Van's Air Force
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Tow bar and pulling on the propeller by the spinner
- Thread starter mikefox
- Start date
"Tow bars" are poorly named. They are for steering. Your propeller sees much higher loads pulling you through the air than you can apply with your hands at the base of a blade.
I've seen one of the cups on the end of a "tow" bar fail (due to age and someone leaning heavily on it to push the plane) and run the free leg down the outside of the wheelpant on a Cessna, carving a line in the paint.
I've seen one of the cups on the end of a "tow" bar fail (due to age and someone leaning heavily on it to push the plane) and run the free leg down the outside of the wheelpant on a Cessna, carving a line in the paint.
Amadeus
Well Known Member
I've been told and I quote: "There is nothing you can do with your bare hands on the ground that even comes close to the forces upon the propeller in the air."
Piper J3
Well Known Member
True... but grasp prop at hub only
PilotjohnS
Well Known Member
Scary
This is scary.
if it is metal prop I think pushing and pulling on the prop is OK.
But if it is a composite prop, PLEASE don't push or pull on the prop blades; I think the trailing edges can be damaged with an overzealous ground crew. My plane is going to be placard, "Do not push or pull on prop blades"
The price we pay for performance
JMHO
This is scary.
if it is metal prop I think pushing and pulling on the prop is OK.
But if it is a composite prop, PLEASE don't push or pull on the prop blades; I think the trailing edges can be damaged with an overzealous ground crew. My plane is going to be placard, "Do not push or pull on prop blades"
The price we pay for performance
JMHO
Over-generalizations are difficult… I’ve watched a creator of composite blade CS props pull and push airplanes in and out of hangars using the prop blades….at the roots, of course…..
I have been working on airplanes for a living for decades and I have a lot of respect for propellers. I don't touch them unless absolutely necessary and prefer to have the spark plugs removed if I need to perform maintenance.
But it's your limbs and life, do as you wish, I doubt you can hurt the prop or engine with your hands.
But it's your limbs and life, do as you wish, I doubt you can hurt the prop or engine with your hands.
Amadeus
Well Known Member
Given the gearing on the Rotax gearbox I'm guessing you could have the master switch on, both lanes on, throttle at 50% and both fuel pumps pumping (which I would NEVER do nor recommend anyone trying) and a human could never spin the propeller fast enough to make the engine start.
The Rotax requires rotating the prop just to check the oil....I'm not removing top and bottom cowling and removing all the spark plugs just to check the oil each time I go to fly.
Last edited:
rcarsey
Well Known Member
Amadeus
Well Known Member
Bottom Line.....................NEVER SAY NEVER!
Piper J3
Well Known Member
Yes, the 80 hp 912 was 1,211cc and 9.1:1 compression ratio. The original flat-four was bumped up to 1,352 cc and 10.8:1 compression ratio to yield 100 hp.
Rotax uses Capacitive Discharge Ignition (CDI) instead of magnetos. The CDI’s are not powered by the battery, but rather by coils on the back of the engine. The engine crankshaft needs to rotate about 250 RPM to generate a spark. So, it’s possible to hand start (especially the lower displacement / lower compression) by back-driving the gearbox. You’d have to spin the prop really hard and fast.
A couple of items….
This minimum speed to attain spark is also why it is advantageous to have battery fully charged for fast cranking. Slow cranking = no spark.
It’s perfectly safe to turn propellor by hand as no spark would be generated at slow speeds.
One add’l item – the Rotax utilizes a Sprag, or overrunning roller clutch, on the starter motor instead of a Bendix drive gear assembly. The reason I mention this is early SN engines do not have automatic spark retard at start. If engine cranks slowly and backfires, it can easily destroy the Sprag clutch. Once again, a fully charged battery is your friend….
Rotax uses Capacitive Discharge Ignition (CDI) instead of magnetos. The CDI’s are not powered by the battery, but rather by coils on the back of the engine. The engine crankshaft needs to rotate about 250 RPM to generate a spark. So, it’s possible to hand start (especially the lower displacement / lower compression) by back-driving the gearbox. You’d have to spin the prop really hard and fast.
A couple of items….
This minimum speed to attain spark is also why it is advantageous to have battery fully charged for fast cranking. Slow cranking = no spark.
It’s perfectly safe to turn propellor by hand as no spark would be generated at slow speeds.
One add’l item – the Rotax utilizes a Sprag, or overrunning roller clutch, on the starter motor instead of a Bendix drive gear assembly. The reason I mention this is early SN engines do not have automatic spark retard at start. If engine cranks slowly and backfires, it can easily destroy the Sprag clutch. Once again, a fully charged battery is your friend….
RFSchaller
Well Known Member
One thing I hate is when people jump in and start pushing to help when I’m pushing it back in the hangar. A small rotation of the nose wheel with the tow bar can quickly send the plane toward a hangar rash incident if you aren’t the only guy pushing and can’t stop the plane.
Amadeus
Well Known Member
The -12s are so lightweight no help is needed even when pushing on an incline! When someone wants to jump in and help I ask them to walk the wing for me instead. They still get to help but in a different role.
NEVER do that!
I've been working in light aviation for quite some time.
Many people I know, including professional motorists, have mangled fingers and hands.
Do not repeat their mistakes!
