Using only 4 out of 6 propeller mounting bolts

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kubark42

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I am constructing a folding prop hub for an electric motor which has 6 mounting holes, spaced evenly at 60 degrees.
Screen Shot 2021-03-01 at 3.35.24 PM.png

I notice some motors only use 4 holes, spaced evenly at 90 degrees. And in fact, for this motor, it is only held onto its mount by 4 bolts. So it seems there's good evidence 4 bolts are sufficient.

Looking at the holes at 6- and 12-o'clock, they don't seem to do a whole lot. Just from appearances, it seems that each blades' torque, centrifugal, and thrust forces are all contained by the two bolts at that blade's root, and that these two do very little.

It's an electric motor, so the torque pulses are almost non-existent. The hub is a CF-infused nylon core wrapped in carbon fiber tow. Safety factors will be around 4-5x.

For the construction of this hub, it would be much easier if I simply omitted the holes at the 6 and 12-o'clock positions. In particular, it lets me pull tow straight across the sides, without having to worry about matching any curvature.
Screen Shot 2021-03-01 at 3.47.10 PM.png


Does this look reasonable or are there considerations I'm not taking into account?
 
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Dana

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Just remember that the engine torque is transmitted by friction between the flange and propeller, not shear on the mounting bolts. The bolts provide the clamping force. If your prop hub is rigid enough to provide adequate clamping from four bolts, you should be OK, and as you say an electric motor will be much smoother.
 

kubark42

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the engine torque is transmitted by friction between the flange and propeller, not shear on the mounting bolts.
I have wondered about that. If it were purely friction, then I would imagine the bolts could be significantly smaller and there would be far more attention paid to the interface between the prop and flange. On the flip side, if it were purely shear the bolts are so much thicker than they'd ever need to be.

Do you have a reference I could read so I can understand what the target clamping force is? I can make educated guesses based on materials, but I'm sure there's some good rules of thumb derived from a century of flight.
 

Hot Wings

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Do you have a reference I could read so I can understand what the target clamping force is?
Not really a reference but if you go through the spread sheet on this page, by one of our HBA members, for using Belleville washers on prop bolts you should be able to get a 'feel' for the clamping pressure needed.

For actual numbers you are probably on your own to figure them out using the parameters for your particular combination of materials and loads.
 

kubark42

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Fascinating! I read through what was there, but the actual pressures required are specifically not addressed: "Check with your prop manufacturer for appropriate bolt torque."

One thing we can conclude from this is that belleville washers are ideal for prop hubs which have a high CTE or sensitivity to humidity.
 

Dana

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All propeller manufacturers specify the appropriate bolt torque. Naturally, it's quite a bit higher for metal props than wood props. This is from Sensenich:

1614648093243.png
 

wsimpso1

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I have wondered about that. If it were purely friction
Friction carries the torque. The bolt preload also has to keep the joint from opening. Nominal prop flange design holds the prop without slipping and keeps the joint from opening against:
Nominal torque;
Thurst;
Firing pulses;
P-factor;
Gyroscopic moments;

And here is the thing, most of them repeat every revolution of the engine and all of them can be at their worst at the same part of the cycle.

You have serious work to do to define all of the loads that can be applied to your propellor and through the hub, estimate their magnitude, and assure yourself that your scheme will hold together. Only firing torques will be smaller in your design than in a conventional engine. The rest will still be big.

A wise scheme to check your design calcs would be to look at existing designs and the full suite of loads encountered, then check stresses and deflections, check margins, and then make sure you have at least that much margin in your design.

If you are not really friendly with fatigue and fastener design, I recommend a revisit of these topics. Shigley is good on both.

Billski
 

kubark42

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Those are good suggestions. I'm fortunate in my application that very little of that is of a high order. My prop blades weigh 200g and my thrust is around 400N in a pusher configuration. RPM is somewhere between 2500 and 2700RPM, so it is a low power, low inertia setup. You dodge a lot of bullets when your glider's MTOW is only 300kg and you think 400fpm is a stellar climb rate.

The same propeller blades used in a 1200N thrust configuration use a 4-bolt pattern on an aluminum hub. I'll ask the designer of that hub what torque spec he used and work from there.
 

Doran Jaffas

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Those are good suggestions. I'm fortunate in my application that very little of that is of a high order. My prop blades weigh 200g and my thrust is around 400N in a pusher configuration. RPM is somewhere between 2500 and 2700RPM, so it is a low power, low inertia setup. You dodge a lot of bullets when your glider's MTOW is only 300kg and you think 400fpm is a stellar climb rate.

The same propeller blades used in a 1200N thrust configuration use a 4-bolt pattern on an aluminum hub. I'll ask the designer of that hub what torque spec he used and work from there.
Don't under think it but no need to overthink it as well. I would say you're on the right track regarding this. Once you get it bolted up obviously the end game will be to do it engine run up of several hours off and on at various thrust settings such as rpm. just my personal thoughts after having built several airplanes is that after 2 or 3 hours of ground running that engine if they are going to be problems with the propeller in the hub you will see it starting to happen. I'd appreciate being kept posted on this design and what you're doing. Sounds interesting. I have been in a glider once more or less for an introductory lesson and I understand the appeal but me liking the fan up front or in back I would shrink and not having a motor to get me airborne or to start up if I didn't like where I was going to land.
 

kubark42

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The hub manufacturer pointed me toward the motor manufacturer torque spec. The motor manufacturer does indeed give a spec, but my guess is that it's derived from the max capacity of the aluminum threads, and not attaining the required interface friction.

My TBO is 25hrs (which should cover the airframe lifetime of 6,000hrs) and I don't expect more than 5 hours a year of operation, so it's trivially easy to test using @Doran Jaffas's recommendations.
 

rv7charlie

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Since clamping force / friction is postulated as what keeps the prop from moving on the flange, I invite you to perform the following exercise:

1. Remove all six drive lugs from the prop flange (the raised lip in the center of the flange mated to the prop bore will keep the prop centered).
2. Span the intersection of the flange & prop in a couple of places with narrow strips of adhesive backed copper, aluminum or stainless tape.
3. Install the prop, using all 6 bolts.
3. Run the engine to max output several times, in fast taxi if necessary (don't leave the ground).
4. Check the tapes for condition.
5. Report your results.

Charlie
 

BBerson

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I don't know about carbon fiber hubs, but a wood hub can be removed and checked for charring from inadequate friction clamping.
I know of a propeller shop that was sued after using a paper shim on the hub and the propeller was lost (c-170).
 

rv7charlie

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To 'track' the prop. Wood props sometimes have blades that aren't in the same same plane. Can occur with composite & metal props, but less common.
On a ~3 foot arm, it takes an amazingly thin shim at the bolt circle to move the tip of the blade a significant distance.
 

BBerson

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Most prop manufacturers require a clean flange. I think that means no adhesive backed foil tapes that perhaps might reduce friction
 
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rv7charlie

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I was saying to put a strip of the tape across the junction between flange prop root, as a 'witness strip' to see if the prop is moving (in rotation) relative to the flange; not between the back side of the prop and the flange.
 

rv7charlie

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For a near-direct replacement for the experiment, here's the installation manual for larger IVO Props, that used to be sold for Lycomings.
https://www.ivoprop.com/images/PDF Instructions for Medium Quick Adjustable Ivoprop 6 Bolt Adapter.pdf
Note the torque spec'd for the 3/8" (AN6) bolts; 30 ft lbs instead of 15-19 ft lbs used for wood props. Note also that the faces of the aluminum hub plates are knurled; not smooth, and when the prop bolts are torqued, the knurling actually digs into the front & back faces of the prop blades. Not mentioned in the document is the fact that there are no drive lugs; the prop is totally dependent on 'friction' between the blade faces and the hub faces to prevent movement of the blades in the hub. Note the instructions for using 'witness tape' across the joints between the blades; my experiment would have the tape spanning between prop and flange, since a more typical prop is a single piece.

Now. See the elaborate testing required for the IVO; the serious concern for blade movement? Originally, IVO used *copper* tape as witness tape, but it kept failing so they switched to stainless (and still have failures).

I've owned and flown an IVO on a Lyc 320. I know how to mount and torque a prop properly (I guess you'll just have to trust that I do...). I can tell you that on a 4 cyl Lyc 320, the blades *will* move. They moved for everyone that flew one on a 4 cyl Lyc. In fact, IVO quit selling them for 4 cyl Lycs, because of that very issue. You can ignore that bit about 'resonance' in the installation doc; resonance isn't the issue.

So, my question to the group: When using the same size bolts (3/8") and torqued at ~30-50% less torque than an IVO, would a wood prop have *more* friction against the smooth flange than the composite IVO blades have against their hub flanges, which are knurled and bite into both faces of the blades?

Charlie

BTW, many (if not all) of the metal Hartzell constant speed props use only 4 bolts. But they *do* have drive lugs.
 
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proppastie

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When using the same size bolts (3/8") and torqued at ~30% less torque than an IVO, would a wood prop have *more* friction against the flange than the composite IVO blades have against their hub flanges, which are knurled and bite into the faces of the blades?
This may have some bearing on the issue......something interesting I found when bending T3 aluminum in a pan break.....the traditional method of setback of the clamp did not produce the proper bend radius because the hard material would lift and bridge to the clamp and the radius would be sharp........

so perhaps the stiffer IVO prop when torqued .....the fibers are bridging (or warping?) between the bolts and not contacting as well as a wood prop....obviously there is a difference. Just an idea.
 
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