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tartare
21st Dec 2009, 19:54
Looking at the A400M, was wondering if there would be any benefit in using the same multi-bladed scimitar shaped props on light aircraft i.e. C172 size. I assume they provide advantages in mitigating vortices (note they are also used on subs to limit cavitation). Obvious cost/complexity implications - but from a pure performance viewpoint, would they provide any benefits?

21st Dec 2009, 22:36
I'm sure someone will give a much better answer, but untill then, perhaps this will tide you over...

There is a formula to work out something called "Propeller Solidity". The actual formula escapes me, but it is a function of the Number of blades, the area, and the Prop RPM at a given power setting. This comes up with a figure that indicates how efficient (or not) a propeller is.

To put it simply, the more power an engine gives off, the more props it needs to put all that power to use. A good example is the Spitfire. The Mk I had a simple two bladed prop, by the time they got to the Griffon Engined monsters, there were big, wide 5 bladed props.

So, to finally answer your question. There is nothing stopping you putting multi-blade scimitar props on your cessna. However i suspect that there would not be enough power in the Lycoming Donk to put enough power into the system to make it work, and as such, your "A152M" probably wouldnt go very far.

However, i have been wrong many many times before, and as prop theory is more of an interest rather than a specialty, I could be wrong...

TTR

enicalyth
22nd Dec 2009, 07:10
The issue is the tip speed I think. If the aircraft is going forward at 3 units per second and the rotational speed of the propeller tip is 4 units prer second then Pythagoras gives us total tip speed of 5 units per second. Provided that this is less than about Mach 0.9 and the airfoil design is good then efficiency in cruise can be of the order of 85% or so. But above Mach 0.90 the efficiency rolls off ultra fast and blade sweep is an attempt to avoid the usual problems in the trans-sonic region. Solidity is not really a measure of efficiency but a necessity to deal with power loading for one thing. The ideal prop would be a disk where air was accelerated by a vanishingly small blade. Of course this can't happen, blades are necessary. 2-bladed props are said to experience awful gyroscopic effects to do with forward motion and rotation that are smoothed out somewhat by three-blades. Early piston engine designers worried about vibration and harmonics so seven- and nine-cylinder engines nestled nicely with four-stroke cycles though machine tooling must have been a nightmare. Seven into 360 degrees goes..... hmmm! Four-bladed props runs a mite against that theory but the issue would be blade loading. The turboprop C-130 for example wasn't happy with three blades and settled down with four. Five blades might be better vibration-wise but machining costs go sky high. Would a really solid prop disk be efficient? I don't think so. You could try Martin Hepperle's website. Remember to choose your airfoil wisely - Clark Y will do - and set your atmospheric options to suit. Martin designs props for model aircraft but as the actor in "Flight of the Phoenix" says, "Ze theory is just ze same" and have found his applet works extremely well. You'll probably find once you have entered all the parameters that a conventional three-blader is the best

Have fun

The "E"

Brian Abraham
22nd Dec 2009, 15:00
Probably not quite what you had in mind, but they do manufacture what they refer to as scimitar props for light aircraft.

Hartzell Propeller Press Room (http://www.hartzellprop.com/pressroom/pr_htm/pr_12_03_03.htm)

bearfoil
22nd Dec 2009, 15:49
One might refer to the Hartzell as a 'Scimitar'. It isn't. The tip is raked (a la) Scimitar. Cheap airplanes need cheap solutions, and marketing makes up for some lack of value, it was ever so.

Curve is a synonym for expensive. Even the Beech, pricy as it is, couldn't sell a 25,000 dollar propellor to Dr. Jones, he'll buy a Caravan or PC-12, and get some real speed and utility. IMO.

bear

B2N2
25th Dec 2009, 13:56
Some very interesting reading on elliptical propellers:

EAA - EAA Experimenter - ELIPPSE Propeller (http://www.eaa.org/experimenter/articles/2009-02_elippse.asp)

http://www.contactmagazine.com/Issue79/RearCover.jpg