Ira and Remi,
Great watching your comments.
Curiosity got to me on how both of you stick by your guns so I decided
to review the DA20 manual and stick my nose where it doesn't belong.
You are comparing apples and oranges.
Do not confuse Thrust horsepower and shaft horsepower.
Ref the DA20 manual figure 5.8(b). Max power is the thrust horsepower
available for that specific prop and engine combination cross referenced
to the altitude, temperature vs the TAS etc...This clearly shows Ira's
point that the shaft horsepower of the engine, as the altitude is
increased, drops off. At sea level the engine prop combination produces
130 ktas, but follow the standard day line up to 6000 feet and the max
power the engine is capable of is 75% and yields 121ktas. Use the
dotted line example and then look at as temperature decreases, the
horsepower increases at a constant altitude, resulting in an increase in
TAS because of an increase in shaft horsepower due to an increase in air
density. Shaft horsepower is directly proportional to fuel air mass
flow and RPM pumping out the watts of power.
Remi, you are right in saying that you get more power, but the real
answer is, thrust horse power or THp. THp goes up as speed increases.
>From one of my previous references "Airplane Aerodynamics", I believe
the problem that IRA confuses is he is talking brake horsepower, where
as you are analyzing thrust power required for level unaccellerated
flight. The equation to derive THp is: THp available = TV/550.
Thrust is generated by the relationship of Brake HP times propeller
efficiency. What this shows is there is no Thrust horsepower when
stationary (V=0). Note on the chart referenced above how flat the
power curve line is. But when the speed of the aircraft is considered,
an increase in THp is directly proportional to the speed. Boy this is
boring... Diamond's charts are to make the pilots life easy and are a
brilliant way to get the most from their prop/engine combination. It
really is a plot of HP avail vs HP required points referenced to
altitude and speed. Pretty slick.
Do not confuse MP with horsepower. Let's face it, MP is just the
pressure measured between the valves and throttle body. When combined
with the rpm, fuel flow and prop efficiency you get a number that
equates to the THp measure above. The IO-240 is a fuel injected,
mixture adjustable, ram air induction, engine with a highly refined
Sensenich cruise prop. (If you think Rotax engine data sucks, try to
get the Continental folks to give you the test material. Essentially,
you must show a need, as they don't want it in general distribution.) As
the speed increases at altitude, the Sensenich prop doesn't loose
efficiency as quickly as a normal prop and with the aid of ram induction
you can achieve 65% thrust horsepower at a higher altitude, but your
shaft horsepower is lower. Normally a fixed prop unloads significantly
with speed increase, whereas the Sensenich W69EK-63 is a highly refined
prop tuned to the DA20 airframe and IO-240 engine combination. It has a
target sweet spot of 65 to 75% Thrust Hp from 4000 to 8500 feet at a
nominal 120KTAS while still allowing good takeoff performance. This is
no off the shelf prop. The net result is a prop that can allow you lean
the mixture to compensate for air density so as to use the extra MP from
the ram air and not swing the prop too fast. With a prop pitched like
the one used on the DA20 attached to a Rotax / Bing equipped engine you
would lug the engine down until the plugs fowled shut.
The Europa 912S, with Airmaster prop and non adjustable mixture is a
different animal. It behaves like a normally aspirated engine lowering
its MAP as altitude increases. The fuel injected IO-240 is blessed with
a better intake and fuel metering system. But alas the shaft horsepower
decreases with altitude just as any other plane.
So you're both right.
Better to just plot data for your plane, figure out what works best for
your engine/prop/airframe combination and tweak it like you should.
Every airplane has a sweet spot, you just gotta find it. I prefer to do
it systematically as IRA does, I just prefer a lot more data points to
plot the HP req vs Velocity curves like Dykins does... From these
curves, better understanding of max range, max endurance, miles per
gallon, speed vs economy decisions can be made.
Have a great holiday.
Bud
----- Original Message -----
From: Remi Guerner<mailto:air.guerner@orange.fr>
To: Europa-List Digest Server<mailto:europa-list@matronics.com>
Sent: Saturday, December 27, 2008 12:56 PM
Subject: Europa-List: power settings
Ira,
You misread me. I said that a given combination of MP and RPM gives
you more power at altitude. I am not talking about max full throttle
power. Of course I agree that full throttle power decreases with
altitude on any normally aspirated engine.
Go back to the SR20 exemple: flying at 8000 ft, 2500 rpm and 21.2"
MAP give you 65% power. Now flying at 2000 ft you need 23.8" of MAP to
get the same 65% power at the same 2500 RPM, so obviously if you fly at
2500 rpm and 21.2" MAP at 2000 ft you will get less than 65% power. At
2000 ft the ISA temperature is 11degC while at 8000ft it is -1 degC.
Therefore the density of the air at the same 21.2" pressure is higher at
8000 ft than it is at 2000ft. That is why this 2500 rpm and 21.2" MAP
setting will give you more power at 8000 ft than the same setting at
2000 ft. From my calculation this setting will give 62% power at 2000
ft. That is just the laws of physics and all aircraft engine
manufacturers seems to agree with them.
Cheers
Remi
>>>>>> Your suggestion that a normally aspirated engine
will develop more power at 8000' than at 1000' is quite
curious.<<<<<<<
http://www.matronics.com/contribution<http://www.matronics.com/contributi
on>
http://www.matronics.com/Navigator?Europa-List<http://www.matronics.com/N
avigator?Europa-List>
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