----- Original Message -----
From: Russell Duffy
Sent: Wednesday, May 05, 2004 7:54 PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: surging / rough idle
1. Do not use injectors that are to large. Go to the RC Engineering web site (www.rceng.com) and calculate the size you need based on HP needed and an 80% duty cycle. (225 HP requires 4 injectors of 38 lb each at 80% duty cycle, fuel pressure of 38 PSI, and a BSFC of 0.5)
I just spent some time looking at the RC engineering site, as well as the other fuel injector sites I could find. In each case, these calculations are made without ever asking about rpm. I just don't see how they can do that ??? How do they know if my 225 HP would be at 2000 or 10000 rpm? The same amount of fuel will have to be delivered in either case, but you'll need a whole lot more flow rate to deliver the fuel at 10000 than you would at 2000.
Now, before anyone says it, I know that Tracy's using those wimpy injectors with his current system, making 200+ HP at 7000+ rpm, and isn't having any problems, so clearly they're big enough. I really hate it when he does that :-)
Rusty (prop and bolts arriving tomorrow)
Rusty, were'd you come up with this theory? True, It takes the same amount of fuel to make X horsepower regardless of rpm. The injector doesn't fire as often at low rpm BUT, it stays open for a longer time (assuming the engine is making the same power). It all comes out the same in terms of flow rate requirement of the injector.