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From: "Monty Roberts" <montyr2157@alltel.net>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
References: <list-1578713@logan.com>
Subject: Duty cyle, was MSD
Date: Wed, 13 Dec 2006 09:44:27 -0600
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        100% of the power available is 100%.

        It is almost never the rated or demonstrated power of the engine in 
some controlled situation, like a dyno room on the ground.


That is the point I was trying to make. The engines rated HP would be what 
you get in a dyno room at SL standard conditions. The duty cycle of the 
engine from a power output standpoint is what time averaged percentage of 
max power it sees in a particular application. For example take the 250 hp 
PPort that makes its power at 9000 rpm. In an aircraft you will never see 
250 hp continuous unless you use a CS prop, a turbo and an appropriate gear 
box. It will always be less, even with the prop stalled. Reducing the 
throttle setting or leaning the engine does count, it reduces the power 
output of the engine and thus mechanical and thermal stress experienced by 
the hardware. Cruising LOP at 12000 feet, even at WOT will produce much less 
than 250 hp. If the engine produces less power, the hardware is happier 
regardless of how it happened. Duty cycle must refer to a fixed point 
somewhere. Available WOT power changes depending on the rpm (determined by 
the prop), altitude, and mixture.

Any change in load on the engine like a driver lifting his foot for a few 
seconds, going into a corner, makes a huge difference in cooling. It gives 
the cooling system time to remove heat from the combustion chamber area. It 
then takes time for that area to heat back up. Then he is into the next 
corner, and the process repeats. In an aircraft this never happens. It is a 
continuous heat flux with no chance to cool.

Difficulty cooling the rotary needs to be taken with a grain of salt. There 
are too many installation variables. Assuming a functioning and properly 
designed cooling system is attached to the engine, I don't think there will 
be a problem cooling the engine as long as continuous power output is kept 
below 100 hp/rotor. That may be a bit conservative, but I think people who 
want to make huge continuous power output are going to have problems cooling 
the combustion chamber area. Of course the argument that a TC needs to be 
placed there has been made by PL and is a good point. People who are 
turbocharging and using a CS prop should do this.

The duty cycle argument is always used by people claiming that automotive 
engines will never work in aircraft. The point is, from a stress on the 
hardware standpoint, what matters is the duty cycle relative to max output. 
In a street car it might see 20% or less. In an aircraft it will probably be 
closer to 50-75% depending on how you cruise, what type of airplane etc. 
This is higher than what the engine in the car sees, but not the often 
touted 100%.  In a boat it might be 50-100% which automotive engines are 
used in and work just fine.

Monty