Prop Crush Plate?

[bashibazouk]

Does an aluminium prop require a crush plate? I don't see anything about it in AC43-13 or maybe I just didn't look in the right place.

Josh Brehm
NC24369
Winterville, NC

[mikeg]

Re: Prop Crush Plate?

Does an aluminium prop require a crush plate? I don't see anything about it in AC43-13 or maybe I just didn't look in the right place.

Josh Brehm
NC24369
Winterville, NC

Crush plate is used only with a wood prop.
Mike

[Guest]

Re: Prop Crush Plate?

I use one with a metal prop because the bolts are too long without it.

[Hank Jarrett]

Re: Prop Crush Plate?

I'm not a prop guy so take this as a question only.
Isn't the metal prop designed to carry the thrust load from the base of the counterbores in the prop hub through those tube spacers that go over the bolts? The bolt heads and washers aren't supposed to bear on the face of the prop hub like the crush plate does on a wood prop, they carry the load to the middle of the prop (front to back). If you use a crush plate on a metal prop aren't you putting all the load on the face of the prop?
Might want to check with the prop manufacturer and make sure you aren't damaging the prop. Whatever you find out, let us all know. I have both a wood prop and a metal one and if I can use the crush plate and longer bolts on the metal prop it would be nice. Those prop bolts are EXPENSIVE!
Hank

[VictorBravo]

Re: Prop Crush Plate?

Our own Terry Bowden, "Barnstormer" will be able to answer this question with authority...

[Guest]

Re: Prop Crush Plate?

I'm not a prop guy so take this as a question only.
Isn't the metal prop designed to carry the thrust load from the base of the counterbores in the prop hub no through those tube spacers that go over the bolts? no, they are merely spacers and for the most part useless, take a look at their fit sometime, they are too short to fill the hole to make a nice fit on a wood installation, I suppose on the metal installation they fit up the diameters ok, they rattle in the holes.The bolt heads and washers aren't supposed to bear on the face of the prop hub why not? like the crush plate does on a wood prop, they carry the load to the middle of the prop (front to back). If you use a crush plate on a metal prop aren't you putting all the load on the face of the prop? yes, that provides the needed fore and aft force, friction is not enough, unbolt a metal prop from a flanged hub and you can take off yourself. Might want to check with the prop manufacturer and make sure you aren't damaging the prop. Whatever you find out, let us all know. I have both a wood prop and a metal one and if I can use the crush plate and longer bolts on the metal prop it would be nice. my airplane has been set up like that for about 35 years that I know of and no problems at all. Those prop bolts are EXPENSIVE!
Hank

Hi Hank, comments in blue.

[Hank Jarrett]

Re: Prop Crush Plate?

As I said, I'm NOT a prop guy, so they are just questions.
I have a metal and a wood prop and with wood I know you need to put some "squeeze" on the wood with a plate to distribute the load.
I don't think the tubes over the bolts are just "spacers". It would be so much cheaper and simpler to just drill straight through and not do the expensive counter bore with that smooth radius at the bottom. If you were trying to pass the prop bolt load through the neutral axis of the prop you would want a counter bore and the spacers shouldn't be tight in the holes (you want all of the load to pass into the prop at the base of the counterbore). All that extra machine work and the spacers are just too much extra cost to not be there for a reason. I have never seen a spacer tube on a wood prop, but the material properties of wood would be really bad if you did. I would expect the prop to delaminate at the base of the holes.
Every fixed pitch metal prop I have worked with bolted to a flange keyed to the crank. Every wood prop I have worked the prop was mounted to a metal hub carrier. The carrier was fastened to the crank. (As a disclamer, the only props I have worked on were on a BIG turbo prop on an E-2C, that had NO relationship to our planes, and the metal and wood props on my Taylorcrafts. I'm NOT a prop expert, but I DO claim some expertise in materials and engineering).
I would still like to hear from a prop designer/expert. Neither of us qualify as the final word. I found things on my 41 while tearing it down and putting it back together that had been there for decades. A couple of them could have distroyed the plane at any time. Just because it has been there for a long time, doesn't make it safe.
I wouldn't want anyone to get hurt if we can do something to stop a failure. Sometimes fatigue can take years or decades.

Hank

[Hank Jarrett]

Re: Prop Crush Plate?

Makes sense on a wood prop. You can't apply the engine torque at the neutral axis on a wood prop because there would be a shear load between the laminations
I have seen a wood prop that was used with insufficient pressure on the front plate and there was actually burning between the laminations at the back from the layers rubbing back and forth with the engine pulses. The only way to get the engine torque pulses into the wood prop would be with a front and rear plate squeezing the wood between them, just like our props work.
Since a metal prop isn't laminated, it makes sense to apply the torque in the middle to eliminate the rocking effect of through bolts.
Fatigue isn't a problem for wood so the rocking load wouldn't be a big problem but slight shifting between the laminations would be deadly.
Hank

[Guest]

Re: Prop Crush Plate?

As I said, I'm NOT a prop guy, so they are just questions.
I have a metal and a wood prop and with wood I know you need to put some "squeeze" on the wood with a plate to distribute the load.
I don't think the tubes over the bolts are just "spacers". It would be so much cheaper and simpler to just drill straight through yes, and they used to do that but after the advent of flanged hubs which require the counterbore they now counter bore all wood props because its cheaper since the prop will work on both installations and not do the expensive counter bore with that smooth radius at the bottom. If you were trying to pass the prop bolt load through the neutral axis of the prop you would want a counter bore and the spacers shouldn't be tight in the holes (you want all of the load to pass into the prop at the base of the counterbore). All that extra machine work and the spacers are just too much extra cost to not be there for a reason. I have never seen a spacer tube on a wood prop, sensenich recommends that you use them with a wood prop and sell them too, they used to be quite adament that you use them, or at least they did 10 years ago but the material properties of wood would be really bad if you did. I would expect the prop to delaminate at the base of the holes. I don't follow here can you elaborate?Every fixed pitch metal prop I have worked with bolted to a flange keyed to the crank. Every wood prop I have worked the prop was mounted to a metal hub carrier. The carrier was fastened to the crank. (As a disclamer, the only props I have worked on were on a BIG turbo prop on an E-2C, that had NO relationship to our planes, and the metal and wood props on my Taylorcrafts. I'm NOT a prop expert, but I DO claim some expertise in materials and engineering).
I would still like to hear from a prop designer/expert. Neither of us qualify as the final word. I found things on my 41 while tearing it down and putting it back together that had been there for decades. A couple of them could have distroyed the plane at any time. Just because it has been there for a long time, doesn't make it safe.
I wouldn't want anyone to get hurt if we can do something to stop a failure. Sometimes fatigue can take years or decades.

Hank

Hi Hank, comments in blue again

[Guest]

Re: Prop Crush Plate?

Hank, your first post about prop hubs refers to thrust load, just checking, did you mean thrust or torque because your later post mentions torque? Dave

[Buell Powell]

Re: Prop Crush Plate?

This is from the Sensenich Installation,Operation, @ Maintenance for wood props "It can be shown that an engine must deliver its driving torque to a wood propeller through static friction. That is, the force which resists movement of the propeller hub on the engine flange is due to compression of the wood against the flange. Therefore is is important to compress the wood to maximum during installation,but also important to avoid crushing the wood. Although the drive bushings incorporated on most flanges provide a back-up system, a load will imposed on them only if there is a movement of the propeller on the flange. The bushings can carry engine driving-torque loads for a short period of time.

[Hank Jarrett]

Re: Prop Crush Plate?

Thrust loads are passed along the prop shaft to the front bearings, then the case, then the engine mounts and on to the airframe. They come from the lift generated by the spinning prop blades. The prop blade thrust load (which tends to bend the blades forward) is the source of the thrust. The blade problem comes in when they bend forward. The front laminations are in compression while the back ones are in compression. This builds a shear load between the laminations. On a wood prop that would tend to cause delamination and rubbing between the now not glued pieces of wood. On a metal prop you would get bending fatigue. The fatigue loads would explain why there is such a generous fillet at the root of the blades. If a metal blade was shaped like a wood one at the root, the blades would fatigue and crack at the root. It also explains why you want the load to be applied in the middle of the blade (front to back). That means there is only half the distance from the load application to the tension extreme fiber. That would really increase the fatigue life.
Torque isn’t really the problem. It’s torque pulses. Torque pulses tend to bend the blades back and forth (trying to rotate the prop in a series of pushes). Again, this would tend to shear the wood prop. Lucky for us the laminations are made front to rear so there are no fore and aft glue bonds to fail. This is why wood props do so well on the old rough running engines. It is also why metal props don’t have any sharp discontinuities on the leading or trailing edges (and why the FAA is so critical about nicks in the edges of metal props). A small nick in the edge of a metal prop is a stress concentration area and a good place for a crack to start.
I’m learning a LOT about props lately. I’m amazed how much of my fracture mechanics, materials and stress analysis applies to props. I still don’t know what the effect of using a wood faceplate on a metal prop would be. I need to get with my IA and pull my prop off the hub and look for the counterbores in the wood. I don’t remember seeing that on mine (although for all I know my prop could be original from 1941!).
The prop I saw with the burns was one from a LeBlond and had a beautiful spinner back plate carved as part of the prop. The piece of wood for the back of the spinner had sheared from the aft blade lamination and had rubbed till it was charred. Made a great wall hanger, but would never fly again.

Hank

[Guest]

Re: Prop Crush Plate?

Well this could be fun. I am thinking only of tapered hubs here and the stress from torque and not blade flex. Engineers and interested parties please throw darts at this and check my arithmetic, we'll see if this flys.

I looked in ANC-18, it's the wood design handbook for aircraft back in the 40's. Shear strength parallel to the grain for Ash, birch... is often in the 1000-1500 psi range, some species drop to 900. I think these are common prop woods.

The same manual states that the perpendicular to the grain strength is about 1.5 times that of that parallel strength. Use the lower of the two.

I am too lazy to walk down and measure a hub but I estimate that its 8" o.d. and 3" i.d. so I come up with an average effective radius (therom of Pappus) of 3.7 ".

[pi * (4**2 -1.5**2)= pi * 13.75, effective radius = sqrt(13.75)=3.7]

A 65 hp. Cont. at 2300 RPM should produce an average torque of about 150 ft-lbf. (average torque). I used a formula on the web and did not check the units conversion, it was torque=hp*5252/rpm.

When I apply that torque and resolve it to a force at 3.7" or (3.7/12 feet) I come up with a force of 150 ft-lbf / (3.7/12)ft = 486 lbf.

That force is spread in shear over the hub area (pi * 13.75 sq-in) or 486/(pi * 13.75)~= 12 psi. Pretty low average shear stress. (is there are error here that I don't see?)

That's only the average, power pulses will spike it to a higher number but assuming I did this nearly correct it seems like shear stress is a low priority.

In terms on friction coef. it is more interesting, I only found one data point and that was for sliding coef. of cast iron on oak was about .5. I interpret that in this way; assuming the face plate carries no torque load then the one needs to impress up to 972 lbf of compressive force on the hub (2*486). That means that each bolt must be tightened (pre-loaded) enough to compress the prop by about 150 lbf . Again these are average numbers and need to be increased for the pulse factors.

I suppose that the bolts and holes could easily carry bearing loads to support a total load of 486 lbf if the load was static, the dynamic nature of it would make things loosen and elongate, perhaps the friction force provides a big dampening effect there.
I want to look up the bolt bearing load allowable later, I am curious.

Now I am thinking about flanged crank hubs with those big lugs in them. The counterbores are associated with the flanged cranks but it's not clear to me if they reduce the need for friction. I suspect that they do not. But I guess that depends on how the prop fits. In other words how much the prop can rotate on the hub with the hub held in position when the bolts are snug but not pre-loaded.

Dave

[Guest]

Re: Prop Crush Plate?

I woke up this am and realized that I made and error in find the effective radius. Corrections in blue & red.

Well this could be fun. I am thinking only of tapered hubs here and wood props. Engineers and ineterested parties please throw darts at this and check my arithmetic, we'll see if this flys.

I looked in ANC-18, it's the wood design handbook for aircraft back in the 40's. Shear strength parallel to the grain for Ash, birch... is often in the 1000-1500 psi range, some species drop to 900. I think these are common prop woods.

The same manual states that the perpendicular to the grain strength is about 1.5 times that of that parallel strength. Use the lower of the two.

I am too lazy to walk down and measure a hub but I estimate that its 8" o.d. and 3" i.d. so I come up with an average effective radius (therom of Pappus) of 2.75"".

[pi * (4**2 -1.5**2)= pi * 13.75, effective radius = sqrt(13.75)=3.7] <=error

effective radius= pi * 13.75/(2*pi*(4-1.5)) = 2.75"

A 65 hp. Cont. at 2300 RPM should produce an average torque of about 150 ft-lbf. (average torque). I used a formula on the web and did not check the units conversion, it was torque=hp*5252/rpm.

When I apply that torque and resolve it to a force at 2.75" or (2.75/12 feet) I come up with a force of 150 ft-lbf / (2.75/12)ft = 654 lbf.

That force is spread in shear over the hub area (pi * 13.75 sq-in) or 654/(pi * 13.75)~= 16 psi. Pretty low average shear stress. (is there are error here that I don't see?)

That's only the average, power pulses will spike it to a higher number but assuming I did this nearly correct it seems like shear stress is a low priority.

In terms on friction coef. it is more interesting, I only found one data point and that was for sliding coef. of cast iron on oak was about .5. I interpret that in this way; assuming the face plate carries not torque load then the one needs to impress up to 1308 lbf of compressive force on the hub (2*654). That means that each bolt must be tightened (pre-loaded) enough to compress the prop by about 220 lbf . Again these are average numbers and need to be increased for the pulse factors.

I suppose that the bolts and holes could easily carry bearing loads to support a total load of 654 lbf if the load was static, the dynamic nature of it would make things loosen and elongate, perhaps the friction force provides a big dampening effect there.
I want to look up the bolt bearing load allowable later, I am curious.

Now I am thinking about flanged crank hubs with those big lugs in them. The counterbores are associated with the flanged cranks but it's not clear to me if they reduce the need for friction. I suspect that they do not. But I guess that depends on how the prop fits. In other words how much the prop can rotate on the hub with the hub held in position when the bolts are snug but not pre-loaded.

[NY86]

Re: Prop Crush Plate?

Damn engineers!

There used to be a "crush plate" called out for metal props, but, I recall, there was a bulletin, somewhere... that said to stop using it. Of course, I cannot find the original info...