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Thread: Constant Speed Prop for my Cub - Advice Needed

  1. #1

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    Constant Speed Prop for my Cub - Advice Needed

    Good day folks

    Im considering making my experimental Cub stand for some TLC. Im currently running a O-320 150hp with a 74 Sensenich cruise prop. Long term plan is to upgrade to a 180hp with a proper pulling prop

    One of the first things I want to focus on is power delivery. On takeoff I get about 2400RPM which means I get airborne with just over 100hp. I can probably repitch my current prop for higher RPM, but the cruise speed will be sacrificed. So I probably want the best of both worlds? This leaves me with one option- constant speed. But now, hydraulic constant speed props are heavy, expensive and they take the original Cub spinner look away. So, only one option left - Electric VP/Constant Speed. But where?

    Here in South Africa we have a company AXSport that specializes in building electric constant speed props. They also build their own LSA composite aerobatic airplane
    One of the props they build is a 74 3 blade specifically for the Lycoming O-320/360 engines. So for the same price(maybe a bit less) than a Mac Borer, you can have a composite 3 blade constant speed prop. But is it worth it?

    My goal is to pull all the available power out of the engine on takeoff. What would your opinion be?
    Maccauley Borer 82 at 2500RPM compared to the 74 3 blade at 2700RPM? I.o.w. will the composite prop pull harder because of the full rpm/power and surface area(3 blades) or will the Borer pull harder due to the longer diameter?
    Will the constant speed really cruise much faster?
    Will the constant speed prop be more efficient?


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  2. #2
    skywagon8a's Avatar
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    Look at this thread: https://www.supercub.org/forum/showt...nt-speed-props

    Keep in mind that if you are eventually going to change to an 0-360 engine you will also have to change the prop again. For one thing the attachment bolts are different sizes.
    You may also find that one of these Airmaster props gives you all the performance that you need.
    N1PA
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  3. #3
    algonquin's Avatar
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    Yo7 might look at Wirlwind props, they are ground adjustable, and very easy to adj. they are very light and will shift the CG back for you. They are $2900.00.

  4. #4

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    Soooo lots of things to think about. No right or wrong just stuff. First is terms, when you say your get 2400 at takeoff what does that mean? Is it before you start the tires rolling, brakes locked with 3-4 sec spool up for max RPM or is it 1/2 way down the runway or when tires leave the ground or 100ft into climb out, everyone seems to have a different definition. All available power from the engine, well that would be 2700 RPM or higher no matter what prop you use. If you go fixed pitch it is your job to adjust that black knob to keep it in proper RPM range through takeoff roll and climb out. Some things to consider are what is your mission, are you going off improved runway/dirt runway/ river bar/tundra/ grass. Are you just doing 1/2 hour from home play dates with your friends or are you doing 6 hour one way flights? Hauling a big load or empty. What length runway are we talking about? What is short to you 500 ft or 1,000 ft, what altitude are we talking. It is hard to beat a Borer for getting a plane in the air fast ,handling brush/ rocks/ crap that gets thrown up on rough strips. But if you want 2500 static with a 150 0320 and a 82 inch Borer you are talking a 38-39 pitch on the prop and that will put a cub at about 77-80 MPH at 2400 rpm. It will get a empty cub in the air in about 125-150 feet, can you land that short? Do you really need to get into anything shorter than 500 ft? I have two Borer props one for the fun stuff and one for everyday use. Swap them out as needed. The nice thing about a 3 blade prop is you have a lot better ground clearance and they tend to be faster. How about one of the ground adjustable Like I said no right or wrong just things to think about. Make sure when you are getting info from others they have the same definition of the terms that you do.
    DENNY
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  5. #5

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    Thanks for the reply guys.

    Denny, I get about 2350 when the wheels roll, 2450 when I rotate. So take an average of 2400. I weigh about 180lbs, airfield is 460ft AMSL with a slight downslope. Temperatures vary from 5 degrees to 35 degrees Centigrade. With all this I typically use about 180-200ft when solo and around 350ft when loaded. Bear in mind that this is still a stock standard Cub setup with no STOL mods. I’m still planning on installing the Thrustline mod, Extended Gear, ABW’s, VG’s, Gap seals, T3 tail suspension and like I said, a 180hp with a hard pulling prop
    Runways I operate from are typically anything from 400ft to 1000ft but thus far I’ve only operated from them when light and solo on the colder days. I’m currently building a runway next to the river with 390 maximum length available so I will need to get all performance available.
    The Borer prop would be my ideal choice since it’s a proven performer, but since my mission is to get up and go as quick as possible, but also do the 100NM trips with some ease, I thought Constant Speed would be the go to option.
    I have some experience with the Cessna 180/185’s. What I can tell from them, the 3 blades pull harder than the 2 blades, cruise speed slightly less but not much, and at 2850RPM the 86” props run just in their sweet spot. So this is the direction I’m trying to go in - to ise as much of the potential power available and turn that into thrust

  6. #6

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    So the planes in the pics are not what you have? That had me questioning the post. No average crap allowed!!! tie the tail down and see what is the max RPM you can get!! If I was to do a 400 ft strip with a 150 hp often, I would run a flat Borer even with a 160 hp I would do the same and suck up whatever else was the price!!!! I can do 400 with a strong 160 hp and a 82/43 with two people fuel and some gear but I DO NOT LIKE IT!!! You will find a day when conditions outperform pilot skill. If you want to go fast you won't go short!!! If you could we all would be doing it!! If your long flight is only 100nm you just need to get a better playlist and fly low.
    DENNY
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  7. #7

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    Most of the stuff you say you will add will not help your takeoff. Extended gear should help. As should tall bushwheels with the correct pressure. VG.s, high tailwheels, etc may help landing but not takeoff. When I see a three blade prop on a cub win at Valdez I will believe it pulls harder!!!! Don't mix apples and oranges. I currently have a killer cub and understand that you want to expand what your plane can do. BUT!!! Understand if you want a STOL that is what you build, you can get speed in tire size, fairings, ect. Bottom line is build a plane that will fly even when your skill is not up to the job.
    DENNY
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