Which for max speed?

[Spikey]

All other things being equal, which of these will give me best speed flat out?

8 x 4

8 x 4.7SF

8 x 4 3-blade

And what's the reasoning behind the answer?

[Denis Watkins]

The 3 blade would travel furthest per revolution, followed by the 4.7

The pitch of the screw per revolution increases from your 1st to last prop, which will travel furthest each rev

[Dave Hopkin]

But the three blade would be the equivalent of 9x4 or even 10x4 - so may well slow down the revs on an IC or increase current draw on an electric set up

[Speedster]

A speed prop would be 8x8

Cheers

Soren

[Denis Watkins]

Exactly Dave, some say it would act like a 9 x 4, putting the diameter up by an inch, but I just assumed practically it was more prop

[Denis Watkins]

8 x 8 is for vandals, enjoy it

[Gary Manuel]

Surely the 8 x 4.7 will travel 4.7cm per revolution, which is more than the other two, which will only travel 4cm per revolution.

In practice, as Dave says, the 3 blade will be the slowest because the revs will drop more than the other 2.

I think 8 x 4.7SF will be the fastest, then 8 x 4 2 blade, then 8 x 4 3 blade.

[Peter Miller]

Posted by Speedster on 10/07/2016 20:17:33:

A speed prop would be 8x8

Cheers

Soren

Try hand starting a Super Tigre G21 29 on a 7X10 wooden prop.

Yes, it can be done and never bit me

[Stuphedd]

the 8x8 Stant Team race "B" wooden prop on an ETA 29 !!

wonderfull combination , full of memories ,

Vandal !!!

[Peter Beeney]

Surely the 4.5SF will result in the fastest speed, assuming that all three props are turning at the same revolutions per minute? Taken that the rpm’s are limited by the the max allowable revs of the Slow Fly?

However, if the other two props were allowed to rev faster but by exactly the same amount then the model would speed up proportionally, but by also exactly the same amount; the 3 blader would move a bit more air but would require a bit more power to do so.

My reasoning behind this is simply that converting the revs per minute to miles per hour is just arithmetic, for example if either of the 4 inch pitch props were turning at 8,000 rpm then the model speed would be 30.3 mph; the 4.5 would yield 34 mph. Or, conversely, the 4.5 would only need to rev at 7,110 rpm for a forward speed of 30.3 mph. This is in the sprit of Spikey’s OP of course, and ignoring any unknown variables such as slip, etc.

We’ve clocked an 8 x 8 on a OS 35 at 13k plus in a Whizza. This tends to make the electric versions rather pedestrian by comparison; nevertheless, there are some very quick electrics too. A small model powered by an inrunner turning a 4.5 x 4.5 prop is revving in excess of 25 thou to travel at 110 mph. It’s also soaking up relatively copious amounts of power so to do… and it’s fairly conspicuous by it’s audio signature, too.

As always, just my take… Nothing set in stone.

PB

[Spikey]

Very interesting, gents. Thank you. At least I know now that it isn't the straightforward question with an obvious answer that I thought maybe it was ...

[Bob Cotsford]

You might even find that the extra blade area of the SF prop loads the motor down and it ends up actually flying more slowly. If the 3 blade had a particularly efficient blade design compared to the other two it might actually prove fastest (unlikely!). There are a lot of factors at play here.

[Slopetrashuk]

If you want to fly fast you need both rpm and pitch.

Pitch comes into play when you decide how long you want to accelerate for and whether you are being hand launched of not.

A typical F3D or Q40 pylon racer (i.c.) would run 7x7 to 7x8 props on a 40 sized motor (they would be carbon). They need to reach top speed quickly and not really slow in the turns. A sporty pylon racer at lower (17K to 18K) rpm would turn 8 or 9 inch diameter props at 8 or 9 inch pitch.

I used to run my speed delta's on 9x9 and later 8x10's. Any more made a hand launch difficult but I could afford to take my time to build up the speed.

The electric speed model I flew at Greenacres on Sunday (220mph) runs a 7x10.

Worlds speed record models would run multiple (5 or 6_ blade props with pitches of 20+ inches on 15+ cells.

Andy Ellison

Still.....British I.C. Speed Record Holder.

Edited By Slopetrashuk on 11/07/2016 11:17:50

Edited By Slopetrashuk on 11/07/2016 11:19:01

[Simon Chaddock]

As others have pointed out obtaining the maximum speed from a given motor/prop combination is complex but remember any prop design is a compromise. If you set one up for just maximum possible plane speed it will not be as good in other respects.

In a extreme case you might find that it struggled to actually get to its maximum speed although ti could maintain it if it ever got there.

[Peter Miller]

The other thing to remember is that a high pitch propeller is like driving your car fixed in top gear. Take offs will be long and acceleration will be very slow.

The first Spitfire with fixed pitch propellers had a lot of trouble, the later one with low pitch for take off worked much better.