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Thread: Propeller 101- does more length make up for a flatter pitch speed wise

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    Propeller 101- does more length make up for a flatter pitch speed wise

    If say an 84/43 propeller is giving you 90 mph cruise, then would an 86/40(longer and flatter), at the same rpm, give you less cruise speed because the pitch is flatter or would the extra length make up for it by any chance? But wouldn't you get off the ground shorter however because you have more prop pulling you and probably about the same rpm as the pitch is flatter?

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    My understanding is the second number tells you how many inches the prop should move forward in one complete revolution. There is some slippage occurring so say 10% less. The longer prop should pull harder but not faster. Another thing to consider is not all props are measured the same, so different props will have different performance depending on which point on the prop the second number is measured. IE Catto prop numbers do not match up to Borer's. What kind of prop are you talking about? DENNY

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    Longer equals larger disc which equals more drag. Clear as mud

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    skywagon8a's Avatar
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    An old rule of thumb for equal performance with same design prop.
    Increase the diameter 1" = reduce the pitch 1". Reduce the diameter 1" = increase the pitch 1".

    In your example you have increased the diameter 2" and reduced the pitch 3". Therefor at the same cruising rpm you should cruise slower. The engine should be able to turn a higher rpm at take off, therefor more horsepower with shorter take off and higher climb rate.

    The longer diameter will move a larger diameter chunk of air, therefor more thrust. Also will provide more drag when the throttle is closed because of the larger prop disc acting as a brake.

    I did some tests on a 150 hp 7GCB seaplane using various diameters 74", 80" & 82". The original prop was a 7456 which produced a cruise speed of 103 mph. The final prop was an 8046, producing 103 mph with a reduction of take off time of 1/3 and an increased climb rate also of about 1/3.

    That did not match the old rule of thumb. +6" diameter & -9" pitch. The other props were 8241 & 8243. The 41 produced a slower cruise, the 43 produced 103 mph. All three produced about the same increased take off and climb improvement.

    The 82" diameter props "felt" as though some of the prop was just thrashing around doing nothing. SO, the diameter was reduced 2" and the pitch increased by 2"+1" for good measure. Very satisfactory.

    At Cub cruise speeds the longer diameter will on it's own will not produce much difference in cruise speed. Higher cruising airplanes, yes.

    The 7GCB with the 8046 would cruise the same speed on floats or wheels. Thus the prop was controlling the speed, not the drag of wheels or floats.
    N1PA
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  5. #5
    Mauleguy's Avatar
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    I know of a person who was on straight floats all the time. One year he decided he wanted to go to Alaska on wheels. He wanted 35" Alaskan Bushwheels. He said it cost him a few mph in cruise. More drag then straight floats

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    In my TCraft on Baumanns with C90...I’m only 4 mph slower than with 6.00x6 wheels. Pretty nice cruise too...96mph with a 76AK-2-40 prop. It just stays on floats since I fly my J3 and soon my new 11 on wheels. Excited to see how my 11 does with O200 and Catto 76-36
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    BC12D-4-85's Avatar
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    My Taylorcraft performs exactly as Dan's does on floats - C-85 Stroker and same prop dragging EDO 1320's at 2350-2400. My experience with props also dupes N1PA's. I ended up with an 8042 with the Citabria 2" spacer on my PA-18 for skis and floats.

    Gary
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    mvivion's Avatar
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    The answer to the original question is easy: Propellers are magic. Simple.

    The problem with this discussion to this point is that length and pitch are only two of several propeller characteristics. Those are just the ones that are easy to measure. Unfortunately, most of the other characteristics, like shape of blade, twist, etc, etc, etc can be really difficult to measure.

    Easy example: Back in the day all Cessna 185 and 206 aircraft came equipped with a three bladed propeller. Since then, propeller manufacturers have developed much more EFFICIENT propeller blade shapes, and thus with similar length but very different shaped blades, we now have much better performing propellers.

    Length and pitch (understanding that pitch measurements can be very subjective) are meaningful, but the shape and chord of blades also make a huge difference in performance.

    MTV

  9. #9
    JimParker256's Avatar
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    Yes, shape and chord matters when it comes to propellers.

    I think I've told the story before, but I had a Grumman Traveler with a brand new McCauley cruise prop. During my annual inspection, the shop (a noted Grumman 'guru') had another Traveler parked next to mine. The owner had brought it in to "get the engine fixed" because he was not obtaining anywhere NEAR book performance. The IA kept telling him it was his worn-out prop that was the issue, and that the engine was fine. But the owner insisted that a propeller shop had overhauled the prop and told him it was "just fine" as it was...

    Looking at the two planes parked nose to nose, his propeller looked like a toothpick compared to mine. It had been filed down so much along the entire length of the blades that it was at least 3/4" narrower than mine, and was substantially "thinner" as well. It had apparently been filed repeatedly to remove dents and dings over the years. Aerodynamically, it was a complete mess, having so much chord and camber from the profile. I honestly have no idea how a competent prop shop would have ever signed off on that prop as being airworthy. Even seeing the side-by-side pictures, the owner simply refused to believe that his poor abused propeller was the reason his airplane was waaaay slower than book...

    Since we had to remove my prop anyway to fix a leaking front seal, the IA asked my permission to temporarily install my prop on the other guy's plane, and take it up for a test flight with the owner. As soon as they landed, the owner forked over the money to buy a new prop for his plane.

    PS - The labor cost for replacing the seal on my engine got "comped" by the shop, since they sold the new prop to the guy. Good deal all around.
    Jim Parker
    2007 Rans S-6ES
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    A friend has a cub he built with a Catto 84/43. If he would go to a Catto 86/40 or 41, would he take off shorter, climb better and cruise maybe a couple Mph slower? Or would he suffer in takeoff or climb, besides cruise, too?

  11. #11

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    Designs/shapes of propellers have changed perhaps more so in recent years. However, there are no props anything like the one on Mike Patey’s Scrappy cub. It’s an interesting concept- looks like very quick exceleration (like an airboat), and the ability to make a steep approach with that huge air brake out front. But what’s missing here? Reduced cruise? Weight? The builders background lends to significant credibility in design/build. Are we going to be seeing more airboat props?

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    skywagon8a's Avatar
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    Quote Originally Posted by RoddyM View Post
    Designs/shapes of propellers have changed perhaps more so in recent years. However, there are no props anything like the one on Mike Patey’s Scrappy cub. It’s an interesting concept- looks like very quick exceleration (like an airboat), and the ability to make a steep approach with that huge air brake out front. But what’s missing here? Reduced cruise? Weight? The builders background lends to significant credibility in design/build. Are we going to be seeing more airboat props?
    One thing to be remembered about that particular prop. Those blades must be able to absorb all that horsepower. It takes a lot of blade area to accomplish that mission.
    N1PA
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    To me the question revolves mostly around prop length. I think most folks understand the pitch situation at least on a basic level.

    Has anyone had experience with two props of differing radius but same pitch - on the same airplane? What is the effect of disc size itself without other consideration?

    I also share RoddyM's curiosity about the basic concept behind unusually wide blades as seen on Mr. Patey's project. Again on a conceptual level of course.

  14. #14
    Crash, Jr.'s Avatar
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    Quote Originally Posted by RedOwlAirfield View Post
    I also share RoddyM's curiosity about the basic concept behind unusually wide blades as seen on Mr. Patey's project. Again on a conceptual level of course.
    I have serious doubts that the Whirlwind airboat prop that he's running is going to be a very efficient design or do well at all. Those props are short and paddle bladed because they need to produce maximum thrust at high RPM's but low air/ground speeds. Airboats rarely go faster than 50mph whereas an aircraft fixed pitch prop has to work from a standstill up to 100mph or more on a single pitch angle. Totally different applications, totally different blade designs. Probably will accelerate from a stop pretty good though.
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    Comparing my McCauley C401 prop blades to my Whirl Wind 200A? The difference is staggering. My Cub’s prop blades are probably 50% wider. Thicker. Significantly different twist. Scimitar shaped with lots of taper. The one thing I know about props is that I don’t know squat about props. But I know who to ask. I’m guessing Mike has good counsel.
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  16. #16
    skywagon8a's Avatar
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    It is my understanding the Whirlwind props are using the design studies of the late Paul Lipps. He had some good ideas, it's too bad he went west at too young an age.
    This is informative. https://www.homebuiltairplanes.com/f...opellers.4911/
    He authored a superb article on the subject for Sport Aviation years ago. I can't seem to locate it now. Somewhere there is an index for past Sport Aviation articles? I know I have that issue in my collection from the past 62 years worth of the magazine.

    Perhaps Joe Norris can provide a link to the article?
    N1PA
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    My biggest challenge for my new project is trying to decide what prop to put behind my O-290D2, I know borer props are for the 320's but never dealt with the D2 before.

  18. #18

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    I had a McCauley 78-41 on an O-290D2. I think it was a cut down Borer, but I sold the plane and don't have the model number. It performed really well on wheels, skis, and floats. I kept it when I upgraded to O-320B2B and repitched to 44.

  19. #19
    BC12D-4-85's Avatar
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    Hi Tim....what airframe? See TCDS P-857 for O-290-D2 and Mac 1A170GM or 1A175GM combination. Also this document for compatible long props. Sensenich offerings

    Gary

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