Rotax vs. traditional (engine shutdown)

RyanB

RyanB

Super Administrator
Management Council Member
PoA Supporter
Joined
Jul 21, 2010
Messages
16,231
Location
Chattanooga, TN
Display Name
Display name:
Ryan
My rookie is going to show here.

Why do Rotax engines stop almost suddenly upon shutdown, where as traditional Lycoming and Continental engines take a second or so to stop, until they don’t have enough energy to roll over the compression stroke.

Why the difference?
 
RyanB said:
My rookie is going to show here.

Why do Rotax engines stop almost suddenly upon shutdown, where as traditional Lycoming and Continental engines take a second or so to stop, until they don’t have enough energy to roll over the compression stroke.

Why the difference?
Click to expand...
Propeller mass via direct drive takes longer to stop. The geared Rotax uses a smaller engine with less rotating mass and runs it faster to make the HP, and the propeller's inertial force is reduced by that gearing so it stops sooner.
 
Dan Thomas said:
Propeller mass via direct drive takes longer to stop. The geared Rotax uses a smaller engine with less rotating mass and runs it faster to make the HP, and the propeller's inertial force is reduced by that gearing so it stops sooner.
Click to expand...
Awesome explanation as always! Thanks Dan!
 
Rotax engines are shut down by killing the ignition systems like a car. Lycomings and Continentals are shutdown by cutting fuel flow. Ignition cutoff is instant. Fuel flow cutoff not so fast and many systems don’t kill the fuel flow completely with the mixture in idle cutoff as they age.
 
Jeff767 said:
Rotax engines are shut down by killing the ignition systems like a car. Lycomings and Continentals are shutdown by cutting fuel flow. Ignition cutoff is instant. Fuel flow cutoff not so fast and many systems don’t kill the fuel flow completely with the mixture in idle cutoff as they age.
Click to expand...
The engine in a Ly/Co doesn't stop instantly if you turn the key off.

The Rotax will also stop suddenly if fuel flow is interrupted.
 
My Continental doesn't have mixture control anymore, hence it gets shut down using the ignition. It does spin to a stop.

Ron Wanttaja
 
dmspilot said:
The engine in a Ly/Co doesn't stop instantly if you turn the key off.

The Rotax will also stop suddenly if fuel flow is interrupted.
Click to expand...
Mine does, I do however have a 3 blade composite prop. A big heavy metal prop will of course rotate longer. Most Rotax aircraft have very light props.
 
If stopped in the air, will a typical stopped Rotax engine installation crank itself for restart from just diving airspeed via the prop torque?
 
Well said by all above - I've nothing really to contribute, but this is the internet, so I'm typing away.....

Some time back, I did the first flight on a beautiful Rans S-7 that a tailwheel student of mine had built. Good design, great build, loved everything about that airplane, except the shutdown. The first time you experience it, you'll swear you just broke something. Really unnerving.

--Tony
 
I wonder the difference in -200 vs -550 cubic inch engines due to engine rotating assembly inertia too. Massive rods, pistons, and crank vs teeny.
 
Prop mass.

My Mooney with McCauly metal prop spins down. My CAP-10 with Hoffman composite stops quite quickly.
 
You must log in or register to reply here.
Back
Top