Fox 35 Horsepower Curves

Fox 35 HP Curves
The Fox 35 has the Classic 4-2 break stunt run.  In level flight the motor runs rich, purring a long in a 4 stroke at around 8500-9000 rpm.  When the plane starts to maneuver the motor switches to a 2 stroke run and picks up a significant amount of power.  Most people use a 10/5 or 10/6 propellor to do this.  It nicely matches many of the old Classic planes like the Nobler, Smoothie, and Chief.

Note that the peak torque, which accellerates the plane in maneuvers, occurs at around 6500-7000 rpm.  Nomal takeoff rpm is around 9000.  It settles into a 4 stroke running pretty much the same rpm once in the air.  A 4 stroke at 9000 rpm produces less horsepower than the 2 stroke power shown in the graph.  The motor is putting out maybe .2-.3 hp. in level flight.  When the plane maneuvers the change in load and change in fuel head combine to make it start to fire on every stroke and usually speed up some too.  Exactly what happens depends exactly how things are trimmed- fuel, prop, glow plug, size of plane, compression ratio. The power goes up significantly, maybe to as much as .4-.5 hp.  The extra horsepower counteracts the extra drag induced from the lift needed to turn and the extra drag of the flaps.

If you go back and look at similar graphs for the Fox 35X, which has a very simililar design, you can see what happens when nitro is increased or the venturi is opened up.  Adding nitro increases the torque across the board, but really doesn't change the shape of the torque curve.  Going from 10% to 25% nitro ups the torque a lot, perhaps from 46 to 50 or so.  That is enough to swing a significantly larger prop or pull a significantly larger plane.  Or you can readjust the needle and get a much broader, more tolerant setting.

Opening up the venturi increase both the peak torque and moves the peak to a higher rpm.  On an engine designed for higher rpm the torque peak might go from 6500 or so up to 8,500 rpm.  Adding nitro will increase the peak torque and rpm even more.  Enough air and nitro can easily double the engines horsepower, using a much smaller prop and running at a much higher rpm.  But we don't want to put the old Fox through that, do we?