follow-up on constant-speed propellers
Marc Adler
controlled by the pilot. What's the point of that? Does the airplane
automatically lower the rpm when cruising?
Marc
SR20GOER
<fly...@gmail.com> wrote in message
news:1161837008.9...@i3g2000cwc.googlegroups.com...
> One word: FADEC
> Here you go: http://en.wikipedia.org/wiki/FADEC
>
> -Nik
Sadly, not a FADEC which is a pity.
Basically just a cable between the throttle and the governor.
Means a Cirrus is high revving in cruise. No manual or auto compensation.
I have never followed it through the maths but I suspect ability to coarsen
the pitch in cruise in a Cirrus would give better speed / economy.
Also means an approach is best at 15-20% power to "spool up" for a go
around.
Brian
Michael
> I've heard that Cirruses have constant-speed propellers which can't be
> controlled by the pilot. What's the point of that?
The point is to convince people that it's easier to fly than it is.
Most people these days learn to fly in airplanes where the gear is
fixed and the prop pitch is fixed. The idea behind the Cirrus is that
it's just as easy to fly - no gear lever, no prop control.
In reality, two extra levers don't add much to the difficulty of flying
a plane - an Arrow is no more demanding than a Cherokee 180, even if
the FAA does require a special endorsement. And the Cirrus demands
more planning in the descent and deceleration than any plane I've ever
flown - more than the high end Mooneys, Bonanzas, etc.
> Does the airplane
> automatically lower the rpm when cruising?
The engine revs at redline (2700, I think) with the throttle full up,
and drops back to 2500 the moment you pull the throttle back a bit.
It's pretty inefficient - most of the time (at lower altitudes and
reduced power settings) you could do a lot better at 2100-2400 RPM.
Basically, the prop control is a poor decision designed to appeal to
low time pilots.
JMNSHO, of course.
Michael
Doug
throttle changes, even when the pilot does not move the prop control,
don't you?
SR20GOER
"Doug" <anoth...@access4less.net> wrote in message
news:1161977388....@h48g2000cwc.googlegroups.com...
> You do realize that the govenor is adjusting the blade angle due to
> throttle changes, even when the pilot does not move the prop control,
> don't you?
>
Yes, but .......
When in cruise throttle changes have virtually no effect on the blade angle.
So you have neither the ability to coarsen it at high altitude and speed nor
ability to fine it on the approach with your PUFF checks.
Certainly it does simplify matters for the low time pilot but also certainly
it does not allow maximum efficient operation and even more certainly it is
wise to carry 15-20% power on final so the prop is ready to spool up on
sudden power application.
Perhaps the 20 could keep the single lever and the 22 get a blue knob in an
ideal world.
Brian
Mxsmanic
> And the Cirrus demands
> more planning in the descent and deceleration than any plane I've ever
> flown - more than the high end Mooneys, Bonanzas, etc.
How come?
> Basically, the prop control is a poor decision designed to appeal to
> low time pilots.
That seems to be the target market for most of the Cirrus features,
from what I've read.
--
Transpose mxsmanic and gmail to reach me by e-mail.
Michael
> > And the Cirrus demands
> > more planning in the descent and deceleration than any plane I've ever
> > flown - more than the high end Mooneys, Bonanzas, etc.
>
> How come?
The plane is slick, and there is nothing to help
the pilot slow down. No gear to drop, very low flap speed and flaps
that don't add much drag, and you can't even push the prop forward
without powering up.
Michael
Roy Smith
"Michael" <usenet...@thisoldairplane.com> wrote:
Are slips prohibited? Or are they just not very effective?
Michael
> Are slips prohibited? Or are they just not very effective?
I'll be honest - I never really considered a slip to be a viable way to
get down or slow down in IMC. Sure, you can do it - but you have to
maintain cross-control pressures, and that requires you to focus too
much attention on controlling the airplane to do much else. Not a huge
deal on landing, but not something that makes sense when you need to
drop 5000+ ft in solid soup.
Slips are generally not a very effective way to lose altitude in glass
anyway - the fuselage tends to be too slick. But that's conjecture -
I've never done a forward slip in a Cirrus.
Michael
Mxsmanic
> The plane is slick, and there is nothing to help
> the pilot slow down. No gear to drop, very low flap speed and flaps
> that don't add much drag, and you can't even push the prop forward
> without powering up.
I understand the utility of flaps and gear to slow the plane, but what
do you mean by pushing the prop forward?
Michael
> I understand the utility of flaps and gear to slow the plane, but what
> do you mean by pushing the prop forward?
I mean pusing the prop control lever forward, which puts the governor
in high RPM mode. This causes increased drag.
Look at it this way - you need a certain amount of power to compress
air and overcome friction in an engine. At low power settings, you're
not actually making enough power to turn the engine and prop at high
(say 2500) RPM even without pushing back the air. If you are running
low power and turning high RPM's, you're actually stealing energy from
the moving air to compress the air in the cylinders and overcome
friction and the drag of the prop blades moving through the air at high
speed (much higher than aircraft speed). The faster the engine and
prop are turning, the more energy you are stealing - meaning there is
more drag. Thus we refer to windmilling - a regime where the prop acts
as a windmill, taking power from the air to turn itself and an engine
that is not producing sufficient power to make high RPM.
So if you are at low power and push the prop control forward to
increase RPM, you are actually increasing drag by windmilling at higher
RPM.
In order to reduce the drag caused by windmilling, multiengine
propeller airplanes and fixed-engine (meaning non-retractable)
motorgliders actually have provisions for stopping the propeller,
either by putting the blades edge-on to the wind, called feathering, so
they can no longer function as windmills, or by a brake somewhere on
the exposed portion of the crankshaft. The latter mechanism is almost
as effective at low speeds (and substantially simpler and cheaper),
because while there is some difference in drag between the blades in
the edge-on (feather) vs. normal position, the majority of the
windmilling drag is actually the compression of the engine the and the
movement of the blade through the air in rotation (at much higher than
aircraft speed).
Michael
Marc Adler
wrote:
> The plane is slick, and there is nothing to help
> the pilot slow down. No gear to drop, very low flap speed and flaps
> that don't add much drag, and you can't even push the prop forward
> without powering up.
Never flown a real one, but the Flight Simulator version certainly
behaves as you describe it. The only solution I've found to slow down
for a landing is to come in real low very early and just point the nose
up until just before the runway. This makes it hard to stay on course,
especially in any kind of crosswind, so I occasionally dip the nose
just to make sure I'm on track.
The reason I asked the question is because the RPMs remain very high
even after throttling back for cruise. The "simpler to fly (drive?
<g>)" explanation makes sense.
Marc
SR20GOER
"Marc Adler" <marc....@gmail.com> wrote in message
news:1162239313.9...@e3g2000cwe.googlegroups.com...
With the Cirrus you are certainly slippery but it also comes down to being
ahead of the aircraft.
Our technique is to have speed on downwind such that one stage of flaps can
be popped abeam the target threshold then pop the second stage after the
base turn at around 15-20% power, monitor the speeds all the way down and no
problems.
From memory the manual is not keen on sideslipping.
Cruise descents are best with planning and I find the ability to set ROD
excellent as you do your maths on the necessary height loss and then set
your target arrival altitude, ROD, and manage the power to suit. The
ability to guarantee a ROD is far ahead of the manual descent where one can
wander around the ROD and need to reassess several times.
Brian
Mxsmanic
Thanks for the detailed explanation. So if I set throttles to idle
without changing the RPM with the prop control, the props are actually
a substantial source of drag rather than simple neutral? I had
somehow assumed that as long as the prop was being turned by the
engine, it would not produce any drag (even if it was not producing
thrust), but from your explanation I can see that I overlooked some
factors.
Unfortunately, it's awkward to adjust the prop control in flight on
the sim, but maybe I can change control assignments or something.
Mxsmanic
> Never flown a real one, but the Flight Simulator version certainly
> behaves as you describe it.
Where did you find a Flight Simulator version of a Cirrus? Who makes
it and how well is it made?
Marc Adler
> Where did you find a Flight Simulator version of a Cirrus?
Here, I'll teach a man to fish -
1. Go to www.google.com
2. In the search field, enter the following phrase, _without_ any
quotation marks:
flight simulator cirrus sr22
3. Look at the first @#^@$^ hit at the top of the list. Presto! Your
answer!
In the future, this is what you do before posting questions like this.
If you have questions related to the information you got from Google,
then post them here. For example, if there were multiple companies
making Cirrus simulators, you could ask which one people recommend, and
why.
See how it works?
> Who makes
> it and how well is it made?
As a simulator, it's a lot of fun. I can't say much beyond that, never
having actually flown a plane.
Marc