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.