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Propeller sizes

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  • Propeller sizes

    Was always told bigger is better by the likes of Jeremy Harris; this was a puzzle, as the best matched prop I've ever encountered was a 1450mm (57") diameter three blade Arplast on a 39hp 447 Chaser.

    Tested varying prop diameters to suit the little Briggs, actually slightly less powerful than the Rotax 447, arrived at 1,525mm (60") as nearest match to perfection; after checking the tip pitch in degrees, and diameter compared with actual distance traveled per turn of the propeller, it was found that hands off cruise rpm had prop tips at slightly negative angle of attack to incoming wind.

    Trimmed another inch off and the climb rate fell, but allowed adding a touch more pitch and ensured it could be flown bar in to 70mph, at this combination full throttle gave 90 kilos static thrust, enough to push a 250kilo SSDR off quite nicely.

    After changing to the fuel injected 61 hp water cooled V-Twin, that prop was way too small; had to pitch tips at 10 to keep rpm below 6000 on climb out, so bought a 1,710 three blade carbon Lugaprop. This was way too big, had to pitch too fine to get full rpm and knew that attempting to take off would see the trike run out of pitch at only just enough speed to allow lift-off. This is awful when it happens as you need bar full out to stop the wing accelerating to the point of no climb rate.

    Trimmed to 1,650 and set to 7.5 tips and got 5,000 rpm, 6.5 gave 5,500 and around one degree negative tips at 60mph cruise

    Trimmed again to 1,625 and returned to 7.5, this gave just positive AOA and 5,500 static, should mean just under 6,000 in flight, or 2,727 propeller rpm
    Lessons learned:-

    Power requirement is related to cube of diameter increase/decrease; big increase in diameter means engine sits like a car climbing a hill in top gear

    Trim half an inch at a time to arrive at 1" change in diameter, got a 500 rpm change; as this is a 2.2:1 reduction that's 227 prop rpm change on a direct drive motor

    Change in engine speed is not linear as the power curve peaks at 6250 rpm, at 5,000rpm the motor makes 50hp, at 6000 it makes 58hp so oversizing has many downsides too

    It's well worth taking circumference of propeller, draw as straight vertical line; work out distance traveled in one turn of prop at cruise and check the angle formed, it is too easy to be so fine that the tips are actually negative and just soaking up power. I did various climb speed and both full throttle and cruise rpm checks, the undersized prop had too big an AOA at 50mph and high rpm, but was almost perfect at 2 AOA at 60mph cruise and 5000 rpm. Static pitch was 10 degrees, prop slip or efficiency was an amazing 81% which is up with the best.

    OK yeah, I'm an anorak, but fun to test all this stuff out on a shoestring budget


    PS hollow carbon props can be trimmed but need the tip refilling, I used Marine epoxy, which works like car body filler but sets very hard, also had to rebalance after every trim-takes patience
    Last edited by Kevin Armstrong; 16th February 2020, 17:44. Reason: more information
    BMAA 5696

    "If anybody ever tells you anything about an aeroplane which is so bloody complicated you can't understand it, take it from me: it's all balls."

    R.J. Mitchell :- Designer of the Supermarine Spitfire

  • #2
    Hi Kevin,
    Interesting reading and always good to see some proper 'one change at a time' experimentation.
    I assume you've stuck to the same 2.2:1 drive ratio for all the prop changes?
    Did you look at the AoA of the blades at any points other than the tip?
    I assume that there must be some washout at the tips, as the prop is still producing thrust at cruise even with negative tip AoA. Interesting to know whether the washout is linear along the length of the blade, or just in the last few inches.
    Pete T.

    "A closed mouth gathers no feet".