View Full Version : Damage due to pulling/pushing on Propeller
24th Jun 2012, 18:00
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.
24th Jun 2012, 18:16
Er do you mean damage to the aeroplane, or to the person doing the pulling?
24th Jun 2012, 18:23
I mean damage to the propeller or to something inside the hub... ;)
A and C
24th Jun 2012, 18:37
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.
24th Jun 2012, 19:06
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.
24th Jun 2012, 19:26
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. .
24th Jun 2012, 19:53
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:
24th Jun 2012, 20:30
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)
24th Jun 2012, 20:36
Do not pull near the end of the blade, the prop is at it's thinnest and you have the longest leverage.
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.
24th Jun 2012, 20:52
Make it a habit to *always* check the mixture and magneto switches before going anywhere near the prop.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.
24th Jun 2012, 22:13
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!
24th Jun 2012, 22:18
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.
24th Jun 2012, 22:45
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.
24th Jun 2012, 23:39
The most thrust is generated by the propellor tips, as they rotate the fastest
Really? I thought blade twist was there to keep it fairly uniform along the blade, but may be wrong!
25th Jun 2012, 05:13
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
25th Jun 2012, 08:52
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. 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..........
25th Jun 2012, 12:25
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/455060-helicopter-crashes-field-glastonbury.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" http://www.youtube.com/watch?v=tO0sRWCf9k4
You may not have to duck when moving under a helicopter rotor, but each person can only make a wrong call once.
25th Jun 2012, 13:31
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.
I always understood that he would keep a little upward load on it as well to make sure it would not flap downwards.That may be true, though I believe naval helicopter pilots sometimes use downthrust after landing on a moving deck.
My favourite youtube rotorblade clip:
25th Jun 2012, 21:37
Could not understand a word!
26th Jun 2012, 09:50
Yes you can bend an ali prop and damage a composite prop by pulling on it. Prop shops do it all the time. It's called cold working. Looks like there's a lot of opinion here so without getting all technical I'll add this.
I've seen damage to both kinds of prop. Ali and composite.
Props are under incredible load during different phases of engine operation. Granted. But they are specifically designed to carry those loads.
If you point load a prop blade in exactly the right spot with enough resistance + force, you will bend it.
But most things people have said ring true and if you stick to your common sense, you'll never have a problem. ie; Don't pull a 421 out of the mud by a single blade tip and take extra caution with wooden or composite blades. Which are easily damaged by mishandling. I've pulled 3 composite props this year that have been damaged without the engine running.
26th Jun 2012, 10:17
Fascinating clip in abgd's post.
Thread drift again, sorry....
Is it just coincidence that - in that clip at least - the blade flaps downwards, fairly consistently, as it passes the tail rotor? I can't think of a reason why it would do that.
26th Jun 2012, 10:59
I've pulled 3 composite props this year that have been damaged without the engine running.
You need to be a bit more careful then!
It does show however that this isn't an old wives tale.
I have seen elsewhere mention of not to pull "wobbly" props as this can damage the hub. Does anyone have any opinion on this (and isn't it difficult to tell for most non-engineers whether a prop is variable pitched or not without looking at the controls?
26th Jun 2012, 12:47
I always imagined propellers as "flimsy" and worthy of delicate handling until I had the opportunity to see one NOT attached to the front of the aircraft. I'm talking about a C172 propeller - nothing fancy. When it is leaning against the hangar wall it transforms from my little propeller to become HUGE. It is built like a girder, but beefier, and I can hardly lift it, because it weighs so much.
Suddenly it went from being the most fragile part of the aircraft to the toughest. So, if you are looking for a part to push or pull, don't choose the cowling, or the spinner, or the strut, or the wing - grab hold of the propeller instead!
26th Jun 2012, 13:32
levers and application of force are not obvious to the majoirty of the population.
I did a consultancy job for a court case for a police force that was sueing a stiff cuff manufacture for supply faulty goos which wern't fit for purpose.
BAsically they were bending and failing while making an arrest.
So we did material tests on them which proved that the metal that they were made out of was over spec'd for the contact.
We put them in a the testile puller under various conditions using scafolding poles as the levers. Again passed with flying colours compared to the spec and contract agreement.
Put them in the hands of one Her Majestys finest drug enforcment officers who has years of practise subjecting people to the maximum legal amount of force as required tackling a druggie who is out of there tits they bent like a twix in the tropics.
With the forces involved we couldn't understand how the person being arrested arm was still on afterwards never mind not broken.
We got asked to foxtrot oscar and they lost the case 9 months later. If they had asked us we could have told them that there specs were wrong and that all they needed to do to the current ones was super glue a metal spacer to torsionally stiffen the cuffs and you could have rolled a car over using its towbar hook using one of them.
Its all in the application of force not the amount or how strong the thing looks that your apply it to. If its not been designed to take a load in a particular way you can break it.
26th Jun 2012, 14:04
I've pulled 3 composite props this year that have been damaged without the engine running.
How (precisely) was it damaged?
26th Jun 2012, 14:22
I would have presumed Peter the matrix will have started delaminating around the stress point down at the hub. As soon as it does that it will start wicking moisture and bit of a freeze a prop failure here we come.
Always did think that making props out of composite was asking for trouble.
A and C
26th Jun 2012, 16:38
I have to totally disagree with you about composite props, the advantages outweight the disadvantages.
As this thread is about damage the main advantage is that you can repair a composite prop and return it to its new size & shape.......you can't do that with a metal prop.
26th Jun 2012, 19:44
I will bow to your experence fixing them.
My none aviation experence of comporsite structures were they were a royal pain in the arse when damaged both fixing them and proving that they were still fit for purpose. But this was in applications which wern't weight sensitive and throwing a couple of tons of steel at it might piss the product design mafia off but wouldn't result in any real penalties.
Has anyone ever made a titanium prop?
26th Jun 2012, 22:51
I have seen props used for pulling aircraft for years, though personally I wouldn't do it to a wobbly. Jemima has a composite ground adjustable prop however since she is a pusher and the prop isn't accessible for pulling or pushing I had not really thought about it for a long time.
I can see how a composite prop might be susceptible to damage even though designed to be robust enough for flight.
Jemima has an IVO 3 blade. Each blade is held by two bolts and goes about 3 inches into the hub behind the front plate. The prop is hollow too with a steel rod down it for adjustment., With the right hub it can be adjusted in flight by twisting along it's length by twisting the rod at the hub. (So the hub end is not just solid composite material.)
During flight there is a strong centripetal force which might mean that the prop is 'advancing' and applying force at the hub but not a leveraging force, (ie, it works like a helicopter blade.) When stopped and a load is applied to the prop the force at the hub would be all leverage and the point of the hub in contact with edge of the front plate could receive a load several times the actual load applied, say, 10 or 12 inches along the prop.
Not saying it could not take it. Just thinking out loud
26th Jun 2012, 23:18
There is a huge difference (well just times it by how far out you put your hands)
Between hauling on the thing next to the root and pulling at the tips.
One hand either side of the hub and close in won't do any damage.
The further your hands are out away from the hub the more chance you have of damaging something. Also as well pulling only one side ain't to bright either.
And that goes for metal, wood and any other material that takes your fancy.
A and C
27th Jun 2012, 07:11
Composite structures are a very different thing from metal and this is a big problem within the industry, a lot of the major players within GA have a very worrying lack of knowlage and ability when it comes to composites.
Almost all of the Daimond and cirrus approved maintenance shops in the UK are not capable of all but the most minor repair work, this leads to the aircraft becoming expensive to insure as the aircraft get written off for damage that could be fixed. One example is a DA40 that was scrapped on the south coast simply because the maintenance company had no idea that the damage was repairable, the aircraft could have been back in the air within a month or so had it been looked at by the right people. (NOTE this sort of damage would be repaired IAW a manufactures repair scheme ). Unfortunatly this also is true of parts, things that should be repaired are being scrapped due to lack of Repair knowlage within the industry and it is the owners that are bearing the cost!
The knowlage of the long term durability of composite structures is mostly to be found in the renewable energy industry as these structures are having to last up to thirty years with minimal inspection, I know of one aviation based company that is in the renewable energy sector and they have fatigue numbers for composite water turbines that say the turbines are good for thirty years running. The boss of this company is of the opinion that compared to water turbines aircraft get a very easy life with lots of inspection and so are far easier to maintan safely.
27th Jun 2012, 07:17
The knowlage of the long term durability of composite structures is mostly to be found in the renewable energy industry
The gliding sector also has a lot more experience with repairing composite structures. In fact, whereas most powered composite aircraft are still built of glass fiber, most high performance gliders will have a substantial amount of carbon fiber content since it's even stronger and lighter than glass fiber. But reportedly even tougher to work and repair.
A and C
27th Jun 2012, 08:49
You are quite correct in saying that the gilding sector has a lot of experience with composite structures, Unfortunatly almost no one from this sector has taken the step of getting the EASA145 approvals enable them to repair whole aircraft.
In terms of calendar life with composite structures the gliding industry has some experience but due to the nature of gliding the airframes don't do a lot of flying hours, this has to be contrasted with some water turbines ( the blades are about the size of a GA aircraft wing) that will operate underwater and have an expected fatigue life of 250,200 operating hours.
I would be interested to know the number hours that the highest time GRP glider has flown.
27th Jun 2012, 08:57
A and C, all true. The only thing I can add is that gliders do have a lot of cycles (starts/landings), which arguably puts more stress on the airframe than flying hours.
And I think gliders suffer minor (repairable) damage more often than other aircraft, due to hitting stuff during outlandings.
27th Jun 2012, 09:27
Its a big problem in every industry that uses them.
I would say that the marine side of things has more experence than the rest put together. And I know a couple of lads from sailing who have been pinched from boat yards to work on wind turbines etc.
But still to this day I believe that most repairs are in the hands of god and also the person that fixes it. The anal attention to detail of the fixer comes into play more than anything else. The same of course is the same with fixing metal structures but at least with them you can NDT them and carry out other checks to prove that they are fit for use.
Unlike the comporsite repairs where you have to trust that it has been carried out in exactly the right temprature, right humidity, there isn't any contamination of the matrix such as hairs or dust etc , the weave direction has been aligned exactly as per requirments (may need a full engineering analysis because it may be different to the original) Then you have issue with the interface between two matrixes and not actually knowing whats been damaged you can have sections where the fibre has gone but the matrix looks fine and you wouldn't have a clue.
I know I am an ex engineer that mainly worked with metal but the state of affairs when I stopped working as an engineer was that the compersite side of things was at the same level as pre liberty boats and the Comet with metal. You asked any of them if they could prove anything and they would start talking statistical proberbilitys of failure.
I know things have improved since then on the production side of things but the fixing them still isn't a science in the same way as fixing metal is.
A and C
27th Jun 2012, 19:25
You do have a very good point about quality control with composite structures, we inspect the job at each stage to make sure that all has been put together correctly, the post cure temperature and time are monitored by two independent systems, a sample of the resin is kept for analissis and if it is a spar boom we are repairing a test sample of the rovings and resin are made as the spar is being assembled. This test sample is tested in the same rig at the manufactures as they test samples of new spar booms.
Composite quality control is not a problem if it is done at the time of assembly but is as good as imposable once the assembly has been compleated, this is unlike most metal repairs were it is clear if a repair is not up to standard just visual inspection.
27th Jun 2012, 19:55
Yep would agree with that.
I would also say as a pilot I can visually inspect a metal prop and "not flying that, its fecked"
Compersite prop you wouldn't have a clue until it failed for 50% of the time. All because some plonker was bouncing the aircraft through a rabbit hole holding onto the prop tips.