Porp nut weights

[Harold Brewer]

I am trying to balance my Ukie 35, and find that I need to add nose weight.  I need a good source for some prop nut weights.  Any help appreciated.

Brew   

[John Tate]

Brodak has one for 1/4X28 thread:
http://brodak.com/spinner-hub-brass-hex-1-4-28.html

[Perry Rose]

Higley has one at 2 ounces. There are others in the hobby shop at one ounce and aluminum ones around a half ounce.

[john e. holliday]

Easier on the engine if you just stuff the back plate.

[Randy Cuberly]

Easier on the engine if you just stuff the back plate.

Absolutely, listen to the Ole' Doc!  Just epoxy some lead weight into the back plate.  Most crankshafts already have enough stress on them, don't add to it.  Keep the epoxy thin on top of the weight so you can add more if necessary.   "Way back when". I used a series of 4 Fox 35 Back covers that I poured molten lead into in half ounce
increments.  Used which ever one was best on my nobler of the moment!

Adding weight to the crank increases precession!  Never a good thing!

Randy Cuberly

[Tim Wescott]

Adding weight to the crank increases precession!  Never a good thing!

Yes to everything else, and using weight that's not rotating is always good, but adding a prop nut increases the moment of inertia of the rotating assembly, and therefore precession, by an almost laughably minuscule amount.

Just as a rough estimate, a 11.5" long, 1/2 ounce stick that's of even weight along its length has a moment of inertia of around 107x10^-6 kg/m².  A real prop gets lighter toward the tips, so that figure is probably more accurate for a wood prop, or an APC.  A one ounce, 3/4" diameter prop nut that's perfectly cylindrical has a moment of inertia of about 5.5x10^-6 kg/m², or about 20 times less.  So a really heavy prop nut has 1/20 the moment of inertia of a typical prop.

So -- no, a prop nut isn't going to increase precession.  It may well cause accelerated engine wear, but I have one plane with about 1 ounce of brass bolted to the engine shaft, and another with a 3/4 ounce Higley Heavy Hub, and both have hundreds of flights and are still going strong, so I wouldn't sign up to that necessarily being an issue.

[Tim Wescott]

On at least one of my planes I took some 1/4" thick brass, drilled holes in it with the same fore-and-aft spacing as my motor mounting lugs, and mounted it along with the motor, with extra-long mounting screws.  It worked great for years.

[Brett Buck]

Adding weight to the crank increases precession!  Never a good thing!

   But only in very tiny amounts. The r^2 in mr^2 is tiny for a nose weight/spinner weight. Say you have a 1 ounce, 12" diameter prop, and a 1 ounce 1" diameter spinner nut. Radius of gyration for the prop is about 2.75", and the radius of gyration for the spinner nut is 1/4", so the prop has 121x the inertia of the  spinner nut, or put another way, the spinner nut adds less than 1%. And that is a very light prop.

   We have done the same thing for crankshafts and electric motor armatures. The prop is essentially all of the inertia in the system, so adding shaft weights is negligible.

     Brett

[Randy Cuberly]

Yes,  I know that guys but a small amount is still an amount.   I actually meant that as somewhat of a joke but didn't add the little laughing faces !  Anyway it's not a good idea to add weight to the shaft of a relatively high RPM engine without adding strength to the shaft (especially on mass produced stuff)  if you want it to last a long time...remember the sucker has acceleration and deceleration forces at a very high rate of speed while running.  Making it heavier, even while possibly adding some damping to those forces, still increases the total stress loads on the shaft and bearings.  That's never a good thing if it can be easily avoided.  Weight, for aircraft balance, in non-moving parts is usually a better choice!

Randy Cuberly

PS:  Also remember this is a "PORP Nut" as specified in the heading and I'm not quite sure how to analyze the forces on one of those!   

Uhhhh I was also thinking in terms of a reciprocating mas as in an IC engine...Dam electric things always confuse the issue!

[FLOYD CARTER]

It's true that a heavier prop nut will not stress the crank, any more than using a Master Airscrew really heavy prop vs a light wood prop!

There are some who will insist that they can feel the difference in trim using a different prop nut!  Now, that is really being sensitive!

[Brent Williams]

There are some who will insist that they can feel the difference in trim using a different prop nut!  Now, that is really being sensitive!

We refer to those people as Walker Cup winners.

[Ken Culbertson]

Am I the only one here that builds in an adjustable nose weight box?  We have one for tail weight and tip weight, why not nose weight.  Before I started  doing this I made a thin sheet metal backplate cover and stuffed it with fishing weights.  Never did like spinner weights and it is nice to add a little to the nose in wind.  Better yet - build light and take out tail weight!  You used to be able to buy sheet lead as thin as 1/32"  You can mold a lot of thin lead into a cowling.

Ken

[Brett Buck]

We refer to those people as Walker Cup winners.

    Howard has accused both Paul Walker and me as being afflicted with "Princess and the Pea" syndrome.

     Brett

[Jim Kraft]

What about the gyroscopic procession of the extra weight not wanting to change directions putting more force on the crankshaft in a tight turn. May or may not have much effect, but could add to it. I ran McCoy 40's for a long time with just the prop nut and never had a problem. Then I built a plane that needed a tad of nose weight and added a heavy spinner nut. A few flights later toward the end of the pattern the crank broke. It broke in the part where the intake hole is cut. May have not been the cause of the spinner nut as that engines had many hours of running on it.

[Tim Wescott]

What about the gyroscopic procession of the extra weight not wanting to change directions putting more force on the crankshaft in a tight turn.

That's exactly what Brett and I are talking about.  My estimate, which was purposely very pessimistic, came up with the prop nut having about 5% as much effect as a prop.  His, which is probably more realistic, came up with the prop nut having less than 1% as much effect as the prop.  If it was such an issue, then you'd hear story after story about engines running APC props wearing out way faster than engines with lighter wood or even lighter carbon fiber props.  You don't, so it's probably not a major cause of engine wear.

Note that gyroscopic precession is a factor in how the plane flies, which is a big part of the reason that the top dawgs run CF props (they're lighter, & hence have a lower moment of inertia).

I'm pretty sure that this whole "heavy prop nuts will kill your engine" thing is a living legend.  The people who are scared of using them don't, and their engines last a long time, and they think that's proof.  The people who aren't scared of using them do, and their engines last a long time, and they don't remark on it.

[Brett Buck]

What about the gyroscopic procession of the extra weight not wanting to change directions putting more force on the crankshaft in a tight turn. May or may not have much effect, but could add to it. I ran McCoy 40's for a long time with just the prop nut and never had a problem. Then I built a plane that needed a tad of nose weight and added a heavy spinner nut. A few flights later toward the end of the pattern the crank broke. It broke in the part where the intake hole is cut. May have not been the cause of the spinner nut as that engines had many hours of running on it.

      I doubt that precession of the spinner nut was an issue, it is a *tiny* torque compared to all the others. It's slightly more likely that it was the additional rotating mass along the rotational axis (which is NOT precession). The difference is mostly that the prop has some "give" in the rotational axis, meaning when the charge fires, the root of the prop accelerates, and the blades lag behind, perhaps softening the response. The weight doesn't do that. It's the difference between beating a hammer on a spring VS an anvil. The fact that it broke in the classic manner from torque around the shaft axis instead of across it is highly suggestive. It breaks at the intake because you have a stress riser - and it usually causes a spiral fracture from the corner around to the front.

   The loads and torque on the internal engine parts is quite remarkable. Even a lowly ST46, which is very wimpy by modern standards, has cylinder pressures up to about 400 psi on a clean firing, which leads to about 200 lbs on the connecting rod and crank pin. That in turn gives you torques up to about 9 ft-lbs peak in very sharp spikes (compared to only about .35 ft-lbs of average torque) and even 3 foot-lbs slowing it down just compressing the charge on the "dead" strokes. That's not terrible impressive until you consider it happens back and forth at nearly 200 times a second.

     No one really does any sort of analysis when designing the engine, they eyeball it, and then see what happens.

      Brett

[Dan McEntee]

   If you can find them, Prather nose weights that fit the shaft behind the stock prop nut are a quick way of checking nose trim at the flying field, They came in several different weight sizes from 1/2 ounce up to one ounce or more, I think. I have a pretty good collection of them, cleaned out the local hobby shops  of those and Prather fuel tubing when they announced that they were going out of business.. They fit under a spinner if you need them to, also. When I was racing carts, the clutches had stackable weights that were used to adjust how much you wanted your clutch to slip, or not slip, before it hooked up in time with the power band of your engine. They weren't very heavy and it didn't take much to make a change. But that was taking advantage of centrifugal force and they were quite a ways away from the center of the crank. I always had a simple belief that even the heavy hubs were too close to the crank center to affect how the engine pulled.
  On off road dirt bikes, you can add weights to the fly wheel and to some crank shaft cheeks to help with torque at low end. I think that spinner weights could have an effect like that if they were used on a throttled engine, adding some fly wheel effect. I guess you would notice it in the engine break if you had a good enough ear. it should make the break slower, longer and softer, I would think, much like putting a longer prop on it, but with out the prop loads, if that makes sense??? Fun to talk about and experiment with.
    Type at you later,
   Dan McEntee

[FLOYD CARTER]

For tail heavy, I would hesitate to put weight on any rotating part, just for good measure.  I simply glue lead on the inside part of the cowling.

[Fredvon4]

Sport hack flier who always tries to balance a plane by recommendations on various forums

That said I have been known to add lead to the rear or the front...most times needing some nose weight ..I adjusted with heavier muffler or some times a tongue muffler....tried many techniques including wrapping a long piece of solder to out board wheel strut and fussing with tip weight.... even drilling with a LONG drill into the area above the engine mounts and adding a piece of lead solder

in fact on some nose lite models I opted for a BB heavier engine of same size or even a larger engine

Some planes I cut off tail and made lighter

Point ...for me is... I Never considered 1, 2 or 3 oz brass heavy hubs....many ways to skin this cat with out compromising a good engine

[Chris Wilson]

I am trying to balance my Ukie 35, and find that I need to add nose weight.  I need a good source for some prop nut weights.  Any help appreciated.

Brew   

Befriend someone with a lathe, obtain some brass hex bar the size of your favourite spanner and make a Tee nut. Ream prop hub to suit.
Or maybe an acorn nut just might be enough.

[L0U CRANE]

Two thoughts about what Brett and Tim posted, and the reputation of Fox Stunts for wearing bearings out or breaking shafts rather quickly.

It did happen, and was more rapid when the engines were fitted with prop shaft extensions. The precessional loads from the prop significantly increase during severe corners. Prop nut and spinner radii keep their contributions to a small percentage of the prop's

The Fox was an 'eyeball' designed and relatively inexpensive product.  If I can assume that the prop apples precession loading at about the half thickness of the hub, it is not only inherently greater, but also further from the main bearing length supporting against it.

Several other designs of the 60's and 70's did use Fox 35s with shaft extensions. It has since become a basic no-no.

Recently reread Al Rabe's Mustunt article. At one point he mentioned loading about 16 oz at a plane's CG to test his ideas on higher lift airfoils. That called for more lift to make turns. Lift makes drag, so he had to run the engine harder, and found that that did little other than "wear out a good Fox 35." Just that testing may have involved 60 or more flights, of over 6 minutes each. Call it 360 minutes, 6 hours under the increased load conditions! Serious stunt fliers practice efforts put in many more long flights than occur in most other model aircraft flight. An economically designed engine of at best modestly precise manufacture did amazingly well, considering...

IMHO...

[Harold Brewer]

Hey guys -

     Thanks for all of the information.  Higley is the supplier that I was trying to remember.  That said, I need to digest all of this information and decide how I am going to go about it.

Best regards,

Brew

[Brett Buck]

Hey guys -

     Thanks for all of the information.  Higley is the supplier that I was trying to remember.  That said, I need to digest all of this information and decide how I am going to go about it.

   This gives me a good product idea, now that the Prather weights are no longer being made. This seems right up Jim Lee's alley, and you get to spend your money with one of us.

   For those not familar with them, the Prather weights went on the shaft but were counterbored to permit conventional spinner nuts and the Tru-Turn prop washer.

    Brett

[Dan McEntee]

   This gives me a good product idea, now that the Prather weights are no longer being made. This seems right up Jim Lee's alley, and you get to spend your money with one of us.

   For those not familar with them, the Prather weights went on the shaft but were counterbored to permit conventional spinner nuts and the Tru-Turn prop washer.

    Brett

  Beat you to it in reply number  17!!