Critical Understanding #3, Rate of Climb, the critical prop evaluation.

Builders:

Go to your EAA meeting, and listen to the first question asked when a guy mentions putting a new prop on his plane: 95% of the time, the first question will be the nearly pointless: “How fast does it go?”

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I say ‘nearly pointless’ because that would be the important question to ask if the plane was a Reno Unlimited racer. Notice that almost no one asks “What is the rate of climb?” which is the critical question to ask.

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Think back on your last 100 flights in a light plane; perhaps we are speaking of 75-150 hours aloft. That is 4,500 to 9,000 minutes of flying.  Realistically, answer this question: How many of those minutes did you spend flying at the absolute top speed of the plane? Now stop and consider that on every single one of those 100 flights, each and every take off and climb out was performed at the maximum rate of climb.

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Thus, the critical understanding of props should be absolutely focused on Rate of Climb. There are other factors, but truth be told, there is surprising little difference between good designs on the factors of speed, smoothness and efficiency. However, there is far greater differences in take off and climb on different prop designs and pitches.  There are very good reasons to focus on differences in climb, and bias your selection for the prop that delivers the highest rate of climb. When I make a prop recommendation, it is focused on having a very good climb rate. If you gave me a choice between a 5% top speed increase or a 10% rate of climb increase, I pick the latter, and would certainly do so if the plane was open cockpit or STOL.

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There will always be someone who claims to want a ‘cruise prop’ because he likes the idea of speed. Consider this: most pilots who choose one of these props select one that will not reach red line rpm in level flight, thus limiting their top speed. If they picked one with lower pitch, the plane would actually speed up, and incidentally it would also have a better climb rate.

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The next time you are looking at a set of logs for an experimental aircraft with more than 40 hours on it, look to see if the builder filled in the FAA required Vx and Vy ( speeds for max rate and max angle of climb) in the logs which is required for the plane to be done with phase one flight testing.  I look at logs of planes all the time, and roughly 50% of the logs don’t have this filled in. If we asked that builder to tell me what these speeds were for his plane, do you think he would be able to give us a specific set of values? If the same guy was on a very short strip do you think that he would fly the correct speed to not hit a 50′ tree at the end of the runway? This lack of taking Vx and Vy seriously goes along with most people focusing on speed rather than climb.

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Combine a guy with too low a static RPM ( Critical Understanding #2, Absolute Minimum Static RPM. ) and a guy who doesn’t know the Vy for his plane, and you have the makings of an accident. This isn’t speculative fearmongering,  just that accident happened in a Corvair powered plane in 2015. I would name the person, and point out his previous accidents in the same plane, but read this Comments on aircraft accidents and understand that I am not always at liberty to say such details, but I want people to understand that when I say certain mistake combinations will not end well, I am speaking about history, not theory. -ww.

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Make the book or comparative ROC  for your plane/engine/prop line 3.1 in your Hand book

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Make the calculated ROC  for your plane/engine/prop for typical conditions at your test airport  line 3.2 in your Hand book

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Make the measured ROC  for your plane/engine/prop  line 3.3 in your Hand book

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