I have seen this mentioned several times, does this mean the engine will naturally speed up or slow down once launched? Thanks, Ray

Hi Ray

The typical stunt engine will unload..as in increase RPMs and go richer...  this happens because the engine has an easier time turning the prop in flight, rather than being held still on the ground, normally you can listen and hear the engine go richer when launched, usually within  1 lap

Randy

I have seen this mentioned several times, does this mean the engine will naturally speed up or slow down once launched? Thanks, Ray

    It will naturally speed up (typically in the area of 500-1000 rpm) AND, it will (in very general terms and assuming it is a properly-working stunt engine) run as if much less load is applied - which it is. There is *much* less torque required to keep it spinning even 1000 rpm faster when its moving at 60 mph than sitting on the ground. As an example, for my PA61 setup, the level flight load of the flight prop,  a 12.5-3.75 Brian Eather 3-blade, is about the same as a 9-4 Rev-up on the ground.

     It will also, typically, be "richer". Not really, it actually has less fuel pressure in the air and slightly lower fuel flow rate. But it will be more prone to 4-stroking, or further from peaked out 2-stroke. This has been massively and almost universally misunderstood over the years. With less load and same mixture is it much more prone to rich misfire which people interpret as "going rich". It's all related to heat and power output.

     As soon as the airplane slows in-flight (like from maneuvering) , the load returns. This extra load is what causes the 4-2 break, for example, or causes a 40VF to boost the power output as it's running close to in-tune for the pipe. This is extremely beneficial since it allows the speed to recover and not drop as much as it could otherwise. The instant the airplane slows down, the fuel delivery pressure actually spikes up dramatically due to hydrostatic pressure (maybe a full G or more), but it nonetheless sounds "leaner".

     There's a whole bunch of very interesting things about this and it's absolutely fundamental to understanding how stunt engines work. Good question. Don't expect a raft of very good answers, nothing we have ever talked about has had more arguments and more utter and complete bilge spewed about it than how power, torque, mixture, "apparent mixture", torque and power curves, and anything associated with engine operation.

     I would suggest digging through old SNs and finding and article called "The Two-Cycle Stunt Engine" by Scott Bair (and caused to be reprinted by Tom Dixon). It's dated as it discusses primarily 4-2 break engines and ST60's in particular, which for the most part have not been a mainline stunt engine for the last 25 years. But it's has so many nuggets of *real engineering information*, instead of the usual misguided random speculation, that its should be required reading.


    Brett

All my profiles lean out -- probably from tank placement.

Ray: the torque that the engine needs to exert on the prop goes down as the prop starts traveling through the air.  That's the "unloading" part.  I didn't know about the going richer part, 'cause I fly all profiles.

Hi Tim

that is normally the tank setup, all my profiles went richer sounding in the air, and if they went leaner I adjusted the fuel delivery untill they did

Randy