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
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of Charlie England
Sent: Tuesday, July 10, 2012 2:20
PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] Re: On the
step
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
From: Rotary motors
in aircraft [mailto:flyrotary@lancaironline.net]
On Behalf Of Charlie England
Sent: Monday, July 09, 2012 3:45
PM
To: Rotary motors in aircraft
Subject: [FlyRotary] Re: On the
step
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
I commented on here recently about how I took a
cross- country flight to Walla Walla from Everett,
Wa
recently and was only seeing 140-150 mph cruise at 11,500 feet. It got me to
thinking- I was getting about 6200 rpm on the engine which is 2175 at the prop.
So, I'm a little overpropped/ underpowered.
So, there's the old chestnut about how you can get a
few more knots from an airplane in cruise if you go a little above your cruise
altitude and then descend to your cruise altitude, thereby getting "on the
step" . Some pilots swear by this, others dismiss it. I have a theory- if
I climb to a cruise altitude and stay at full throttle while doing so, I will
only get the engine to run up to a certain RPM and therefore a certain hp will
be achieved, and no more and so the airplane will settle into an equilibrium at
a certain speed- say at whatever hp/speed - 140 mph at 6200 rpm- lt's say x hp.
Now, If I were to descend to my cruise altitude in a
shallow dive, I will be acheiving a higher rpm and hence horsepower, say 6800
rpm and horsepower will be x + 9%x for a hp of 109% of the hp achieved at 6200
rpm, so I should be able to achieve and maintain a higher cruise speed if
I descend to my cruising altitude. ( I picked 9% arbitrarily for the sake of
argument) .
Anyone want to shoot me down on this one?
This is just an theory, If I want to go faster first I
have to clean up my drag. My cooling scoop is effective, but obviously draggy.
And It needs a flap on it. That's a project for this winter.