engine/prop combos on LSA, and CTs in particular, for best "all around" performance.  Ed seems to be talking about the special case of best cruise performance at a relatively high altitude, and optimizing for that case.  Since that is the most important aspect of performance for Ed, it makes sense that he's viewing performance through that lens.

 

I guess my take on it is that you might *both* be right for what you are trying to do.

[Ed Cesnalis]

As others have pointed out both can't be right on the point of best speed at 5,500 requires WOT.  The prop doesn't have to be producing optimal thrust. Roger admits that additional throttle produces another 150 RPM so the prop is behaving as expected.

[ctfarmer]

Easy to work out angle of attack. Same as pulling your stick back. You get more lift, to a point at which it stalls. And if you go faster, aka prop speed, more again.

Andy, like any airfoil, there would be a point where your prop would stall and lift lost.

 

Hard to believe we are there but it is worth a quick calculation. As well the faster your ac travels, the lower the angle of attack for the same pitch and rpm. And if we were close, we would feel/see the prop stall on takeoff everytime.

 

So we are back to simple physics. For continuous operation,wot, 5500 rpm with highest prop aoa gives most thrust. Hence more speed.