Damage due to pulling/pushing on Propeller
 

Hello everyone!

A few weeks ago I started to work on the ramp at an airport. It is part of our job to pull out small GA airplanes out of the hangar in the morning and put them back in the evening (the pilots are not allowed to do that for some ridiculous reasons I don't know). Of course not every rampy is a private pilot and sometimes I can hardly watch the other guys when they pull on the propeller with one hand in the middle and the other near the end of the blade. Long story short:

Are there any known incidents or accidents due to pulling on the prop? Maybe there's a mechanic in the forum knowing about this.

Thanks!

Er do you mean damage to the aeroplane, or to the person doing the pulling?
;)
H

I mean damage to the propeller or to something inside the hub... ;)

You are very unlikely to do any damage to the aircraft by pulling or pushing the propeller with your hands as long as you use the blade root.

Of course you must make sure that the mags are dead and I would be very careful with a warm engine as there are the most likely to fire if a mag is live.

It was hammered into me in basic training that a hot aircraft piston engine can fire (as opposed to "start") if a cylinder goes over compression regardless of whether one or both magnetos is(are) live. Therefore pulling on a hot prop, or even turning one for any reason apart from starting using the proper technique, was an absolute no-no, dead magnetos notwithstanding.

I always assumed that this would simply be the result of compressing the hot gas until ignition occurs (a degree or two after the spark would have occurred?) because the gas is heated in the hot cylinder to begin with, which sounds plausible to me.

Do not pull near the end of the blade, the prop is at it's thinnest and you have the longest leverage.
It takes some practice but with a hand on either side closest to the spinner you can even steer a SE airplane as you push it back.
DO NOT push on the spinner, depending on make and model it can be fairly easy to bend the spinner back plate which can cause the spinner to (ever so slightly) wobble which will mean cracking over time.
In very extreme (rare) cases it can cause spinner separation in flight.

I forgot to mention that I am a private pilot. :) I am wondering if there are any incident or accident reports that were caused by wrong handling of props. Would be easier to convince co-workers to only pull on the blade root. .

I'd have thought that if you could damage a prop by pulling on it, any use of the engine (a large amount of horsepower being converted into thrust) would destroy it instantly. After all, the aircraft is pulled/pushed through the air by the prop in the first place. What do you think all those wire-locked bolts are for?

I can see the sense in pulling by the hub area, as it's the strongest part with the least mechanical advantage. Also, tying a rope to the tip and towing it with a car isn't the greatest idea.

I think it's one of those things that has grown over time from "take reasonable care when doing this" to "it'll come off in your hands!". There's another one doing the rounds about not checking the compression by hand because you might damage the engine. I hate to think what happens when the starter motor engages... :rolleyes:

If there are no examples of damage - is this actually an old wives tale?

At my club instructors regularly pull the planes by the props when the tow bar isn't handy (but I've always been told to do it at the root). Or is it that damage is likely to not be noticed? Of course, mags are checked and the prop isn't rotated when trying to pull it.

I do note that some prop manuals specifically state that you are never to try and move the aircraft by the prop. I don't know how widespread that limitation is (all the manuals I have looked at say this, eg http://www.mccauley.textron.com/tech_guide.pdf page 10; or section 5B http://www.hartzellprop.com/public_dl.php?id=4)

The most thrust is generated by the propellor tips, as they rotate the fastest, so to suggest that the half horsepower or so that you are able to generate by hand would be sufficient to bend the tips... I don't believe it.

Nevertheless, there is no mechanical advantage to pushing or pulling on the tips, over pushing or pulling on the root, so use the root. Better safe than sorry.

Another contentious issue is whether you are allowed to rotate the propellor so that it's horizontal (and thus clear of the tow bar) or not. And if so, which way to rotate it.

Some engines (Thielerts come to mind) do not like to be rotated by hand whatsoever, because the gearbox now works in the opposite direction. Other engines, including the traditional direct-drive avgas Lycomings/Continentals fitted to most of the GA fleet, can be turned by hand. But I have not seen a clear, official statement from the engine manufacturer regarding the direction.

Some argue that you should always rotate the prop backwards. Reason is that this reduces the chance of the magnetos firing with an air/fuel mixture in the engine. Others argue that you should never rotate the prop backwards, as the ancillary stuff bolted to the engine (such as the vacuum pump) can be damaged by rotating it in the wrong direction.

To be absolutely on the safe side, as a rampie, ask the owner about the correct procedure/rotation direction and stick to it.

Furthermore, three more tips:

Never put a body part inside the propellor arc that doesn't have to be there. And if you rotate a propellor, do it so that if the propellor accidentally moves on its own, your hand will leave the prop arc by its own momentum.

Make it a habit to *always* check the mixture and magneto switches before going anywhere near the prop.

Never leave a towbar attached to the nosewheel assembly when you leave the vicinity of the aircraft. Disconnect it and put it across the nosewheel path as an improv chock. Starting an engine with the towbar still attached may cause major damage, not just to the towbar, but also to the prop and innocent bystanders.

Mixture, Master, Mags. ;) As after every flight.

I think we can't compare the force on a propeller during operation to being pulled by hand. The prop is twisted to produce equal (as far as possible) thrust from root to tip. If you pull on the tip you bend it because you only apply force on this particular point. But I don't know if it is actually possible to bend a blade by hand. I think I just have to live with it that airplanes are being pushed around like this... I can't always be there to correct everybody...:8

I would wager it is possible to damage a prop by pushing from the tip. Or at least, I wouldn't take it for granted that it isn't.

Take the example of a helicopter blade... They also develop most of the lift near the tip, yet depending on the rotor type they are designed to allow movement at the blade root, and 'cone' and flap significantly in flight. They are held rigid primarily by centripetal force and if you let the rotor RPM drop too much, the blades may experience structural failure due to excessive flapping.

So the disingeneous answer is that it's fine to push by the blade tips, so long as the propeller is spinning.

I'm not going to volunteer to be the guinea pig to test that!

Composite prop damaged by pulling
 

You can damage a composite prop by pulling on it, if done badly. My composite prop was damaged on the rear edge of the blades (where they are thin), where a rampie pulling hard caused the rear edge to chip where his stiff gloves twisted/rolled over the thin edge and broke some chips off in a few places. (Had to get repairs done.)
So I never push or pull by the prop, and insist that any rampies don't do so either.

This old one again :)

It is hard to say it is never possible, but traditional ally props are very strong. At the max rated RPM the stress in the metal is roughly 25% of the tensile yield point - many tons for a normal sized GA prop. This indicates the required strength of the hub too....

There is I suppose a possible weakness in that during operation the blades "come out" a little and compress the tapered roller bearings in which they sit, ensuring that any clearances are taken up. During ground movements, this doesn't happen.

But if you saw the internals of say a Hartzell CS prop, you would realise that not even Geoff Capes could do anything to it, by orders of magnitude. They are massive.

Backplate/spinner damage is much more likely and is very common, and becomes evident long after the event(s). The problem is that if somebody is sufficiently unclued-up to push on the spinner, they are probably pushing on the cowling and cracking it, and elsewhere too.

Really? I thought blade twist was there to keep it fairly uniform along the blade, but may be wrong!

You are correct Halfbaked, in addition there are tip losses at the tips, the distribution is almost elliptical like on the wings

İ met a very experienced instructor, a former champion who refused to use the tow bars and always used the prop for pulling and pushing, the prop he had was composite and 3 blades prop , now i am doing the same

Once upon a time us pongos were told all about "blade sailing", as an awful warning never to go under a rotating rotor unless/until signalled by the pilot who, presumably, would do so only when the rotor speed was enough to avoid decapitating the entire load? I always understood that he would keep a little upward load on it as well to make sure it would not flap downwards.

And even then we kept our heads down as we ran from the 2.00 o'clock position to the door, for no rational purpose. I've noticed that people still do that.

Back to the thread..........

Helicopter blade sailing.....

There are numerous recorded instances of the helicopter blades chopping off the tail, smashing into the cockpit roof (see http://www.pprune.org/rotorheads/455...astonbury.html post 4 for a picture) and there is a Youtube video of a US military CH-53 helicopter chopping off the refuling probe with his rotor after a little "rough handling"
You may not have to duck when moving under a helicopter rotor, but each person can only make a wrong call once.

The twist in a prop blade is to keep the AOA of the blade something sensible along the length, rather than manage lift distribution per-se. The tips are travelling 'round' a *lot* faster than the roots, whereas both are travelling 'forward' at the same speed. Consequently the tips 'see' the airflow coming from a rather different direction.

As for pushing, I don't believe it. Think for a second what forces go through a prop when you're manouvering with a lot of power on. The only scenario I can see is that the composite/wooden props on some of the microlight/LSA types could be susceptible to (trailing edge) damage perhaps, just through rough handling? Spinners certainly fragile, but in most cases I'd be more worried about people pushing on inappropriate airframe parts than the prop.