Propellers

I notice there is a Big difference from one propeller to another. The angle changes the closer you get to the hub and the angle seems to have nothing to do the with pitch marked on the propeller.

Measurements were taken 1" apart starting at the tip. 0=tip

Master Air Screw 11x6 Plastic
0 = 6 degrees
1 = 7 degrees
2 = 8 degrees
3 = 15 degrees
4 = 25 degrees
average = 12.2 degrees

Master Air Screw 11x6 Wood
0 = 10 deg
1 = 12
2 = 17
3 = 20
4 = 27
average = 17.2

Master Air Screw 10x6 Wood
0 = 7 deg
1 = 10
2 = 12
3 = 22
4 = 35
average = 17.2

Top Flite 10x6 Wood
0 = 8 deg
1 = 12
2 = 20
3 = 25
4 = 25
average = 18

APC 8x6 plastic
0 = 6 deg
1 = 12
2 = 20
3 = 30
average = 17

Simitar 8x6 plastic
0 = 8 deg
1 = 20
2 = 20
3 = 25
average = 18.25

Power Prop 12x4 wood
0 = 4 deg
1 = 6
2 = 8
3 = 10
4 = 20
average = 9.6

propeller, device consisting of a hub with one or more blades that propels a craft to which it is attached by rotating its blades in a fluid such as air or water. In the latter part of the 1830s the Swedish-American engineer John Ericsson and the English inventor Sir Francis P. Smith independently patented screw propellers. Propellers on airplanes generally have from two to six blades. These are usually made of wood, aluminum alloy, steel, or composite materials. At first, all were of fixed pitch, i.e., the angle of the blades was not variable. Later, advantages in speed and power brought variable-pitch propellers into general use; their blades are set into sockets in the hub with gear arrangements capable of altering the pitch in flight. The development of automatic equipment to alter the pitch as needed for maintaining a predetermined speed produced the constant-speed propeller. When the number of blades was increased from two to three, then from three to six, to achieve greater thrust or propulsion or to keep the blade size down, new stress problems arose. These were met by the development of contrarotating propellers, in which the blades were arranged as two separate three-bladed units rotating in opposite directions.

Wilber and Orville Wright were the inventers of the air tunned. They tested several 100 air foil shapes and designed propellers according to the air foil shapes. Their propellers are some of the most efficent propeller ever made.

The numbers on a propeller, say 10 x 6, refer to the 10 inch diameter and the 6 inch pitch. Supposedly the angles at each station out from the hub are such that the prop unformly moves forward 6 inches with each rotation. So the angles decrease propertionately as the distance from the hub increases.

As the Wrights figured out, a propeller is actually a rotating wing and how far forward it moves in a rotation is determied by how much lift it produces vs how much drag it is hauling along. Because it is a wing, one can do anything to a propeller that can be done to any other wing: for example warps, wash in, wash out, etc.

Now I understand the concept of the constantly changing angle. Example, if the cutting edge of a drill bit is sharpened at a 5 deg angle and it is used to drill a hole the metal, it will cut at an even rate and continious speed. If the drill bit were sharpened at a constantly changing angle like a propeller it would try to drill faster in the center and slower on the outside edge. The slow outside edge would hold back the faster cutting center and put the drill bit in a bind. It would require more power to turn the drill bit and it would be less efficent. The speed of the propeller moving through the air like an airplane wing produces lift. The faster it moves through the air the more lift it produces. If the angle were the same for the full length of the propeller blade the faster moving tip part of the blade would produce faster moving air and the center of the blade would produce slow moving air and the propeller would get in a bind like the drill bid did. The angle of the blade has to constantly change so it produces the same speed air over the entire length of the blade. Airplane speed is determined by the propeller pitch and the engine rpm. Torque is determined by the length of the propeller.

>I notice there is a Big difference from one propeller to
>another. The angle changes the closer you get to the hub
>and the angle seems to have nothing to do the with pitch
>marked on the propeller.

The correct angle, alpha, at any point along the radius r, for a given pitch, p, is

alpha = atan(p/2*pi*r)

for a 6" pitch prop, for example:

r alpha
1.......43.6793
1.5....32.4816
2.......25.5228
2.5....20.9055
3.......17.6568
3.5....15.2610
4.......13.4270
4.5....11.9808
5.......10.8125
5.5......9.8497
6.........9.0431

The angle is NOT the pitch. Averaging the angle over the span of the blade will not tell you anything much at all. The angle is higher near the root because the distance is travels to go one revolution is shorter, and thus the angle to move forward one "pitch" over the shorter distance is higher. See numerous previous postings on the topic, or the recent SN article on the topic of measuring the pitch with a laser pointer, for more details.

That they are not the same from manufacturer to manufacturer is not remotely a revelation. Most serious stunt fliers (and many other disciplines, too) carry propellor pitch gauges with them and use them frequently, both to measure manufactured props to see what they really are, and to change the pitch to match the engine/airplane/conditions. For example. Ted Fancher repitched at least 3 different props since Last tuesday, one *at the field at the W/C*.

Note that varying the pitch along the length of the blade is one of many, many tricks that are commonly used to tune the propellor/engine for the desired performance.

BTW, the Wright Brothers understood this perfectly well, and their propellors had a distribution of angle along the blade very close to the relationship above.

One misstatement above. The Wrights did not invent the wind tunnel. The first person to test a model in a wind tunnel was Edme Mariotte of Dijon, France, in the mid-1600's. He also invented the uniflow tank at about the same time.

No doubt the Wrights were the first to do a number of things with a wind tunnel, but the wind tunnel was invented some 250 years earlier.

Who invented the chicken hopper tank?

Herman Chickenhoffer

:-0

Colonel Harlan Sanders

Which of these tables did you use to determine the pitch of the prop that you made to power your bicycle?

Gary,

If you are thinking about a ducted fan stunt ship, you are going to be surprised by the power loss. Thrust of a dusted fan is minimal and totally unsuited to stunt. The setup is very inefficient and duct losses extreme. Ducted fan airplanes use very large engines turning multi-blade impellers, not props, at 20,000 RPM or so. Talk to someone who flies ducted fan before you waste a lot of effort in a futile attempt to do something neat.