YellowHawk

Is there any particular reason why there is no lock washer under the prop nut? A few times I have had it come loose...not in the air yet. I put a lock washer under it today because to me it just seems insane not to have one there. On a real aircraft you never install hardware without some locking feature. I was just wondering if there was some reason that I am not aware of that they don't come with one. I know that even if it comes off in the air you can still glide the aircraft in and land it but seems like it would be preferred that it not come off at all. If there is some reason for not using one, I will remove the one I put on.

HighPlains

My Feedback: (1)

They are not used because they are not needed. If you are throwing props, it is a sign that you are doing something wrong.

So from the description of on happening in the air, I would get it happens when you are starting. It is very easy to over prime an engine and when you put the starter to it, the engine kicks back and you loosen or remove the prop entirely. Is this what happen?

stevenmax50

My Feedback: (1)

I have thrown a prop during starting that was not primed at all. It is a .91 4 stroke and I never prime it since I use a starter. And it has the prop nut and the lock nut from Tru Turn for the aluminum spinner. Yet it has come loose at least 3 times during starting last season. So I thought the lock nut question is very valid. I'd like to hear more opinions. So long as it doesnt affect engine balance, why not? Just as an FYI, Iam a 30 year A&P so Iam pretty sure of my installation techniques. Yet obviously something is not right.

OzMo

My Feedback: (3)

All fuel engines have a detonation point in the cycle ( timing). If this is off do to a bad setting, to rich, to lean or wrong plug it can cause the prop to become loose.
So can not having it tight enough in the begining. A lock washer digs into the material around it causing damage to those surfaces...not a nice thing to do to your prop!

proptop

My Feedback: (8)

What type (material ) props are you using? All props will take a set after a while, and then the prop nut will need re-torquing. (especially when using wood props )

I have noticed, over the years, that many beginners don't get the prop nut quite tight enough. Don't be afaid to tighten that sucker down good 'n tight...the crank threads and prop nut are hardened Steel...they can take it.

I've never had the need for a lock washer...or loc-tite, or anything like that. I would think that the compressability of a lock washer just might make things worse?

Villa

After a few props got lose on me at starting(a few also came off) I concluded I was not tightening them enough. I had always used the small prop cross wrench also used to tighten the glow plug. I believe most people use this wrench to tighten the prop also. I have a very large and strong hand but I was still not tightening the nut enough because that wrench is too small for tightening the prop nut. Using a torque wrench occasionally is not a bad idea. Of course, I do not know what torque to tighten it to. Now I use a 10" adjustable wrench. An 8" is the size I would purchase, but I had an extra 10" wrench so it is now part of my flight tools. I tighten it very tight. I do not recall having a problem since.

oldtyme

Robart makes a GREAT!! prop wrench. It has sizes in it for almost any prop nut you could imagine and both in SAE and Metric. I keep two of them in my field box for when you have a setup like in some OS engines where there are two prop nuts.

YellowHawk

I am sure I probably didn't have it tight enough. It came loose when starting. I had that problem before and ended up having to really tighten it. More than I really felt comfortable with. Seems like a simple lock washer would solve the problem and keep the nut from loosening. Anyhow, I put one on it today and flew it. It worked just fine.

sir_flies_alot

Use a 6 inch wrench on your prop nut, and torque it down.  it is hardened steel, I've never stripped one, nor have I had one come off.

YellowHawk

Just seems like it shouldn't have to be that tight. I may have to remove the lock washer because I just ordered an aluminum spinner and I am not sure how that goes on yet. When I get it I may remove to lock washer. The lock washer didn't seem to cause any problem with the aircraft today. It did keep the nut from spinning loose without having to tighten it with a cheater bar.

downunder

The main reason is because it shouldn't be necessary. When the engine is running the firing impulses act to tighten the prop nut. It's only when starting an engine that the torque involved tries to loosen the nut but so long as the friction between the parts holding the prop in place is high enough then the prop won't loosen. Not all engines have hardened crankshafts or prop nuts so it's probably wise to assume they don't and just use the torque needed to tighten an unhardened nut which is about 7 ft/lbs for a 1/4" nut. Spinners can sometimes cause props to come loose when starting because you have a hard plastic prop against smooth alloy which doesn't give much friction but a cure for that is to make a washer from wet or dry paper to increase friction.

jester_s1

The only issue I can think of, aside from simply not needing them if you're tight enough, is that they could give the impression that the prop is tight enough when it's really not. Then instead of spinning the prop or throwing the prop on startup you do it at high RPM.

Downunder mentioned making a washer out of sandpaper to add friction, but I have something even better. Get some open mesh sand cloth from the plumbing section in your hardware store. It's rough on both sides, so any two things you clamp it between will not move. Since friction is what actually holds your prop in place, the open mesh sand cloth really makes a difference in how much torque you need.

HighPlains

My Feedback: (1)

Throwing a prop in the air mostly happens with 4 cycle engines, and is an indication that you are running slightly lean. When it happens to a 2C engine, it is usually because the engine kicked slightly when it was started.

With composite props, it happens less often because of the stability of the material. Wood props are prone to changes in dimensional thickness of the hub due to change in moisture contend, so they need to be check more often.

landeck

My Feedback: (1)

YellowHawk, are you using a spinner with a plastic backplate? Those have been known to cause engines to throw props. If you are using one switch to a spinner with an aluminum backplate and really torque down on the prop nut. That should not come off.

Bruce

ameyam

Actually, I had a couple of props come loose while starting the engine. One came off during flying when my engine seized after a crank pin failure but nothing much else.

Earlier I used to use a 4-way spanner to tighten the nuts on the 55ax. However, on the 75ax and fs91sII, the nuts were larger and I couldnt use the 4-way. Then I broke the spanner socket on both my 4-ways and began to use an adjustable spanner. Though this is more secure, I found that, inspite of adjusting tightly to match the nut size, it slipped several times and finally rounded the nut. I had custom nuts made for the 55ax (the crank doesnt have standard threads) which were a bit smaller so that I could use a locknut but the problem of slippage is still present. As yet I dont see a way around this issue

Ameyam

OzMo

My Feedback: (3)

YellowHawk,
Loose the lock washer please. If it damages the prop hub it can fail and throw a prop at high RPM. NO fun at all when that happens
Also excess fuel can get into the cylnder and cause similar problems. Pull the glow plug and disconct the fuel line and (cover the plug hole with a paper towel )spin the engine well
making sure no fuel build up in the muffler as well. Always disconect the exhaust presure line when fueling to avoid this fuel problem.

You have probably already done his but remove the mold flashing from the prop. Ballancing the prop is a good idea too, it reduces vibration.
I read on this site years ago that a wide open 40 with a 10/6 prop has tips moving at the same speed as a 45 calibur bullet.

HighPlains

My Feedback: (1)

Interesting claim, with some merit. However the speed of a .45 bullet varies all over the place, from low 800ft/sec to as much as 1150 ft/sec. A very healthy Jett Engineering .40 can turn a 10-6 APC at 16,500 giving a static tip speed of only 720 ft/sec.

So I guess the .45 bullet still wins, and the old saying about never taking a knife to a gun fight continues to be true.

OzMo

My Feedback: (3)

Hi HighPlains,
WOW you actually did the math? OK so I can change the statement to "almost as fast". I never checked, and fingers don't look like they know the differnce afterwards[:@]

HighPlains

My Feedback: (1)

Yeah, a couple of clicks on the HP calculator. Finding out what the bullet speed of a .45 took longer, and Dub Jett's engines are the most powerful sport engines on the planet (worst case scenario). Most RC engines fall far short of a Jett in terms of power and rpm, so it is not even close.

The slowest bullet will be 40% faster than the prop tips on a typical RC engine, and the fastest .45 about double the speed.

While APC props are certainly as sharp as a knife, well..............

Villa

Hi ameyan
I suggest you determine the thread diameter and number of threads per inch for the engine on your plane and look up the chart on http://www.clevelandvibrator.com/vib...bolttorque.pdf . If it is metric Google the same chart but for metric. Column #6 gives you the torque to tighten the nut to. I have assumed a Grade 5 bolt as being representative of the tensile strength of the engine crankshaft threaded section. It could very well be stronger, so do not take what I am offering as gospel. It is a LOT more than most people tighten the nut to. If you don't have a torque wrench get a wrench long enough to allow you to hang a weight on it equal to the number you get from column #6 above, at exactly 12 inches from the center of the bolt. The wrench must be horizontal so the weight pulls vertically down. You would be surprised how many mechanics who use a torque wrench in their daily work do not understand what I just described.

HighPlains

My Feedback: (1)

I might agree to a torque value if we were clamping metal. But you can split a wood prop at the hub by over torquing. Plastic props also undergo a bit of material flow if over torqued.

Villa

Hi HighPlains
You are absolutely right, over torquing is bad. It is bad when clamping metal plates, or our props. How do you prevent over torquing, but also use the correct torque to prevent problems? Interesting. Who should supply the torque #? Maybe the prop manufacturer. I'm not suggesting anything.

HighPlains

My Feedback: (1)

That is the sticky wickit. Over time you develop a feel for it, but to my knowledge nobody has addressed this problem. What with different materials in props, spinners, and engine drive systems, there may be no definitive answer. I only use a small 4 way wrench for most glow engines, and depend on good technique to avoid the problem. The techniques differ slightly between 2C and 4C engines.

On 2C engines my technique is to avoid putting a starter to the engine until I feel for the bump. When it is bumping correctly then it starts with no problem.

For 4 stroke engines, always set them with a tach about 300 down from peak so they don't detonate and toss the prop in the air.

One thing for sure. If you are about to help a new flier with their airplane, check the prop (if on the off chance they actually set it level when coming up to compression).

Villa

Hi HighPlains and others
On the OS engines I use, a special hardened and tapered washer is supplied/used, against which the prop nut will bear and transfer the nut load to the prop. The washer is of a generous diameter so that the pressure the nut produces is distributed over an area sufficiently large enough to protect the prop from being crushed. The washer is of a sufficient thickness, strength, and hardness, to prevent it from deforming under the action of the nut being torqued. All of this, of course, is my opinion, since I did not design the engines or props. If you do not use the special prop washer I described, you can expect problems. An ordinary mild steel flat washer is absolutely worthless as a replacement for the special washer supplied with the engine. When I wear out or destroy an engine, I always save the special prop washer. I would not be surprised to learn that the prop nut can be torqued sufficiently high enough to shear the crankshaft or prop nut, without damaging an APC prop. Please do not do that.

jester_s1

Torque isn't the only part of the equation either. That point of the torque is to create enough friction so the prop doesn't slip. It really doesn't take a lot to do that if you're tightening against the drive washer with those grooves in it, or against a metal backplate made the same way. But if the surface is smooth or plastic, a piece of the open mesh sand cloth makes a decent substitute for the grooved metal surface. Villa is 100% right about using the factory supplied washer too. The farther out on the prop hub the torque is applied the more effective it is at holding the prop on.