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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About William Wynne
I have been continuously building, testing and flying Corvair engines since 1989. Information, parts and components that we developed and tested are now flying on several hundred Corvair powered aircraft. I earned a Bachelor of Science in Professional Aeronautics and an A&P license from Embry-Riddle Aeronautical University, and have a proven 20 year track record of effectively teaching homebuilders how to create and fly their own Corvair powered planes. Much of this is chronicled at www.FlyCorvair.com and in more than 50 magazine articles.

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

  1. Reminds me of a commercial I seen years ago with a little compact fuel sipping car waiting to enter a very busy road with some large fast traffic zooming by as the voice over announcer asks, “what good is 50 mpg on the highway if you can’t get on the highway?”

    This follows along well with a story you wrote where you noted:

    “Dan Weseman has a personal rule I find interesting. He will not fly a plane that can not out
    climb it’s glide slope. If he takes off and climbs out at full rate, he has to know that the plane
    can turn around and glide back to the airport.”

    That story is here:

    https://flycorvair.net/2012/11/17/steel-tube-fuselages-safe-planes-and-250mph-accidents/

    I have to admit that I adopted that line of thinking for myself. If it can’t get up, it can’t get gone. I like to fly into some shorter grass fields now and again and I need to know that I have the ability to get back out without picking my feet up when I get to the treeline at the end of the runway.

    BTW … Thanks for this series of articles. Gives us all things we need to consider. Even those of us that believe we are diligent in trying to avoid all the pitfalls can sometimes miss a trick or become less attentive than we should be. A kick in the shorts is needed now and again.

  2. Howard Horner says:

    I have a question: I live at over 5000 feet and go up from there. There are a few Zenith ch 750 STOL S-LSA that I believe had the 0-200 and a specific prop. One of those ended up at an airport near me. Before it was damaged in a wind storm, I was told it was a poor performing, one person airplane on a COOL day! (I took a demo ride in that same plane at the Open House at Zenith after it was repaired and I am still grinning.) In light of the information offered in this blog, I am assuming that because of the restrictions on the S-LSA that the engine/ prop combo were not optimal for this altitude??

    I am using a 2700 Corvair with a “66” Whirlwind prop. Would you predict It is possible to get adequate climb performance in the environment I have described using that combination if I set my prop to give me the correct static RPM and maximum rate of climb and then accept whatever “cruise” I get? (I suspect that it could be 70-85 mph??)

    Thanks for investing in the aviation community by writing these great articles.

    • Howard Horner says:

      Please forgive me: William Wynne. Whirlwind. Warp Drive. George W. So many “W’s”!!
      I have a Warp Drive prop.

      HH

  3. Sarah Ashmore says:

    I learned to fly in the C-150 like many others and with two on board I was never overly impressed with climb so I can appreciate the need to stay with a prop more optimized for climb then cruise. You can never be sure just what airfields and what conditions you might find yourself in so good is cheap insurance against tall trees on a hot day.

    As a side note my career in developing software for pilot training simulators used in the military I have occasion to sample a variety of aircraft performance ranges. Without a doubt the most humbling of my experiences was performing a max weight takeoff in a KC-135A at Sea Level Std Day conditions. With 10K of runway in front and water injection to enhance available power there was a note in the flight manual on takeoffs that was very much applicable. It cautioned that early rotation would lengthen the takeoff roll but it cautioned to not delay rotation past the 1000′ to go point. I did get off and after the aircraft was cleaned up and accelerated it had a reasonable climb rate but it really showed the the importance of good acceleration and climb rate in any aircraft. The Air Force was wise to re-engine the fleet nearly doubling the available thrust and the same takeoff in the “R” model was no sweat even with the higher allowed gross weight but I must say all those pilots who spent decades with the “A” model must have been damned good pilots.

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