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Model Forum / General / Models / February 2010Propellor design
"The real troubles in your life are apt to be things that never crossed your worried mind; the kind that blindside you at 4pm on some idle Tuesday" Well... more of an idle Friday. Today I was sat at the computer and I looked at the shelf next to my workbench, where I place my works in progress. One, a Griffon Spitfire, is currently wearing a a resin prop that I haven't glued into position yet. The blades are loosely held on with Blutak. Most of them have shifted and are slightly out of true, giving the prop a very odd appearance. It struck me that every prop I have ever seen has the centreline of the blades aligned at 90° to and passing through the hub of the prop. It occured to me that if each blade were to be positioned with an advanced offset with respect to the direction of rotation, then the effective diameter of the prop would be increased without an increase in blade length. This would result in an increase in prop disc diameter and therefore (theoretically) an increase in power. I cannot believe that I am the first person to have thought of this, so there must be some engineering reason why it was never tried. Or... *was* it tried at some point? If it was, the prop wpould certainly provide an eye-catching appearance.  SignatureEnzo I wear the cheese. It does not wear me. > "The real troubles in your life are apt to be things that never crossed > your worried mind; the kind that blindside you at 4pm on some idle Tuesday" [quoted text clipped - 20 lines] > *was* it tried at some point? If it was, the prop wpould certainly > provide an eye-catching appearance. ...something like this, maybe? http://en.wikivisual.com/images/a/a3/GE-36_UDF_Domke.jpg I used to sit in a cube with the two aero-geeks that designed these blades...back when I was a aero-geek myself. But that was a LONG time a go... ...but what you'll probably find analytically is that prop blades have the blade's center of pressure aligned through the center of rotation; both for these and for a conventional propeller. That would help minimize the force required to rotate the blade under load for a constant speed prop. Signature- Rufus > "The real troubles in your life are apt to be things that never crossed > your worried mind; the kind that blindside you at 4pm on some idle Tuesday" [quoted text clipped - 20 lines] > *was* it tried at some point? If it was, the prop wpould certainly > provide an eye-catching appearance. This would put a considerable bending moment on the root of the blade. However, since there is already a considerable bending moment in the forward direction, it is likely this additional moment would be inconsequential, but don't know for sure. I would think it might just be easier to make the blade longer and keep it aligned. I am sure that the designers make the blades for the power they want to absorb anyway. So basically, I am not sure there is an advantage. > > "The real troubles in your life are apt to be things that never crossed > > your worried mind; the kind that blindside you at 4pm on some idle Tuesday" [quoted text clipped - 31 lines] > > So basically, I am not sure there is an advantage. Odd. My comment shortly after the Enzo's post disappeared. The basic argument against is a propeller blade at speed is subject to tremendous centrifugal forces. The propeller blades' center of mass will want to be at 90 deg to the engine shaft. Since it is unlikely that out-of-center blades will straighten out equally the imbalance will tear the engine out of its mount. > It struck me that every prop I have ever seen has the centreline of the > blades aligned at 90° to and passing through the hub of the prop. It [quoted text clipped - 8 lines] > *was* it tried at some point? If it was, the prop wpould certainly > provide an eye-catching appearance. I can't picture what you are trying to describe here - especially since any mucking about with the propeller blade alignment will produce a smaller disc diameter, as far as I can tell. What am I missing? As to propeller design: the most efficient propeller has anly one blade. Some have been made that way, for model aircraft and in few fullscale instances where maximum range with relatively small engine power was paramount. As you increase the engine power you need to increase the amount of propeller to turn that power into thrust. You can increase the diameter, the number of blades or make it turn faster. Diameter and rotational speed run into mach problems at the propellor tip which destroy the blades efficiency. Increasing the number of blades is the only way out, even though they interfere with each other. Eventually you need contra-rotating propellers, which balance out the torque experienced by the airframe and make it possible to absorb really large amounts of engine power and stay controllable. Some modern propellers have experimented with curved, swept-back scimitar blades, which give better efficiency at high subsonic speeds. >> It struck me that every prop I have ever seen has the centreline of >> the blades aligned at 90° to and passing through the hub of the prop. [quoted text clipped - 30 lines] > Some modern propellers have experimented with curved, swept-back > scimitar blades, which give better efficiency at high subsonic speeds. IIRC, when the tips of propeller blades become supersonic, they lose effiency rapidly. SignatureAM http://sctuser.home.comcast.net http://www.novac.com > >> It struck me that every prop I have ever seen has the centreline of > >> the blades aligned at 90 to and passing through the hub of the prop. [quoted text clipped - 40 lines] > > http://www.novac.com There was some research an experimentation after WWII, but once jets came of age, it all disappeared. NASA is doing some research on this, not sure what is online. Check either Ames or Dryden. They also have some good photo archives you can really get lost in. Since we're talking models, there was also some work on sub props. At one time, props on US subs were classified. Since the Cold War has ended that's pretty much gone away. Having gotten paid to do aero work in the good old days, never knew much about marine engineering. Lots there, not sure who has a decent degree in it. Besides Canoe U. |
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