Pulling/pushing aircraft by the prop

[FlyingLapinou]

I'm not a great fan of moving aircraft around by pulling/pushing on the prop, but it's a common occurrence at clubs here in France - well, tow bar plus prop, mostly.

Given that the blades have to absorb huge stresses in flight, and I think I'm right in saying that propeller tip speeds will approach the speed of sound (?), how much truth is there in the remark that one should push/pull close to the hub, rather than further out, towards the propeller tip? The alleged reason being that pulling/pushing nearer the tip does the prop no good? Does it really make any difference? I just can't see how a pair of weedy female arms could possibly exert enough force to do any kind of "damage" to a prop by handling it (slightly) closer to the tip than the hub? But always willing to learn...

As I said, I'm not really a fan of touching the prop at all.

[Aesir]

I agree that the argument that the propeller could be damaged is very unlikely.

However I do tell people not to pull or push on the prop and I normally don´t even explain why to non aviation guests.

My main reasoning is that people could hurt themselves on the sharp blade edges or fall if the propeller moves. And generally I want non-pilot guests to have a healthy respect for the propeller and learn to stay away from it.

edit..
http://i190.photobucket.com/albums/z171/Icepicture/prophandling_zps29698d9e.jpg

Google it. It´s hard to find any examples of damage to props. However for example the manufacturers advise against pulling on the prop but don´t really explain why. But they have one mean looking airplane in their guide for propeller care :)

[abgd]

There was a recent thread about this.

As a thought experiment, if you were to lift a helicopter by the blade tips, would you be surprised if the blades were damaged?

In flight, the spinning of the blades causes the rotor disc to remain rigid, and whilst moving, the blades can withstand forces that would otherwise cause them to break.

Aircraft propellers are smaller and more rigid, but will be protected by their rotation in much the same manner. I would play safe and pull props by the hub.

[mad_jock]

Its basic engineering to be honest.

A force applied a distance away from a pivot point will create a lever. Which creats what we call a bending moment. The further you are away from the hub the larger the bending moment acka if it gets to big its instant damage and even if its not enough it adds to the fatigue history of the prop.

And a little weedy girl is more than capable of bending anything she likes with the correct application of force.

[peterh337]

I cannot see that pushing/pulling a normal light single by the prop by hand can possibly do any damage. The internal prop components are massive. At max rated rpm, the stress on the prop blades is around 1/4 of the yield strength of the blade root...

But I wouldn't push/pull on the tip; there is no point, and one could possibly bend the tip if one tried really hard.

[mad_jock]

Its not just the prop that it may harm.

The bearings arn't designed for an off center load so if you just pull on one side you are squishing hardened bearings into the soft race which will start giving play on one side if you do it often enough.

The engine is designed though to have the force through the shaft so the closer you are to symetric the better so a hand either side close to the hub won't do anything but a pull at the tip one side won't be healthy.

The complexitys of the stress level down near the root in flight is completely different to a static point load. Props just arn't designed to be point loaded they are designed to have a pressure applied over the length and the pitch is altered to vary the pressure. And also to take the centripetal forces of it spinning.

Also a symetric straight pull is OK but when folk try and steer by pulling one side or the other it won't be doing any good.

[A and C]

I agree with Peter.

[strake]

I would suggest that the idea came from the days of stringbags and flimsy aero structures. I would have thought that now it's more to do with handling a prop which might "go live" rather than being able to break anything.

[patowalker]

The internal prop components are massive.

Not in the broken carbon fibre prop in my garage. I am sure the blade would break if I tried to move my VLA by pushing on a prop tip.

[Armchairflyer]

Recently saw one of the mechs at the aerodrome pull a 152 with one hand about halfway between hub and tip, and I suppose that guy knows what he's doing. Still, personally, anything other than both hands right at the hub makes me cringe (and I fail to see the point, grabbing the prop further out makes the task no easier and just puts additional stress on the prop and other structures, even if not outright harmful).

BTW, I am no physicist or technician, but I imagine that if all the thrust force generated by the prop in flight (especially during takeoff and climb) was brought to bear at the tip(s), props would be somewhat short-lived.

[peterh337]

Not in the broken carbon fibre prop in my garage. I am sure the blade would break if I tried to move my VLA by pushing on a prop tip.

That is probably true. The composite props seem much more flimsy and clearly rely on centrifugal force to keep them stiff when delivering thrust.

[FlyingLapinou]

Just to be clear:

Asymmetric pulling/pushing - nope :=
Pulling/pushing on anything that needs extra care, e.g. the prop on "my" trusty Jodel D112 - definite nope :=
Pulling on the actual tip - nope again :=

I was really interested to know if there's any appreciable difference in pushing/pulling about 2/3 out from the hub, as opposed to right up close to the hub? I didn't think there would be enough to warrant a remark from the chap on the other side of the spinner, but clearly there might be.

Anyway, thanks for all the comments, explanations and nice drawing, Aesir :)
Obviously, I might as well err on the side of caution and stick close to the hub. I'll remember that :ok:

[Pilot DAR]

For my experience, pulling on any propeller blade tip is an absolute no no. I cannot see any good reason to not grasp the blade as close the hub as is possible. When I pull on a prop, my hand will be touching the spinner.

Now, if you have a variable pitch propeller, more caution is warranted - but, there are few variable pitch propeller equipped aircraft which are still light enough that moving them solely by hand on the propeller works well. Yeah, Cardinals, Arrows and a few others are on the size line, but you get the point... Find a tow bar!

I was treated to a tour of the MT propeller factory last week - I was extremely impressed with the quality and care of manufacture. They are good, well designed, tough propellers, but I really cannot see myself pulling on one of their blades. Not because I'm concerned I would damage it, but more 'cause it's kind of like pulling on a piece of art!

I'll pull on a solid metal prop, on a very light aircraft. Any vintage wooden propeller aircraft I have flown has other better places to pull, and any variable pitch usually has a towbar, or other suitable means of moving it as intended.

I have zero concern that when moving a light aircraft with ONE HAND on the INNER part of the blade of a SOLID propeller, I will harm the engine. Imagine the gyroscopic forces put into the crankshaft by the inertial of the heavy propeller during hard maneuvering. You could never equal those with one hand on the inner blade.

Helicopter rotors? Very little comparison there. True, we do not handle helicopters by their blades. But their blades are much more delicate than any propeller, and employ tip weights to benefit from centrifugal forces. Propellers don't really work that way.

If you ever get the chance to ride as a passenger in a Dash 8, sit in the plane of the prop, and watch the blades flex forward as the pilot puts up the power, they move a surprising amount forward!

[VP-F__]

putting aside the stress issues on the prop there is a reasonable safety argument here for not pulling/pushing the outer part of the prop. If you are holding the outer part you will inevitably be holding only one blade, if you then slip you will be far more likely to turn the prop and if you have a live mag you have an aircraft moving toward you or you will be falling towards the aircraft with possibly a very nasty outcome! By holding the prop at the root you will not only minimize stress on the bearings, blades etc you will also have far less risk of turning the prop.

[thing]

What's wrong with using a towbar? They're not exactly Charles Atlas stuff. Any 172/28 lookalike is no problem to shift once you get it rolling, plus they are a damn sight more accurate if you are lining the a/c up on a tight, marked out pan. Saves having to lean on the rear fuselage to spin the a/c if you are a bit out hand pushing it.

[FlyingLapinou]

...it's a common occurrence at clubs here in France - well, tow bar plus prop, mostly.

Yes, I should have made that clearer: we were using a tow bar, as well as pushing on the prop.

...a reasonable safety argument...[..]...By holding the prop at the root you will not only minimize stress on the bearings, blades etc you will also have far less risk of turning the prop.

That sounds logical, I like it.

[Cobalt]

When discussing stresses at the blade root, it might be userful to know the forces involved.

I am pretty certain, comparing the acceleration I am able to impose on the aircraft when pulling by the prop with the acceleration on take-off, that the forces involved are one or two magnitudes lower than what the prop has been designed for [assuming I am pulling at the root].

Anyone around here who can caluclate
- the propeller thrust during take off
- the corresponding bending moment at the wing root

and compare this to the 100lbs and 3,000 lb in or so that I might be able to impose on the poor thing on a Sunday morning after a good breakfast?

[flyinkiwi]

My best advice on this subject is, take your lead from the person who owns the plane. If they have a towbar, ask them to demonstrate it's use to you. If they do not, ask them how they would like their aircraft handled when not under its own power.

[Big Pistons Forever]

Like was said above the best way to move an aircraft is with a towbar. However IMO opinion there is no harm in pushing or pulling on the prop as long as you hold the prop just outside the spinner. What is really bad however is moving the aircraft by pushing on the front of the cowl, especially Cessna singles as you will break the flexible cowl mount fixtures.

As always you should never turn a prop without making sure you will not be hurt if the engine were to fire.

[abgd]

I think you could make a good argument that you're more likely to have an accident due to leaving the towbar in situ than you are to damage the prop by pulling on it, especially if you pull close to the hub.

I've heard of a lot of accidents of this type, though none were fatal. I've never actually heard of anybody hurt in recent times whilst pulling an aircraft by the propeller, though I'm sure it must have happened and would be likely to injure the pilot. I've heard very few tales of non-wobbly propellers failing in flight or departing the aircraft or developing severe vibration which might be caused by somebody hand-bending the prop. I'd be interested to be hear if I'm underestimating the frequency of any of these types of accidents.

p.s. I realise that aircraft propellers and helicopter rotors are constructed very differently... Just trying to point out that dynamic loads are different from static loads.

According to:

aircraft-license.com/Demo/17.pdf

the centrifugal forces at the root of a 10kg blade at 2700 rpm may be in the region of 50 tons whereas the total thrust from the propeller is only likely to be in the region of a few hundred lbs. The resulting vector will therefore be within a fraction of a degree of the axis of the propeller blade.

It would probably be possible to design a propeller blade root that was very strong in tension and could resist torsion acceptably, but that could still be damaged by relatively modest bending moments that you might apply by hand. On the other hand, most aircraft propellers look pretty beefy. Without actually sitting down and doing an analysis (which is well beyond my ability) I don't know how you would actually answer the question of whether it is possible to damage them by pulling on the tips. Unless Jeff Capes is on hand to tie one in a knot.

[mad_jock]

the centrifugal forces at the root of a 10kg blade at 2700 rpm may be in the region of 50 tons whereas the total thrust from the propeller is only likely to be in the region of a few hundred lbs. The resulting vector will therefore be within a fraction of a degree of the axis of the propeller blade.


Might be a bit more than a few hundred but will still keep keep the vector well inside the blade geometry. Where as the further out you go it quite quickly gets non-linear if its not rotating.

Also as well the whole blade is in tension while rotating and all bending moments will do is reduce one side of it. A static pull will cause compression on some bits.

If the prop is a single unit type ie its bolted on the front and both blades are a single unit it is extremely unlikely your going to do anything to the prop. Fibre props are a different kettle of fish because they arn't very strong in compression and you don't know if the matrix has been crushed with the bending moment.

Some of the bigger props which I call knife props but thats not the proper namr for them have all sorts of handling instructions on them for supporting them off aircraft and also not to use prop stands what ever they are.

[tdbristol]

The rear edges on the composite prop on my aircraft are very fine and taper down to maybe 3 mm thick (just about this thin all along the rear edge, even towards the prop root).
I have personally seen these rear edges damaged when pulling on the prop, by a rampie with rings on his fingers, where the rings rolled over the rear edge when pulling - chipped the rear edge in a few places, requiring repair.
Pushing at the root seems ok [where you are pushing on the wide front edge], but having seen this myself I would never pull on a composite prop (and to save misunderstanding I insist that rampies never push or pull on my prop).

[Crash one]

I would go with MJ on this. The helicopter analogy is a bit extreme but is perfectly relevant. When the prop is rotating, by & large it is generating an even load on both sides of the disc.
Pulling/pushing on one side static is a completely different can of worms, even for a skinny girl.
Who was it said "Give me a lever long enough & a place to stand & I will move the world"?
I, at 75kg, can easily lift one side of a two ton machine tool with a 6ft crowbar. Does that mean I can apply 1/2 ton side load to a prop hub with a blade length of 3ft?
Stay in the middle no problem.