Propeller drag - how to stop it

by Had Robinson
updated October 9, 2024

Some years back I rigged up a propeller brake to stop it  from windmilling when the engine is stopped or at idle.  However, it has some machined parts that are unique for each engine i.e., taking a bicycle brake disk and machining holes in it and drilling and tapping holes in the propeller hub. Then there is the custom mount for the bicycle disk brake, etc.  It is a lot of fiddling around just to eliminate drag.

There is an easier way to stop windmilling.  A bonus of eliminating the clutch is that there is less chance of getting glider lines in the propeller when landing.

First of all, clutches on paramotors are for new/beginner pilots who see a spinning prop and think how horrible it would be to get into it. A prop at engine idle is not particularly dangerous! A prop at any other speed is a vertical lawn mower - and the clutch does not help as it is fully engaged. The best engines do not have clutches. Once we get over having a clutch or not, we go to the next step. I prefer non-clutched engines for the simple reasons that they are much easier to start, are way easier on the starter whether electric or manual, and there much less drag if the engine is stopped.

The following will work on either a 100% enclosed clutch (which often runs in oil) or an open, dry clutch. The former will just take more work to do and requires a meticulous cleanup.

Supplies/tools:

 Steps:

  1. Using a #19 drill bit, drill (2) holes exactly opposite into the clutch bell and the clutch. The entire clutch must be exposed and the bell and clutch in their respective positions as when the engine is running. The distance of the holes from the bell rim is determined by the rim's relative location from a part of the clutch pads that has no steel structure that goes to the anchor point on the engine shaft. E.g., if the clutch pads are 40mm in width, a spot 10mm in will probably be free of structure beneath the pads. To determine the exact radial location of the holes, wrap the bell with 4 or 5 layers of masking tape, use a box cutter to cut the tape, peel off the tape, measure the length, divide by 2, make a mark, replace the tape on the bell, and drill away.
  2. Remove the clutch bell and enlarge the holes to 5mm with a reamer or a 5mm bit.
  3. Use a 5M x .8mm tap on the holes in the clutch. Use cutting fluid (NEVER ordinary oil).  Try to keep the oil off the clutch material.
  4. Reassemble everything and install the panhead screws with threadlock. Unfortunately, the screws must be installed from the outside and they will tend to loosen if not thoroughly locked in with threadlock.
  5. Run the engine about 10 hours and check the screws that they have not loosened. Check again at 25 hours and then every 50 hours.

If the pilot changes his mind and does not want a non-clutched propeller, he can take the screws out and the clutch will return to normal operation. It is a fully reversible mod.

vulture