Today's Message Index:
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1. 05:20 AM - Re: amphibious floats (Thom Riddle)
2. 06:02 AM - Re: Allegro Cruise Speeds (Thom Riddle)
Message 1
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| Subject: | Re: amphibious floats |
--> Allegro-List message posted by: Thom Riddle <jtriddle@adelphia.net>
Bob,
I'm glad to hear about the amphibious floats. I hope the FAA does what
I've heard they are going to do, change the ill-conceived rule about
not being able to change the amphibious gear in flight. Sort of makes
the idea of amphibian ridiculous if that is disallowed. Let me know if
you guys get an amphibian Allegro. I'd have to fly over there to see
it.
Thom in Buffalo
do not archive
Message 2
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| Subject: | Re: Allegro Cruise Speeds |
--> Allegro-List message posted by: "Thom Riddle" <jtriddle@adelphia.net>
Paul,
Rotax uses Max. Continuous Cruise RPM and power as the base or 100% since anything
higher than 5,500 is limited to 5 minutes. There are two curves on one chart
shown in the Rotax manual. One curve is for power availalbe and the other is
for power required, if my memory is correct. There is also a table for RPM and
Manifold Pressure settings which equate % power. With an in-flight adjustable
prop, like on my Titan Tornado, or constant speed prop. this table is the
key to setting power. But this table is of little use for fixed-pitch props. That
is why I stated in the data I gave you for the power/speed of 912 engine,
that a change in prop. pitch will invalidate all the data.
5,400 rpm on a 912 series engine (AT SEA LEVEL) is about 98% power if the prop
is pitched in the normal range, i.e. it is loaded. At least one article I've
read about the Allegro states this rpm to be 75%. Just not true and a greviously
mis-leading statement in my opinion.
That said, the way a ground adjustable prop is pitched is one factor that will
determine at what rpm a given amount of power is actually being delivered and
thus fuel consumed. The load is also a big factor. For example, go to straight
and level flight and set your rpm to 4,800 RPM. Notice that in level flight the
throttle is no where near wide open. Set your trim for Vy and advance throttle
to wide open. With the prop set like mine is, the Allegro will start climbing
and at Vy the RPM is still about 4,800 rpm. Which one of these conditions
is consuming more fuel? They are at the same rpm, so why not the same fuel? The
load is far greater during climb at this rpm and the throttle is wide open too.
As I've stated before, my prop is set for best cruise speeds at relatively low
density altitudes. That is, wide open throttle at DA of less than 7,000 feet will
produce 5,500 rpm. At this pitch, our engine turns only 4,800 rpm at Vy and
wide open throttle. At wide open throttle, 4,800 rpm at sea level, the engine
is producing about 84% power. If I wanted/needed better climb performance I
could reduce the pitch a little bit which would result in higher RPM under the
same conditions, say 5,000 rpm, which would yield about 89% power and higher
climb rate. BUT, this means that in straight and level I could pass the 5,500
rpm limit for max. continuous cruise, because with less ptich there is less load
on the engine at a given throttle setting. However, this would also allow for
higher rpm at higher altitudes this producing a little higher cruise speeds
at higher density altitudes at wide open throttle.
As you can see the relationship between power, RPM, and airspeed is complex and
the main reason that airplane performance can benefit greatly from an in-flight
controllable ptich propellor. I can pitch the prop. on my Titan Tornado for
5,500 or even 5,8000 rpm at wide open throttle at Vy climb speed and get 1,400
ft./min. climb rate (5,500 rpm), even with its very short 20' wingspan. When
I reach cruise altitude, I adjust the pitch so that I get 5,500 rpm at wide open
throttle, then reduce the throttle to about 4,200 rpm for low fuel consumption,
and low noise level cruising at nice comfortable cruise speed. Even though
both airplanes have very close to the same max. cruise speed, my normal cruise
speed in the Titan with the adjustable pitch prop is only 4,200 rpm but 4,700
rpm in the Allegro, all because of the in-flight adjustable prop.
In the USA, aircraft with Light Sport Aircraft air-worthiness certificates are
not allowed to have in-flight adjustable propellors. So this is not an option
for Allegro owners in the USA. I don't know the rules in other countries. That
said, if one wanted a good compromise between climb and cruise, I would set the
prop. pitch so that the eingine would turn about 5,000-5,100 rpm with wide
open throttle at Vy.
Sorry for the long-winded response but the relationship bewteen prop pitch, engine
RPM, power and thrust produced is complex.
Thom in Buffalo
Read this topic online here:
http://forums.matronics.com/viewtopic.php?p=10149#10149
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