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
From: Rotary motors in aircraft
[mailto:flyrotary@lancaironline.net] On
Behalf Of bktrub@aol.com
Sent: Monday, July 09, 2012 12:43
PM
To: Rotary
motors in aircraft
Subject: [FlyRotary] On the step
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.