NGH GTT70 advice please

[Manish Chandrayan]

Need some considered advice from the group please.

Wanting to run the NGH GTT 70 twin petrol on one of my models. The prop hub on the engine is keyed with a woodruff key, and the prop is secured to the prop hub by means of a substantial sleeve nut together with a lock nut of 3/8"-24 size. The prop hub additionally has four M5 bolts that fix the prop to the prop hub and I think are designed to prevent the prop slipping on the hub for any reason.

The prop I want to run on this engine was already drilled for a DLE 61 where the multi bolts are spaced at 30mm while the NGH is spaced at 29mm. If I were to use the multi bolts on the NGH I will either have to enlarge the existing holes (bad ideaI think) or drill new holes making a swiss cheese of the prop hub.  

 

The question being, can I run the engine where the prop is only secured with the sleeve nut and the lock nut ? Or maybe use only two of the multi bolts by drilling new holes?

 

A picture of the prop hub area of the engine is attached for reference       

 

[Engine Doctor]

High Manis . Just my thoughts as have run quite a few bigger engines over the years.

Lots of engines use a single center nut , sometimes with a lock nut system to hold props on with no problem. Four stroke engines give the bigest shock to the hub on the firing stroke and many of these  up to 30c + use a center nut fixing without issue. 

Your flat twin should be quite smooth but have plenty of torque

As your prop hub has already been drilled it has already been weakened and if its a wooden prop will crush more easily if using a single center nut. Certainly do not re-drill the hub for different pattern multi bolt fixing whether it made from wood or composite as it will weaken the hub further and could throw a blade causing engine damage or possible injury.

Wooden props and some composite with a wooden infill wiĺ crush and loosen . Tighten the nut and all is fine for a while . Then as the wood fibres compress the prop loosenes . 

If its a composite prop it should be ok  a try in a safe place so that it it loosens no harm will come , Beware though as not all composite props are the same quality and strength.

I would opt for a new prop and try the center nut fixing but get used to checking it regularly incase it loosens. Unless you intend to use a spinner with an ally backplate then it must be a multi bolt fixing.

If you opt for new prop and go for multi bolt then check bolts regularly and change the supplied bolts to a higher quality high tensile steel type as the Chinese butter bolts have been known to shear.

 

Edited March 7 by Engine Doctor

[Manish Chandrayan]

Hello Engine Doctor.

It is a wooden prop a Falcon 24x8. I do employ a standard practice of regularly checking the multi bolts on all my DLE. They do compress and then tend to stabilize after a few cycles. But I still check regularly just for my satisfaction.  

Why do you say that use of a spinner with ally backplate will mandatorily necessitate the multi bolts? The way the engine is designed, while the multi bolts could be an option  the sleeve and locking nuts will be mandatory.       

[Jon H]

I run my 60cc laser V's with a single middle bolt and never have any issues. I am not a big fan of multi bolt hubs personally but engines in the past have used knock pins in the prop driver and these can sometimes be helpful. 

[Manish Chandrayan]

Yes Jon, even the Zenoah GT 80 twin use a single bolt. Wondering why NGH went this way of using both. I do see that the latest NGH 70 have redesigned the hub to eliminate the sleeve nut and fix the prop hub to the shaft like DLE and the prop is fixed on to the hub with multi bolts.     

[Engine Doctor]

The size of spinner that engine uses will likely have a serious alloy backplate that will easily slip on the prop driver allowing the prop to move. You could always use a couple of studs screwed into the prop driver to lock spinner backplate then try the center nut to hold the prop on .

[Manish Chandrayan]

Got your point Engine Doctor👍

[Jon H]

1 hour ago, Engine Doctor said:

The size of spinner that engine uses will likely have a serious alloy backplate that will easily slip on the prop driver allowing the prop to move

 

I take a sheet of 200-400 grit (whatever is handy) wet/dry paper and fold it in half back to back. I then smear it up with glue and stick it to itself so its gritty on both sides. once dry i cut 'washers' from it and install them between the prop driver and spinner backplate, and between the spinner backplate and prop. I have also used this on smaller engines with plastic spinners and nylon props. Works a treat in all instances. 

[extra slim]

1 hour ago, Jon - Laser Engines said:

 

I take a sheet of 200-400 grit (whatever is handy) wet/dry paper and fold it in half back to back. I then smear it up with glue and stick it to itself so its gritty on both sides. once dry i cut 'washers' from it and install them between the prop driver and spinner backplate, and between the spinner backplate and prop. I have also used this on smaller engines with plastic spinners and nylon props. Works a treat in all instances. 

 

I do the exact same thing!!, after an experience with a particularly grunty YS.. as Jon says works a treat