Good point, John

 

Nothing like personal experience.  I failed to take into consideration that the “minimum” pitch setting of the IVO prop may/could be optimum.  However, I would not bet my life on it.  I don’t know the IVO prop, but it could be that you can “set” the minimum pitch limit such that you will always generate sufficient thrust – but you want to convince yourself that your prop “Minimum” setting will provide sufficient thrust for good take off performance.  Also the 70 kt marker that John mentions is another method to check your thrust/acceleration.  There no doubt many ways to make this check, just do one or more of them before flight.

 

Also David makes a good point, if you have the “constant speed” module then the prop controller will automatically compensate for prop load.  But, again, I think you want to check that Minimum pitch condition.

 

Ed

 

 

Hi Ed, Chris,
That all makes sense, but I can add something from experience with the IVO prop - The minimum pitch setting seems to optimum for take-off power. When I changed from a big fixed pitch prop to an IVO at minimum pitch the difference in acceleration was very marked. I have never taken off with the pitch at a higher setting than minimum, so I can't attest to the effect, but if you forget to put the pitch on full fine on the downwind, a go-around is much less doable.

Something else about the IVO that might help, Chris, is that when you throttle back on final the airplane will want to slow down quite quickly. This is somewhat unexpected if you're used to a fixed pitch prop that tends to make the airplane float on final. The combination of fine pitch at idle, both rudders and the landing brake make a Cozy (or Velocity I would think) come down like an elevator.

One way to check that you have enough power would be to measure how much runway you use to reach, say, 70kts. If it's more than 2000' I'd say you're under powered. With a turbo and an IVO the acceleration should feel like a good sports car, and the increase from 70 kts to flying speed will only take a few seconds.
My 2c.
John

to Ed Anderson wrote:

No easy answer – to all the nuances of producing power, Chris.

 

200 HP is a possibility depending on lots of factors (like your total induction system set up, however, I believe that 180 HP is more in line with the likely maximum on the older 13B.  The Renesis a bit more/

 

Fuel flow is as much an indication of power as anything short of a dyno and/or extensive aircraft performance numbers.

 

 RPM (in my opinion)  has too many variables – like I have a 74x88 prop with a 2.85:1 gearbox  and can turn 6000-6200 rpm static – does that mean I am making more or less HP than you at 6200 rpm at 35” Hg.  It all depends on what prop load each of our engines are seeing at that rpm.  Generally as a rough rule of thumb you can figure 10 hp per gallon/hour of fuel flow.  So it you have a fuel flow of 16 gallon/hour then your engine could be producing around 160 HP.  18 GPH = 180 HP, etc.  Now your power won’t be more than that, but it could be less.  The rotary can flow considerably more fuel without making useful power than a piston engine, but it is a useful rule of thumb.

 

I street ported my 91 Turbo block myself using a Mazdatrix street port template.  I went through 5 intake designs and several muffler experiments before finding one that appear to give me the power I wanted.  On a cold morning with OAT < 50F, I can get up to 6200 rpm and 18-20 GPH fuel flow with the 74x88 prop.  So I feel I have my set up just about as good as its going to get – short of a forced induction system.

 

With your prop set at max  fine pitch, you have reduced the prop load on your engine to a minimum – that permits your rpm to be at a maximum – but, that does NOT mean you are producing the power and more importantly - the thrust needed for safe flight.

 

 Don’t be mislead by rpm.  As an extreme example to make my point, Without a prop (almost no load) my engine will turn 5500 rpm at idle throttle setting – but I am only flowing something like 1.5 – 2 GPH which means producing something like 15-20 HP even though the rpm without prop is higher than my static was when I had my old 68x72 with a 2.17:1 gear box.   So even though the rpm is the same or slightly greater, the engine not making any where near the  150-160 HP I made with the prop on at the same rpm.  Fuel flow is not a perfect indicator but much more useful than RPM alone in estimating your power particularly with a variable pitch prop.

 

This is important, Chris.  Years ago, there was a long EZ builder who had a non-rotary auto engine who unfortunately ended his first flight in a fatal crash into a cactus plant.  He understood the electronics just fine, but did not understand the relationship between rpm and thrust.  He set his adjustable pitch prop for maximum RPM – which mean minimum prop load, which in his case mean minimum thrust.  He managed to get airborne with this limited thrust, but could not apparently climb out of ground effect with the flight ending on impact with a cactus.

 

 For example, you would get your highest rpm with a prop that absolutely no pitch which would produce minimum load on the engine – however, I think you would agree there would not be much “push/Pull” by such a prop set up.  With a variable pitch prop, you need to find the optimum balance between rpm and thrust.  Too much rpm could indicate too little pitch (and too little thrust), too low rpm could indicate too much pitch (and too little thrust) – neither condition gives you maximum thrust. 

 

The only way I can think of to find that optimum balance between rpm and thrust is to attach your airframe through a scale to an anchor.  Then measure the pull on the scale at various rpm and prop pitch settings.  This should help you find where combination of engine rpm and prop pitch provides maximum thrust (at least maximum static thrust – which is a good start).  Perhaps someone else can offer a better and easier method.  Yes, you should be able to get a feel during taxi test – although some folks frown on the idea of high-speed taxi test.  Theoretically you could use an accelerometer, some accurate speed measurements the weight of  your aircraft and calculate the effective HP – that has been done with automobiles.

 

In cases were folks are using fixed pitch props  with parameters similar to those used by others, you can make a comparison and get a rough feel for engine performance based on RPM.  Unfortunately, with a variable pitch prop making such comparisons is more difficult and questionable.  Now if you can find someone using the same variable pitch prop you are using and compare your rpm and prop settings, that is certainly something worth checking into.

 

Just continue to ask these kinds of questions and to think about the issues in producing power and thrust – you’ll be ready.

 

Ed

 

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