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# Why a prop is twisted?

the propeller on a plane

6 years ago

ok my flight instructor asked me this question and the clues he gave me was that he wanted an answer and it had to have something to do with relative wind and angle of attack..

6 years ago

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January 13, 2006
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At first glance it appears that a propeller is just a big strong fan that pushes the airplane forward by blowing lots of air out the back. Not so. A propeller is actually an airfoil that pulls the airplane forward with exactly the same forces by which a wing pulls the airplane upward.

The propeller blade moving through the air experiences a relative wind in exactly the same way the airplane's wing experiences a relative wind as it moves forward. So, if the angle of attack of a wing affects the amount of lift the wing produces (up until it stalls), then the angle of attack of a propeller blade affects the amount of "lift" or pulling force the propeller produces.

For much more information, with pictures and diagrams, check out the Pilot's Handbook of Aeronautical Knowledge (which is what I am sure your instructor intends for you to do, rather than asking on Yahoo!), or, in a pinch, try an internet search on "propeller aerodynamics."

If you don't have a copy of the Handbook of Aeronautical Knowledge, try the public library.

Good luck.

### Source(s):

retired airline captain
• 2 people rated this as good
Awesome

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• Member since:
June 29, 2008
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A prop is essentially an airfoil just like the wing on an airplane. An airfoil produces lift by spinning through the air like a wing produces lift by traveling through the air. Maximum lift is achieved when the angle of attack on the airfoil is optimum, which is approximately straight at the leading edge of the airfoil. If the angle of attack (direction of the air) turns to the underside of the airfoil then the airfoil is said to be in "stall" and produces less lift. If the angle of attack is from above the wing then the airfoil is said to be in "choke" and produces less lift.

Since the prop is spinning about an axis the speed of the prop is slower at the root (inner diameter) and faster at the tip. As such the speed and direction of the air relative to the prop is different between the root and the tip. To assure that the air angle of attack on the prop is optimum from the root to the tip, the prop is twisted to the correct angle.

The Wright Brothers were the first to properly define the correct prop angle and shape which is closed to what is used today.
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A propeller is twisted to keep the thrust produced relatively equal along its entire length. The tip of the prop is traveling much faster than the area near the hub, so without twist, the tip would produce more thrust (think horizontal lift). This would create a lot of stress on the blade to the point of eventual structural failure.

The amount of twist built into a blade is proportional to the speed of that section of the propeller when it is rotating. The inboard sections have a greater pitch, so they can travel at lower speeds and produce about the same amount of thrust as the less-pitched tips traveling at higher speeds.
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This is a function of the advance ratio, i.e. the pitch.
Take a propeller that is 10 ft in diameter with a tip advance angle of say 5 degree. One full rotation of the propeller would thus see that blade going around 31.4 feet (pi times the diameter), and at 5 degree, this would make the tip advance 2.74 feet (I am rounding up my numbers; the actual equation would be SIN(5) * pi * diameter). We can assimilate this advance rate to a screw advancing in solid material (note that the advanced angle is not necessarily the same as the plade angle itself, as there are camber and angle of attack issues, but for this simple analysis, this is good enough)
Now, let's take a point 2.5 feet from the hub, i.e, in the middle of a blade. This point will go 15.7 feet around during a full rotation, and if its advance rate was also 5 degree, would want to move forward only 1.37 feet, i.e. it would work *against* the tip, dragging it down. Since we want the whole propeller to move at the same rate (evidently) for reason of internal stress and efficiency, we have to solve for the advance angle at that point by using the same advance rate per rotation as for the tip, i.e. 2.74 feet.
In this instance, we have to solve for the equation
SIN (angle) * pi * diameter-of-a-point = 2.74
In the case of a point 2.5 feet from the hub, this is
SIN(angle) = 2.74/(pi*5)
and "angle" has to be 10.04 degree.
Repeat this for all the points on the propeller, noticing that the angle has to be 90 degree exactly at the center by the way, and you just defined the required propeller twist.

About 20 years ago, I was involved in wind tunnel testing, and some of the technicians building the aircraft model decided to make a small rubber band powered aircraft in their off hours. The first prop they made (very smooth, those guys were real craftsmen) had the advance angle at zero at the hub and maximum at the tip; and when they showed it to me, I pointed out that the prop would be fingthing against itself. So they made another one, with the angles correct, and they reported the wind through the small prop was so much higher with the second model than the first. At the end of the wind tunnel test weeks, they tested their airplane, and it went magnificiently. With their first prop, I doubt it would even have picked up speed.

### Source(s):

Aerospace engineer
• 1 person rated this as good
• by arryl26
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May 27, 2008
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Simply put, a propeller is angled in order to pull air from the front and dispense it back. Most planes that have propellers are able to glide and what the propeller does is create a pull that can keep the aircraft in the air. Personally I think you are getting some great answers here but you really need to know your aerodynamics. Read your book! =)
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because if it wasn't it wouldn't create no thrust,and without that the plane wouldn't move through the air,it has to force air over the wings in order to be able to fly,good luck.

### Source(s):

been a certified mechanic for 37 yrs.
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August 15, 2006
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Because the wing is warped......

Inside joke...I hear some aero guys giggling...