Propellers get damaged more easily than any other item on an aircraft. A propeller can be written off in a couple of seconds when high power is used in unfavourable conditions.
Surprisingly, a lot of prop damage happens on sealed tarmac where there is still a smattering of small loose stones on top which can quite often be invisible to the pilot.
Last time we looked at hot and cold starting. In that blog we discussed the concept of ‘waking the beast’ or starting from idle and gradually advancing the throttle just enough to have the engine start, and then keeping the engine at low RPM, certainly below 1,000. On a cold engine I initially aim for 850 RPM because as the engine warms, oil thins out, and the metal expands it will soon enough increase on its own to 1,000 RPM.
Apart from keeping the RPM as low as possible when operating where stones or if gravel or loose seal is present it is very, VERY important to keep the elevator control in the full nose up position. Even one centimetre additional clearance between the propeller and the ground can make all the difference between getting a nasty stone nick or not.
This fact might surprise you: A Cirrus SR aircraft at full power and the brakes on has less than 2” of propeller ground clearance! Which is why mechanics put the trim at full nose up prior to a maximum power test run.
Every time you add RPM against brakes (or ground friction) the propeller clearance reduces. So keeping the RPM as low as possible as well as the stick fully back when manoeuvring at low speed is very important in reducing propeller stone damage.
Why do stones get sucked up into the propeller? Recently I watched our SR20 commence taxi over a shallow puddle of water and I only wish I had my camera ready. The vortex generated looked like a mini-tornado as the water was sucked upwards towards the propeller.
So let’s imagine you have flown into a sheep station and the runway and surrounding areas are all gravel. When taxiing in after landing it’s ok to ask your passenger to assist you in keeping the stick fully aft. If possible, try to keep your speed up until you reach your parking position. If you get stuck or need to use more than 1200 RPM then it’s better to go back to idle and shut down and get the tow bar out, rather than risking severe propeller stone chips and nicks.
When it’s time to start up and depart remember to ‘wake the beast’ as described previously keeping as low RPM as possible. Again, ask your passenger to maintain full back stick for you.
Getting moving can be tricky but breaking initial parking friction can often be achieved by moving the elevator control back and forward in a pumping motion while slowly increasing RPM. As soon as you are moving full back stick again.
Engine run-ups on gravel could easily wreck your propeller and on balance may not be in the interests of safety. Instead, a simple mag check while taxiing is normally sufficient to establish both magnetos are functioning normally.
If runway length permits, increase power gradually and allow the speed to build up before going to full power for takeoff. Remember to keep the stick well back and keep the nose up at least a bit throughout, remembering the normal reduction of prop clearance with power application. There is a risk of a premature lift-off with the stall warning sounding using this technique. If this happens reduce the angle of attack and fly in ground effect (a few feet above the runway) until the airspeed is sufficient for the aircraft to continue climbing.
In summary, propeller damage can be largely avoided by using the above techniques which unfortunately do not seem to be taught by most flying schools.
By Charles Gunter, Cirrus Melbourne