snips...
Paul and Jerry,
I would love to see the variable PP design installed and
tested. Jerry, would you
be interested in installing the variable PP intake valves
that Paul built? I would be
happy to send them to you if you are interested.
It is thought by some that a PP motor with large amounts
of overlap has
somewhat poor idle characteristics due to exhaust gas
dilution of the intake charge.
In order to minimize this intake charge dilution we decided
to use the same
technique that are used on 2-stroke racing motorcycles,
i.e., vary the port timing on
the intake port. A secondary benefit is that at small
throttle settings the velocity of
the intake charge is accelerated while in the intake port,
further promoting fuel
atomization. Also note that the BSFC should improve at
cruise due to the more
conservative port settings.
A slower idle is important because even at 700 - 800
prop rpm a highly pitched
prop will generate lots of thrust. Since the Fast Glass
and the RV's are difficult to
slow down a slower prop speed is beneficial. Note that a
1200 rpm idle (engine)
when used with a 2.85 redrive will result in a prop rpm of
only 421 rpm, a
substancial reduction over a 1800 rpm idle and a 2.17
redrive wich results in a prop
rpm of 830 rpm.
Ken Powell
If you are good you can get a high sink rate by putting the
airplane on the
back side of the power curve. Pull the nose up short of stall
and watch
the vertical sink rate increase. Its called "dragging it in".
Also a slip will increase the sink rate.
Paul Lamar
This has come up before. Paul, don't you think you should be
careful about what you might be saying to low-time pilots
out there?
Before you guys make that a regular practice, it would be a
good idea to find an old gray haired instructor who's spent
his career doing primary instruction in simple a/c. Ask him
what he thinks about the idea.
Charlie England
You have a very good point there Charlie. I forgot to say;
"practice at altitude!".
Paul Lamar
And, practice at altitude with an instructor who will, at
some point in the simulated 'behind the curve' approach,
pull the mixture to simulate an engine failure. Enjoy the
ride...
Charlie E
Another good point Charlie. I wonder what the altitude window is
to survive? :-) I am sure it depends on how far behind the power
curve you
are. If you are making a conventional approach you can still die
if the engine quits on final. I see no real difference.
The only safe approach if the engine is going to die is the slip.
Paul Lamar
cha-CHING...
There's a world of difference between hitting the ground
with the plane trimmed for best glide, & hitting it half way
through the first turn of a spin.
CE
I agree Charlie but who said you will necessarily stall spin
when the motor quits on the back side of the power curve. We are only
talking a few MPH difference in the approach speed. Say 66 rather than 70 MPH
I talked to an old pilot flying a Martin twin engine cargo plane down in
Baja and he told me he always near stalled the airplane at a comfortable
altitude to see exactly what the stall speed was so he could adjust his
approach speed accordingly to minimize the landing distance. The point
is the optimum approach speed is a strong function of your gross weight.
If you stick to a fixed approach speed all of the time you may or may
not be making and optimum approach. If your gross weight is high
you may stall spin out of a recommended approach if the engine quits
as well. You just need to be aware of the possibility and be prepared
to shove the nose over right now. If you are ten feet over the trees there
is not much you can do in any case.
If I'm on final approach (particularly at only 10 ft over the trees) and
the engine quits I'd much rather be at a best glide AoA than at a behind
the power curve AoA. If you are going to be making max performance landings
you should be looking at AoA, not airspeed. Too many variables as you
mention that make airspeed too inexact for a max performance (minimum
landing speed/distance) landing. Its like playing Russian roulette. Add an
AoA instrument to your airplane. It eliminates the variables. A good AoA
indicator is accurate regardless of weight, G-load, etc... The guys who
define max performance landings (the US Navy) fly AoA, not airspeed. And
even with AoA I'd fly a nose low, full flaps, slipping approach rather than
a nose high behind the power curve approach.
Mike WIlls
Somehow I think we've gotten to an apples and oranges discussion here. The power
curve is crossed when you reach an angle of attack that requires more power to
maintain level flight. Let's take the 172 as an example. You can reduce power
until you reach an airspeed of 65 kts and an engine speed of 1600 RPM and maintain
altitude. If you wish to go slower AND maintain altitude you must add power.
Voile, you are on the backside of the power curve. If you do not add power you go
down. As my instructor used to say, "to land, you must first go down". If you're
going into our tree lined, power lined, short field you must increase sink rate to
carry the minimum forward energy over the fence (plus a safety margin). If you use
55kts (5 kts slower than POH recommended short field approach speed ) you are at a
sink rate that puts you well below recommended glide slope, assuming our field has a
VASI or PAPI to let you know. To get behind the power curve on approach you would
have to take the 172 below the very bottom of the white arc on the airspeed
indicator, in effect making your approach in a falling leaf stall, which would, of
course, be nuts. I don't have the luxury of an AoA indicator in the club 172's.
Anyone eaves dropping on the cockpit when I'm on approach would hear the same chant
every time, "airspeed, airspeed, airspeed."
Rick Girard
13B + LEZ
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