Motorglider Propellers

Motorglider Propellers

This is the remains of a pitch change mechanism bearing from a Dimona H36. It is desirable that the clearance from the pitch change plate on these bearings be part of the DI.
The clearance should be around 1mm.

H36 Hoffman Prop static rpm: (G109 about the same)
Propeller fine static max RPM 2700-2900
Propeller coarse static max RPM 2300

HOFFMAN PROPELLER
Prop pitch change lever bearings for Dimona H36  – 608 2RS (two off). These are larger than G109.
Prop flange nuts differ to those used on G109.
H36 and G109A accept same prop, but spinner and spinner baseplates differ, G109 is larger. I think the prop flange nuts are different sizes too, from memory.

Prop nuts on G109 32.5 ft/lb 44nm torque

Propeller Pitch Actuator Bearings
Bearings: Propeller pitch actuator fork 626ZZ  6x19x6. (some A models take 2 bearings, some 4)
Grob109B bearings differ (like almost everything else) and are 6x22x6

 

G109A Propeller Pitch Adjustment

Tools: 13mm ring/open, small shifter or approx 8mm openender, and a good quality Philips for the spinner screws.

  • Remove spinner
  • Slacken 8mm nut about 4 to 5 mm so that rollpin is free to rotate (use 13mm open ender)
  • Feather prop
  • Rotate inner ring counter-clockwise to coarsen. (mark it first)
  • Rotate only one or two positions (ie about 1 or 2/12ths)
  • Ensure roll pin is back in position before tightening 8mm nut.

 

 

Fixed Pitch Hoffman Repair
by McPhee

Personally I would not send ASK14 prop to [a professional aero prop repairer] as their glass work and painting is not as good as we do in gliding and really it is a job [someone qualified in GRP repairs can do] so long as
there are no other issues.

The glass lifting on Hoffman wooden fixed pitch prop is a known problem in Australia’s dry and wet climate. The end of glass lifts 3 or 4cm from leading edge on MotorFalke, Pik 20E etc).

Take prop off and using a stanley blade lift up as much glass as you can (ie the glass that is ready to lift)

Take some 400 wet and dry and sand under glass that had lifted. Mix up some good epoxy (I use Araldite 3600 from Jabiru, 3:1 by weight and is aviation approved) warm a little and let it run in crack between glass and prop and pupp [prep?] the glass surface in and out to let it run all the way in.  You will have leading edge of one prop blade down.
Remove excess resin.Then using foil tape work from bottom up push the foil back in place and rub really hard to level it all.
Do same to other blade and let it set for a day.
Once foil tape is removed you will need a bit of touchup and a bit of sanding to restore profile.  Rather than Q cell (do not use car bog) I now prefer Akzo Noble International 2 pack Perfection undercoat.  It is the best paint you will ever use for sanding and bonds like nothing I have ever used (there is even etch in the system).

You then need to grey paint same but first it is best to set up a balance system.  You need a tube which fits tight thru the centre of prop.  I sometimes use foil tape as a shim to get a tight fit. Set up 2 steel rulers level and start balancing using a spray can to add weight to one tip.  As paint drys it will become lighter so you need to go a bit more with spray can. Try inverting the prop and end for ending but the same tip will need the extra paint – proves your balance is correct.

Balance is very important. I have not noticed much difference with Falke props, but made a huge difference to PIK prop.

Slowly tighten up prop using a pattern [ie 1-3-4-2] and working up to the correct tension.  Personally I like using paired bevel washers – two little dish washers used in pairs cup part outside. Jabiru now use 4 pair of such washers on each bolt

Once refitted check the prop tracking.  If packing is needed (never have) I think I would aluminium foil tape layered evenly.  Do not try uneven tensioning of prop bolts.

Job is now done and if done properly it has a lifetime guarantee (my lifetime!!!) plus you have the satisfaction of a job well done.

PS 25 years ago I did a motorglider course at Parafield and one of the most interesting things was the prop worshop + Hank Kaufmann told me how to balance prop.

Ian McPhee

One Response to Motorglider Propellers

  • Noel Roediger

    Hello Ian.

    A few problems with maintenance of S.A Cadet’s A/C and I’ve taken responsibility for them.

    Documentation is a real can of worms, particularly in regard to time/life components.

    At the beginning of the Falke’s propellor log book Section 1 – Propeller Installation and Removal Record. 1000hrs has been pencilled in as the Reconditioning Period.

    Is this correct?

    Prop. is HO 11-150B-75L.

    Thanks

    Noel

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