Science/Mechanics Question

[Richard in Smithville]

Thinking of an airplane propeller. When it is power up to speed, how much(if any) will the prop bend or warp in reaction to the amount of air that is being push through? I know large buildings have a sway factor so they can survive high winds so how about props? If they stay rigid, will they snap?

[Tom Slick]

They do twist, especially on a variable pitch propeller, but I don't know how much. The blades in a jet/turbine engine twist and stretch quite a bit during operation also. The wings on a large plane are quite flexible. A B-52's wings flex about 3 feet just from empty to full of fuel.

[LCHIEN]

all materials are elastic to some degree. Metal blades of finite thickness are too, so when forces are applied they will bend. I don't know how much. Airplane wings are elastic also, they help absorb some of the air turbulence.

[cabinetman]

It's an interesting question. It seems that the blades have the propensity to deflect, and the causes for that can be somewhat minimized by the blade angle and shape in relation to its speed and resistance to the airflow.

http://www.aboutflight.com/handbook-...ler-principles
.
http://www.faatest.com/books/FLT/Cha...Principles.htm

.

[leehljp]

While this is not large props, I was into flying models since the late '50s. It was interesting to watch the props from the side (while holding the planes, especially ones with 10 to 12 inch props) and as a quick speed increase, the tips would flex forward. Nylon props flexed slightly more than wood props.

I am sure real metal props flex as well.

[jackellis]

We tend to think of large structures as being almost perfectly rigid but as Loring points out, all materials possess some elasticity - even concrete and stone. A metal propeller will flex just like a wood or nylon propeller, only much less so because aluminum is less elastic than the other materials.

The same thing happens with pipes used to drill oil and gas wells. Loring probably knows more about this but I would bet the end of a 15,000 foot string of drill pipe that's attached to the driving mechanism twists once or twice before the bottom of the pipe that's attached to the cutting tool starts turning.

Another example is the Golden Gate bridge. On the occasion of its 50th anniversary, the bridge was closed to vehicles and open to pedestrians. As I recall, more than 400,000 people were on the bridge at one point and the crown in the roadway across the length of the bridge had flattened so much officials limited the number of people on the bridge (using the 175 pound FAA average weight standard, 400,000 average-sized people weigh about 35,000 tons, or the equivalent of 900 fully loaded big rigs).

[TK421]

I used to be a jet mech on P-3's when I was in the Navy.
Yes the props bend. Torque bending, when the blades bend in the opposite direction that the blades are spinning. Thrust bending, when the blades "bite" into the air like leehljp describes. Stretching as well, the blades want pull themselves out of the hub and stretch. Also on variable pitch props the blades want to "twist" themselves back to the angle of least resistance (edge on instead of at an angle)

P-3's use the same engines as C-130's. Allison T-56 turboprop engines. The blades are made of aluminum and weigh about 200 lbs. each. The props spin at roughly 1000 rpms.

[gerti]

Speaking of wings bending, the 787 Dreamliner is pretty extreme at that. Here the flex test:

http://www.wired.com/autopia/2010/03...ing-flex-test/

And here a landing, check those wings when they unload after touchdown!

http://www.youtube.com/watch?v=ojMlgFnbvK4

[LCHIEN]

Jack is completely right about oil well drilling. A 15,000 foot string of drill pipe is not pushed into the ground to drill, at that length it would be like pushing a drill bit with string.

The way that deep wells are drilled is by putting a length of drill collars just above the bit - the collars can weight several hundred thousand pounds and keeps the drill string in tension. THe drilling rig actually pulls up on the drill string to keep a overall tension on the entire drill string and perhaps leave 10,000 pounds force on the bit which is enough to do the drilling. The pipe twists up torsionally but eventually it all rotates nicely being under tension to power the bit into the ground. To help clear the bore drilling fluid is forced into the center of the drill pipe and exits through jet holes in the bit which has toothed rotating wheels. Between the rotating teeth, the fluid jets and the drilling force the bottom chips away and teh fluid carries the cuttings to the top of the well for filtering and disposal. SOmetimes the pressure in the formation helps flake off material from the bottom of the bore as well if the fluid is carefully under-pressured. Drilling rates can be in fractions of a foot per hour to several feet per hour, depending upon the hardness of the earth at that point and the bit type and weight-on-bit and fluids used.

The drill pipe is 3-5 inches in diameter with a wall thickness probably around half an inch but like I said, at 15,000 feet it's like cooked spaghetti.

[JimD]

An airfoil like a propeller is a particularly interesting mechanical design challenge. It receives significant centrifugal forces, as has been mentioned, and forced from moving the air it moves, as has been mentioned, and resonant frequencies must also be considered. Most will also experience a super to sub-sonic transition somewhere along the propeller length that must be considered in design. The modulus of elasticity for different materials will tell you something about how much they will deflect but the cross section resisting the stress is also a key parameter. Materials with a lower modulus of elasticity (like wood) also tend to dampen vibratory stresses well while metal, with a much higher resistance to bending, does not. Interesting design issues.

Jim

[jdon]

Watching the development of this erudite thread from the sidelines, all I can do is quote from Wayne's World: "We're not worthy!"

[twistsol]

Watching the development of this erudite thread from the sidelines, all I can do is quote from Wayne's World: "We're not worthy!"

I've said before that I believe the sum of all human knowledge may be represented on this board. I have no useful information to provide to this thread but it's an interesting topic.

[eccentrictinkerer]

My 25 year old nephew, Whit Coyle, is chief mechanic on a B-17 and a B-24 now campaigning around the country in the "Wings of Freedom" show.

he showed up here in Minneapolis last week and my family had a blast checking out these old 'birds.

I've attached photos that Whit posted on his Facebook page. They show either really warped props or camera distortion.

I hope it's camera distortion!

[gsmittle]

I've attached photos that Whit posted on his Facebook page. They show either really warped props or camera distortion.

I hope it's camera distortion!

Assuming the last photo was taken by an SLR, it could be distortion due to a horizontal curtain shutter. I've seen similar distortion of fast-moving objects taken with my vintage Canon A1.

g.