The hp and thrust of a prop is based on the amount of air which EACH blade sweeps out in one revolution. Since that is a function of disc area, which is proportional to the square of the diameter, if you change from a two-blade prop to a three-blade prop, you can reduce the diameter to 82%, or to 71% with a four-blade, and still have the same swept area. So if you have a 72" 2-blade, you should be able to get away with a 59" three-blade, or even a 51" four blade, and still have the same or better efficiency IF the prop blade has a streamlined shape where it enters the spinner. The reason that the myth about multi-blade props which give better take-off and climb but are not as good in cruise, is that they had such terribly klunky shapes in the root sections, which gives a lot of drag. Tom Aberle's Phantom biplane racer had a 64" diameter two-blade prop in 2003 which gave him 221 mph. In 2004 he had my three-blade design of 59" diameter which gave him 241 mph at 250 rpm less. This year he did 252 mph with my 59" four-blade design with the same rpm he had with the two-blade. The advantage of a shorter multi-blade prop, spinning at the same rpm, is that it will have less noise from less tip loss which adds to its efficiency. So if you want more ground clearance, reduced noise, more efficiency for better cruise, and better takeoff and climb, go find a prop manufacturer who understands this and buy a three or four-blade prop from them! And no, scimitar tips are in the same league as the swept vertical stab on the Cessnas. If your prop has any chord whatsoever at the tip, it just generates drag proportional to the chord and absolutely no thrust. If it's a thin or symmetrical airfoil, its parasite drag will be much greater than a 12% or even 15% thick airfoil. And the hp loss is greater the farther out on the blade you go since hp is the product of torque and rpm, and the greater the radius where you have drag, the greater the multiplier of that drag through the radius to get torque. One pound of drag at 6" radius is 0.5 lb-ft, at 12" radius it's 1 lb-ft, at 24" radius it's 2 lb-ft, and at 36" radius it's 3 lb-ft. Now that drag can be either parasite or induced, so if your planform generates higher lift with the squared dynamic pressure at the outboard radii, the rearward lift vector, the so-called induced drag, uses much more power to generate thrust than do the inboard sections.

Paul,

 

Verrrry interrrrresting!

 

Prop first -

 

2 questions

 

1.  I have an unusual prop on my Lancair 320.  A long time ago Hartzell determined that a certain 84 inch prop could be cut to 70 inches, leaving the tip square, would be quite a good constant-speed prop match for the 320 equipped hi-speed Lancairs.  Of course, when they manufactured the prop, they specified a taper thickness at two chord positions out near the tip (I have forgotten, like 31" and 33") since the extra metal was unnecessary to support the missing 7".  A few years ago I had new blades "installed" since I managed to, uh, "hurt" the old ones.  An unnamed prop shop got usable 84" prop blades and cut them down to 70" without the tapering.  Thus, I have a very thick airfoil at the tips.  Does this mean I am benefiting from less parasitic drag than more normal 320 folk(non-institutionalized like me)?

 

2. I make use of Landoll's Harmonic damper (12# of steel and silicone contained in aluminum and secured to the face of the starter ring/flywheel.  This gives me an unknown boost (hidden by the CS prop) by reducing the instantaneous negative rotational torque inherent in 4-cyl engines (moves the CG a bit forward, too).  Fixed pitch prop users may see as much as 100 rpm increases because of the added weight flywheel momentum and damping of the neg torque.  Have you tried one of these with your props?  If so, do you have an opinion on same?

 

Wee wheel on the nose gear -

 

1 question

 

Sometimes there are unexpected consequences.  If the nose is lowered too much, the AOA of the wing is also lowered during the takeoff run.  This may require a greater ground speed in order to provide enough elevator control to get the nose off and the plane into a takeoff attitude.  However, it may be beneficial on landing in order to make the plane stick once the nose wheel touches down. 

 

A friend with a 2-lifting surface aircraft - an E-Racer that used to have a devil of a time building enough speed to make the canard "elevator" effective because of the heavy front end and rake of the nose wheel landing gear arm.  After raising the front end a couple of inches (changing the rake), the takeoff runs were much shorter and the plane was easier to fly off the runway.

 

Did you notice a takeoff run change with the smaller nose wheel?