Adding mass to a engine shaft

[Andy J]

After a quick check of the CoG position on my large Sparrowhawk I found that I needed to add about 1lb of lead up front if positioned immediately behind the Thor 45 engine.  This is despite the airframe having two large lumps of lead already attached by the original builder.

 

Then I wondered if it would be better to add the mass further forward possibly as a thick steel washer on the engine shaft in front of the prop as this would reduce the auw of the model.  Found on ebay a person selling the correct size diameter washers which is 40mm with a thickness of 10mm which should at least add a significant mass up front.

 

The query I have is will the increased mass on the engine shaft have any adverse affect on the engine as have not seen others adding mass in this way to rectify CoG issues.

[Rich Griff]

Blimey, and additional pound of weight.

 

I would not add anything to the crankshaft.

 

Maybe a bracket via engine mount bolts ( the engine is radial mount ? ) Or similar going forward to mount the needed weight to balance c of g.

 

Weight loss in tail would be better but I don't know your "over weight" model ?

[Paul De Tourtoulon]

My Enya 90C powered Spitfire loves it, and it gives me a good idle and no more prop throwing,,,

[Martin Harris - Moderator]

Heavy brass spinners are a fairly popular option. In a scale type model, the increased gyroscopic effects will add to realism in that P factor and other torque related factors would be slightly more apparent, requiring a little more rudder control for good take-offs. I suspect that a 3D style model might find reduced bearing life and strain on the engine mounting as a consideration. 

[Glenn Philbrick]

I had this brass weight made to fit an os120 it weighs about 200grms. It never gave any problems and ran for several years until the inevitable bad landing.

[Jon H]

With the pound of lead added what does the model weigh? If its not excessively heavy for its size just forget it. Lead is cheaper than an engine rebuild and thor prop hubs are not known for their high strength. The design of the crank extension is pretty poor and out of balance props/heavy spinners have been known to cause them to fail. I would not add any weight to the crankshaft in this case as the weight will be miles ahead of the bearing and completely unsupported. 

 

Another alternative would be to mount the lead further forward. On my DB Hurricane i made a cradle from two glass/nylon engine mounts and this hold my 2lbs of lead very nicely. 

 

 

 

There is so much hysteria about adding lead. Just dont worry about it. Our engines are very light vs full size when you consider the engine mass vs total weight of the aircraft. Adding lead is more or less inevitable and if you have done all you can to minimise it, just screw it on and go flying. 

Edited November 5, 2021 by Jon - Laser Engines

[Andy J]

Just tried to measure the airframe auw but it was well above the limits of the kitchen scales. Would guess the model is in the region of 19 - 20lbs. Given your advice Jon, I will revert to attaching the required amount of lead to the engine standoffs as the prop does indeed sit a long way forward of the front bearing on a Thor 45.

[Jon H]

What size is the model? If its around 80 inch then 20lbs is fine. 

[Craig Carr]

Hello Andy,

 

I had a similar issue with my seagull edge.  Needed about 400g.

 

I used some brackets that fit via the engine stand offs as shown below (a picture paints a thousand words ).  Drilled to accept cable ties and the lead wrapped in electricans tape to tidy it all up a bit 

 

Loads of flights with it now.  The other day i had the cowl off and all was still tickety boo with the installation 

 

craig 

 

Edited November 5, 2021 by Craig Carr

[Stuphedd]

My lead nose cone insert of at least 40 years ago , then I just made solid alum spinners if I needed some weight 

[Rich Griff]

I will admit that the shadow needed and engine stand off plate between the engine mount and fire wall, about 1/2 inch thick wood, and one layer of lead sheet the same size as the stand off, about 1/4 inch larger than the engine mounting.

 

I could have lightened the tail a bit as it's all sheet, tail fin, rudder, tail plane and elevators.

 

Those layers of lead, nice to see nyloc nuts, but flattened sheets and some penny washers would be my "addition ".

 

Build light but strong.

[Paul De Tourtoulon]

Your two nuts and washers may keep the lead at it's place Jon, but the way you have done it I would give you less than one hours flying before it would be rattling around in the engine bay with a 20cc petrol and one flight with an Rcgf 20cc engine

[Simon Chaddock]

I would be cautious of adding lead to any part rotating at engine speed. Lead is technically a liquid at room temperature, a very viscous one certainly but it can and does creep under load. I wonder how many 'g' is it under rotating at 6000 rpm. 

[Paul De Tourtoulon]

Yes I second that NO Lead on anything moving it's too soft,,,

[John Stainforth]

Quite an easy solution that stays fixed is a tailor-made wooden box to house the lead (or more user-friendly alternatives). One can even use lead shot in one of those.

[Stuphedd]

My lead "Bullet" was actually contained within a nose cone , but I to take on board the fact that being soft  it would tend to creep under load and would therefor have a finite creep life as many rotating parts do ! ( one of the major factors governing a turbines life ).

As adding weight to the shaft on our little engines ?? , our boat and car friends use flywheels ! Not lead ones !!

[Martin Harris - Moderator]

I suspect there’s a big difference between a typical purpose designed model engine with the prop driver immediately in front of the front bearing and a converted chainsaw engine or similar with a relatively long extension shaft - like, as I’ve discovered (on checking, 2 stroke petrols not being of much interest to me) the Thor 45 mentioned in the OP.  

[Don Fry]

3 hours ago, Simon Chaddock said:

I would be cautious of adding lead to any part rotating at engine speed. Lead is technically a liquid at room temperature, a very viscous one certainly but it can and does creep under load. I wonder how many 'g' is it under rotating at 6000 rpm. 

There are 2 elements liquid at room temperature, mercury and bromine. Lead is a solid. I think you are getting confused with its yield strength.

[Jon H]

21 hours ago, Paul De Tourtoulon said:

Your two nuts and washers may keep the lead at it's place Jon, but the way you have done it I would give you less than one hours flying before it would be rattling around in the engine bay with a 20cc petrol and one flight with an Rcgf 20cc engine

 

Its fine the way it is. My Sea Fury has the same setup. 8 years old and they have never come loose. My small hurricane has a slightly different setup with captine nuts hammered into the lead. That is 5 years old and they have never come loose either although they are thread locked. 

 

 

 

 

[Ron Gray]

Ah but yours are Laser powered, not 20cc petrol (RCGF 20cc)!!

[Martin McIntosh]

Well, this will make some of you cringe. Many years ago I built a 96" Vailley Aviation Hurri. The plan stated 1/8thx1/4 spruce stringers topped with 1/8th sq. balsa for shaping, about 52 of them I seem to remember. Not wishing the Solartex to create a starved horse look I followed the plan. Needless to say that with a RCG 50 up front and a home made GRP cowl it was somewhat tail heavy and really nowhere to attach lead so, using spirit levels, I very carefully filled the front of the 5" spinner with 1kg of molten lead.

After very many flights I still cannot detect any problems with the motor. A well balanced rotating mass should be no problem since the load is inertial only; look at a large electric outrunner for instance. The only drawback I can see is that slamming the throttle open could break the shaft but I have not managed to do so yet.

[Geoff S]

6 hours ago, Don Fry said:

There are 2 elements liquid at room temperature, mercury and bromine. Lead is a solid. I think you are getting confused with its yield strength.

 

Plate glass also flows at normal temperatures.  When our shop window was smashed (with a brick!) it had been installed for perhaps 20 years (probably more - it was over 60 years ago) it was noticeably thicker at the bottom than the top and I'm sure it wasn't made like that.

Edited November 6, 2021 by Geoff S

[Don Fry]

I would point out, plate glass is a supercooled liquid, it is also not an element, SiO2. Sorry, can’t do a subscript 2

And I think over that timescale, it was thicker on installation. At that time, local glass factories were not high tech, and it doesn’t flow that quickly, or evenly.

[Martin McIntosh]

I did melt the lead first of course!