Discus Woes

[flihijohn]

I am the proud owner of a 4m Discus that I built years ago from the Graupner kit. The model is set up for aerotowing, but interest in aerotowing has waned in this area and, as a consequence, the model has not been flown for a couple of years - a waste of a beautiful aeroplane.
I have decided to fit a FES. I did all the calculations (I am not new to electric flight) and decide on a suitable motor: this is where my problem begins.
The motor, a Turnigy SK3-4250-410, comes configured for prop drive. I needed to reverse the shaft so that I can fit a collet drive in order to use a folding prop. I looked at all the videos on u tube and, with the help of an engineering friend, we set to. The grub screws that grip the shaft have proved impossible to shift - we have even tried a bit of finely-directed heat to the grub screws. We have now damaged the motor boss in or efforts to release the grub screws! Please, is there anybody in the community who has had a similar experience and/or can suggest a solution to the problem?

[Barry_Cole]

If you want to send it to me, I will have a look at it.

E-Mail me:- modelgliding@aol.com



BC

[terry white]

Hi John, This is not an unusual problem, but one that is easily overcome. The grub screws you are having trouble with would have most probably been fitted in with a liquid thread lock at the factory. Some are and some are not. Axi motors for instance are. The way to overcome the thread lock is to heat the whole of the back end of the motor with a fine flame heatgun. The type used for jewelry making. Play the very fine flame to the motor around the grub screw. This will expand the aluminum of the casing more than the grub screw and break the seal of the thread lock. You must be careful to apply enough heat to do the job without damaging the motor its self. Wrap the rest of the motor in a wet cloth for protection. To get the grub screw out if the Allen key head has been stripped it will be necessary to cut down and through it from its top with a slicing cutter on a dermal drill, then use a screw driver in the now made slot the normal way.

Have you thought of fitting the motor to a removable bulk head which screw mounts to a bonded bulk head within the cockpit. This will not necessitate the removal of the shaft that you are having problems with. If fitted well back from the nose you can then decide whether to have the motor shaft protruding through a small hole made in the nose or first lengthen the shaft with a shaft coupler. With the coupler installed the shaft holding the the folding prop can then be taken on and off at will via the coupler thus returning the model to true scale.
This is the way most scale guiders do it these days making use of the cavernous room at the models nose to give them the benefits of both worlds. And also fly where props are not allowed. E.G. The Long Mynd.

Hope this help, if not send it to Barry Cole. Terry

[flihijohn]

Many thanks to Barry and Terry for your swift replies. We managed to grind out the grub screws but now we have no means of mechanically tying the shaft to the body of the motor! A replacement motor has been ordered and is on its way. I hope that the replacement has grub screws which are a bit more amenable to treatment. We will follow Terry's suggestions and hope that we can resolve the problem. If we have no luck I will gladly take up Barry's kind offer. Two scrap motors would be too much to bare!
With regard to alternative mounting of the motor: I have already bonded in the bulkhead, having removed the nose of the glider, so I am committed to the shaft reversal route. I will mold a sturdy fibreglass cowl to cover the motor and to blend in with the spinner.
I will post an up-date when I have something to report.

Kind regards,
John

[terry white]

Hi John, any chance of some photo's of both plane and motor.

[flihijohn]

Hi Terry,
I'll endeavor to attach a couple of pics of the motor. We finally managed to get the shaft out by grinding out the grub screws but this has trashed the motor boss - a new motor has been ordered and is on its way. I might be possible to permanently fix the rotating bell to the shaft, with the shaft in the reversed position, with the correct brand of Locktite (?)
The only pics I have of the Discus are from the pre digital era!
Regards,
John

[terry white]

Hi John, although the shaft is hard it is possible to drill with a good sharp bit taken very slow and frequently brushed with oil to keep things nice and cool. How about drilling right through the damaged boss and shaft and pining it with a good quality steel pin. If you require a new shaft for any reason you can obtain it from Stan, Phoenix models in Newton Abbott. Has a brilliant Web page on line with lots of motor bits. Terry.

[Elliot Howells]

I've just this afternoon replaced the shaft on a hacker A40 motor for a 4m discus, coincidently...

It required a lot of heat to break the thread lock, I use a heat gun on full throttle, I prefer not to use a naked flame as it might tarnish the anodised finish. As a mechanical engineer and more recently, heating engineer, ye cannae beat the heat...

FES is definitely the way ahead, I'm tending toward the Torcman system as it's a stealth fit and easily removed so as not to anger the brummies who seem to think they own the long mynd...

Ell.

[terry white]

Hi Elliot, great news that you removed the shaft, and as I expected lots of well placed heat was required. (I was also once a heating engineer in my first life. However I've had 3 others since then)
You can save a great deal of money going to a shaft coupler to lengthen the shaft through the nose instead of installing the pricey Torcman system. When change over is needed, undo two allan screws on the shaft coupler situated inside the cockpit and withdraw the shaft (simples) Not difficult for a man of your caliber and costs about £25.00 or less all in. The money you save can buy you some more lovely lipo batteries, or you can send it to me.
I have all the details of where to buy the necessary bits and pieces with a few tips as well if anyone out there is interest. As you say it's the way to go in the 21 century L.Mynd or not.

[flihijohn]

The replacement motor has just arrived, so it was with some trepidation that I set to for another go at shaft reversing. Having read all the excellent advise on this forum and having gleaned a bit of extra gen from other learned sources, here is what was done. By all accounts, the sealing compound needs a goodly amount of heat for it to break its grip (400 deg C) - the heat also expands the screw hole more than the screw. A very thin flame, from a torch used for silver soldering, was applied to the tip of the set screw for 4 or 5 seconds. The screw yielded at the first try with the Allen key! I made a small, circular shield, from a piece of litho plate, to protect the innards of the motor and wrapped the entire motor in a wet cloth. The second set screw came out with equal ease. The rest was easy, the circlip was removed and the shaft tapped out and reversed. The supplied collar was fitted to the rear of the motor and the set screws reinserted into the front. I put a light dab of blue Loctite on the screws to keep everything secure.
Thank you to all who have helped with this little problem. Perhaps others will benefit too..

Merry Christmas,
John