Airscrew, Propellor, Propeller Tuning

[Head in the clouds]

I can see that. In the series of pictures (15 +16) the blade has been recut to a smaller airfoil shape within the outline of the original airfoil . Is this the only way to de pitch a wood prop? I really need to know.

Yup - can't force twist 'em like metal ones... but if you need to reduce the pitch it's because you want more revs, so the blade area reduction will help that too, that's why you go at it gradually, no need to varnish it between tests, until you have it just right.

If you want much less pitch and big paddle blades for a very slow STOL plane that's nothing like the one that the original prop was intended for then you need to start again from a laminated blank.

 

 

[facthunter]

Toothpick blades don't work on draggy planes. I feel some of the props tend this way more than they should. Comments? Also any "washout" of blade pitch ( the angle changes but the pitch may not) towards the tips would be (extent?) and where commenced? Nev

 

 

[Head in the clouds]

Toothpick blades don't work on draggy planes. I feel some of the props tend this way more than they should. Comments? Also any "washout" of blade pitch ( the angle changes but the pitch may not) towards the tips would be (extent?) and where commenced? Nev

That's right, the draggier the plane the more solidity required, with commensurately less pitch so as to allow larger diameter. Everything's a compromise...

 

Geometric washout (twist) is very inefficient and should never be used on an airscrew. And aerodynamic washout (airfoil change toward foils nearer the tip which have increasingly negative zero lift angle) is not necessary because we're not trying to make the tips keep 'flying' while the hub (root) is stalled, so if it's used at all the only washout should be the last inch or so of the blade's span for the sole purpose of reducing the strength of the local tip vortex, achieved as described in the last sentence of post #24.

 

 

[facthunter]

I was only concerned with unloading tip vortices but I take it you can manage that by keeping the end edge sharp. Nev

 

 

[geoffreywh]

Well I'm going to try, actually, not just try, I will change the pitch using the guidelines from the prop advisor H.I.T.C...................... My aeroplane is not really draggy. I have a good 0-200 that pulls it along quite quickly but the maximum rpm I can attain in level flight is only 2600, (this could be 2800 or more) Thus I am only generating 85 or so HP. Max ground rpm is 2350. So, I have too much pitch!....While it all goes along pretty nicely I like the Jodel to be as good as it can be.................I am a terrible tinkerer...Plus it's a good opportunity to get rid of the lumps and bumps I can see along the prop. (that's what I'm going to tell my wife!)

 

 

[Head in the clouds]

........... I will change the pitch using the guidelines from the prop advisor H.I.T.C....................

Good luck with it and don't forget my comment in Part One - I'm not providing advice, just passing on what I've learned...

 

 

[spacesailor]

It all sounds good, NOW.

 

Who's doing a "scimitar" style prop, and how to calculate the extra pitch near the tip,

 

spacesailor

 

 

[Head in the clouds]

It all sounds good, NOW.Who's doing a "scimitar" style prop, and how to calculate the extra pitch near the tip,

spacesailor

Yes, it would be very much more of a trial and error exercise for most of us. For anyone that missed the point, ss is referring to the way that scimitar blades act like swept wings and reduce their AoA as loading increases. Hence the scimitar blade very cleverly acts a little like a constant speed/inflight adjustable pitch prop, when highly loaded at WOT the blades flex forward (as with any prop) but that flexing reduces the pitch toward the tips and so also reduces their drag allowing further flex etc... the outcome being a finer pitched prop when highly loaded, ideal for climb. As the throttle is reduced in the cruise the load on the prop reduces and so the flexing reduces and the pitch increases, making the prop better suited to the cruise configuration.

 

Calculating (by trial) the tip region pitch isn't all that complex, at least to achieve a starting point. A scimitar shaped test prop needs to be carved and then loaded in the same way as a wing is sandbagged, the load must be representative in total load and spanwise load distribution, as the thrust generation would be. Physical measurement of the flex can then be made as the load is applied, and the pitch reduction per millimetre of flex can be measured at the same time. Then you have your benchmarks and can set your pitch for the 'real one' according to your requirements for the climb and cruise conditions.

 

Prop design software would take all that into account but it's way beyond me. If anyone wants more involved prop design help just let me know and I'll send you a link to those who would know.

 

 

[spacesailor]

Hi HiC, I was hoping to use the known pitch from the old prop, then make the Scimiter as a course cruise prop, The proplem of couse, is the pitch reduction from the hub to the tip.

 

Do I make the as lofted for the straight prop or make it courser towards the tip.

 

spacesailor

 

 

[Head in the clouds]

Hi HiC, I was hoping to use the known pitch from the old prop, then make the Scimiter as a course cruise prop, The proplem of couse, is the pitch reduction from the hub to the tip.Do I make the as lofted for the straight prop or make it courser towards the tip.

 

spacesailor

Hi ss, unfortunately there are too many variables to give you an exact answer. Certainly on the scimitar you would progressively reduce the pitch less than on a straight blade, so that if you started with the same pitch at the hub you would end up with more pitch at the tip with the scimitar blade than with the straight blade. The amount of difference would depend on the amount of tip offset the scimitar has compared with the straight version i.e. how curved the scimitar blades are, and the flexibility of the timber you choose. I would go for a very hard timber like mountain ash so that it doesn't shed pitch too easily under load.

 

Also, perhaps start out with fairly wide blades, especially near the tip, and more pitch than you really expect to need, that way you can reduce the pitch and blade width as described previously should you need to. If you start out with narrow blades and too little pitch it'd be much harder to fix.

 

 

[spacesailor]

Hi, HITC, As i've only 32hp with a cruise over 80 (hopefully) @3000rpm, I'm thinking of softwood & plenty of shape (offset) about 32pX50".

 

Maybe you could send my idea to your freinds with more knowledge than I.

 

spacesailer

 

 

[Head in the clouds]

[ATTACH]20835[/ATTACH] Hi, HITC, As i've only 32hp with a cruise over 80 (hopefully) @3000rpm, I'm thinking of softwood & plenty of shape (offset) about 32pX50".Maybe you could send my idea to your freinds with more knowledge than I.

spacesailer

PM sent

 

 

[spacesailor]

many thanks:oh yeah:

 

 

[Guest Maj Millard]

Ok, Sorry for the delay but I can now post photos of the prop mentioned in posts 1 & 6. This is the prop used on the Hirth F23. It was originally a canadian product of unknown brand and stamped 60 x 28. It is additionally stamped on the other side 21...which i may have done after cutting the prop down. i was pretty good with my pitch calculations back then using recommended formulas and this may have been the effective pitch that i came up with. The prop length now is 54 ". So it is now a 54 x 21. Although some take-off were less than inspiring, I did do many hours of quality flying behind this prop, once off the ground, and up and running.

 

 

 

 

 

 

 

 

 

 

 

 

[Head in the clouds]

many thanks:oh yeah:

Hi Spacesailor, I was wondering if you ever got in touch with Jan Carlsson who I sent you the intro to? You may be interested to see he's the one who has designed the props for the magnificent Bugatti 100P restoration. There's a link to his site with more info, on this EAA page about the Bugatti

 

 

[spacesailor]

Hi, Hitc, No, not yet got around to doing things about the prop, my daughter was rearended so we have to argue with the insurance about writing off her car,

 

Would there be any interest in a low mileage, 2.5L/ gen 3 Subaru motor, just had its 100,000 service the week earlier, their payout is Rude to say the lease.

 

Yes: I checked the Bugatti site its awesome.

 

spacesaor