Charlie,

You could be right about this.  I think I remember him telling me that he runs the engine at 2450 rpm at cruise.  I don’t know what his fuel burn is, but I looked at several of his flights on FlightAware and he is doing a ground speed of 150 knots just about everywhere he goes.  I realize that ground and true are only the same under no wind conditions, but I haven’t seen anything higher than 159 anywhere.

 

Bill

 

 

Here you go:
http://www.vansaircraft.com/public/rv-7per.htm

Van's numbers are: 75% cruise at 8k ft is 207 statute mph. This is with a constant speed prop, but even with a wood fixed pitch, he shouldn't lose more than 5 mph unless he has it pitched for climb (not much need for that with 200 hp...). If google's math is right, assuming 200 mph cruise that would be 173+ kts. Assuming 150 hp, .45 lbs/HP/Hr properly leaned fuel burn & 6 lbs per gallon fuel weight, that's ~11.25 gal per hour fuel burn (say 11.75 gph for sloppy and/or paranoid leaning practice). I bet he's burning a lot less than that, right?

I bet that he's running at far less than full throttle at 8000 ft & turning closer to 2400 rpm than the rated 2700 required to get 75% power at that altitude.

Charlie


On 07/09/2012 10:38 PM, Bill Bradburry wrote:

Hi Charlie,

 

I don’t have any idea what Van’s performance numbers are??  Where would I find those?

 

Bill B

 

 

Hi Bill,

There's something seriously outside the norm if he's only getting 150 kts at 8k ft full throttle. Any properly built -7 will come very close to Van's spec performance numbers. The fixed pitch prop would have little effect on cruise, if pitched properly for the plane & HP. 160 hp -7's with FP props can easily hit that cruise figure.

Charlie


On 07/09/2012 02:02 PM, Bill Bradburry wrote:

Brian,

 

Getting “on the step” might get you at your terminal velocity/hp/rpm equilibrium sooner, but no matter when, at some time it will all level out at the equilibrium point.  You might be able to dive and pick up 9% more, but it will not hold and will eventually bleed off the same point no matter how you got there.

 

It seems true that you can judge the rotary hp by its rpm, but in the case of the shallow dive, you are actually reducing the load on the prop and the engine uses the hp not needed to pull the plane to increase the prop rpm.  When the load from the plane comes back, when you level out, the rpm will drop back because the hp is now being used to pull the plane as before.  If this didn’t work this way, you could carry a plane with a 3 hp lawnmower engine up to 30K feet and drop it out and the engine would be producing 10,000 hp by the time it got down to sea level!!  It would then level out and maintain mach 2!!!  :>)

 

My next door hangar neighbor has an RV-7 with a 200HP fixed pitch prop and he is pretty consistently running 150 knots of true airspeed at 8K feet.  You could probably use him as a guide for performance on your plane.  At 8K feet he is at close to 75% power.  Say 150 HP.

 

B2