Another one for Uncle Keith – Pitch & Torque?

RE: Another one for Uncle Keith – Pitch & Torque?

Hi Keith,

I understand what you’re saying. I think I was treating the pitch, torque & number of blades as one issue. When in fact it’s several. Any more information gratefully received.

Cheers,
Neilly

RE: Another one for Uncle Keith – Pitch & Torque?

Hi Neilly. What you’re doing with a prop is converting your engine power into thrust. Your engines power output (and torque) is dependent on it’s efficiency and how far you open it up. For a given power a coarser pitch will lower the rev’s. When your talking the number of blades, your talking about “Solidity” this is the ratio of blade to air in the disc area of the prop. The number of blades you need is dependent on how much power you have to convert. A 3 bladed prop on a course pitch can convert as much power as a 4 bladed prop on a finer pitch. Really big engines require lots of blades, or even counter rotating co axial props. But all these bit’s of info can get a bit confusing if you don’t really know the basics. The pitch you choose is more about how you want to fly your aircraft. Having a fixed pitch prop is a bit like having a car with one gear! A course pitch is like driving in top gear all the time, great on a motorway, but a bugger if your in traffic. Fine pitch is like 1st or 2nd, great for a good take off, but no good for the high speed dash!

I’ll put a few drawings together and do a mini series on the propeller, might bore the pants off a few people, but they can ignore them if they don’t want the info.

KeithMac.

RE: Another one for Uncle Keith – Pitch & Torque?

[updated:LAST EDITED ON 04-10-02 AT 07:51 PM (GMT)]Hi Keith,

Thanks for the reply.

Will max torque still be at the same engine revs, too? Are you also saying, it doesn’t matter whether you use 2, 3 or 4 blade propellers the effect will stil be the same? (Pehaps the Tempest will have a 4 bladder, after all! I just hope I never break one!).

I think a future posting on propellers, is a good idea.

Cheers,
Neilly

RE: Another one for Uncle Keith – Pitch & Torque?

Hi Neilly, Believe it or not there’s a SIMPLE answer to your question, propeller torque = engine torque, but in the opposite direction. It’s not the size of the prop that matters, it’s how much torque the engine is producing. Increase the blade angle (pitch) and you will slow the revs for a given power, but the torque will be the same! A bigger diameter prop with the same blade angle will have the same effect. Basically what I’m saying is that the torque effect on your aeroplane is dependent on power output of your engine – not on the propeller you fit.
Looks like I might have to do a few posts specifically on propellers.

KeithMac

RE: Another one for Uncle Keith – Pitch & Torque?

Hi Gary,

Thanks for that one. It’s always better when someone’s had first hand experience. I’m more interested in the torque effect – taking off with a model of this size has the same characteristics of the full size & one of the few problems with the Mosquito, was it’s take off swing. So what I was wondering was what effect a prop with a bigger pitch would have on the torque, as well as rpm & speed ( although I’m not that bothered by a high top speed. Just as long as there’s enough get up & go to mke the model fly like the real thing). By the sounds of it I should be trying a bigger pitch, so the engine revs take longer to build up.

So I’m hoping Keith will come up with a technical answer that I can file away for future use (ready for the next building project, which is in the loft awaiting construction, a Hawker Tempest V. That’ll have a 75 cc engine up front. Now I definitly can’t see a hugh 4 blade prop on it. I might have to settle for a 3 blader, but we’ll see!).

Cheers,
Neilly

RE: Another one for Uncle Keith – Pitch & Torque?

Neilly

Not too uptodate on model props but I am a Safety Boat Cox out here for one of the Island clubs and we have a newish (okay we’ve had it about eight months) Rib…bit like the RNLI use…you know a boat with a huge inflatable ring round most of it). On it backside is a 100HP fourstroke that was originally fitted with a 19″ pitch prop (on boats a 19″ pitch prop will move forward…less slippage…19 inches for one revolution) and this used to give us a top whack of around 32 MPH. One day it got totalled by a tyro so we experimented with a 21″ pitch prop. initial pick up once we firewalled the loud lever was marginally slower but the top end increase to 43 MPH was pretty spectacular. Interestingly, max revs dropped to around 5500 with the 21 but could exceed 6750 with the smaller pitch prop so we had to introduce a rev limit so as not to wreck the outboard. The pickup with the smaller prop was a damn site faster than with the bigger one right across the rev range and the engine , while not ‘bogging down’ with the 21, ‘felt’ as though some of its getup and go had got up and gone and was noticeably slower with the same amount of persons onboard at the same revs.

Dont know if the above is any use, but thought I’d pass it on anyways.

Cheers

Gary