Trim for speed? Pitch for speed? Power for altitude

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BrianL99
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby BrianL99 » Tue Jun 16, 2015 6:40 pm

3Dreaming wrote:
BrianL99 wrote:
Nomore767 wrote:
In any airplane with a fixed pitch propeller will change RPM with pitch changes. The direct drive engines have a smaller change in RPM than the Rotax because of the gearbox. When you factor in the 2.43 to 1 gear reduction you will find the change in propeller RPM is about the same.
What I tell students is set the RPM and don't try and chase it. Just fly the airplane. I will pull the power back a little if the RPM gets into the yellow.


That's interesting. I wasn't all that aware of the fluctuations of a fixed pitch prop. I had less than 50 hours experience with a fixed pitch before I bought my LSA. The Cirrus prop has a "clutch" and reacts differently to pitch.

Why are you reluctant to run a Rotax in the "yellow"? Rotax says the engine will run forever under 5800 rpm's.

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FastEddieB
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby FastEddieB » Tue Jun 16, 2015 8:17 pm

BrianL99 wrote:The Cirrus prop has a "clutch" and reacts differently to pitch.


More proper to say a governor that controls rpm.

Why are you reluctant to run a Rotax in the "yellow"? Rotax says the engine will run forever under 5800 rpm's.

Nope. 5 minute limitation between 5,500 and 5,800 rpm.

From the 912 Owner's Manual:

1. Speed:
Take-off speed .................................... 5800 [rpm] (5 min.)
Max. continuous speed ....................... 5500 [rpm]
Idle speed ........................................... approx. 1400 [rpm]
Fast Eddie B.
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BrianL99
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby BrianL99 » Tue Jun 16, 2015 8:30 pm

FastEddieB wrote:
BrianL99 wrote:The Cirrus prop has a "clutch" and reacts differently to pitch.


More proper to say a governor that controls rpm.

Why are you reluctant to run a Rotax in the "yellow"? Rotax says the engine will run forever under 5800 rpm's.

Nope. 5 minute limitation between 5,500 and 5,800 rpm.

From the 912 Owner's Manual:

1. Speed:
Take-off speed .................................... 5800 [rpm] (5 min.)
Max. continuous speed ....................... 5500 [rpm]
Idle speed ........................................... approx. 1400 [rpm]



Yes, "Governor" :)

I just checked my Rotax Owner's Manual and you're right Eddie, thank you for the correction. I could have sworn I read somewhere that they could run all day under 5800, but I must have been delusional when I read it.

Does anyone run their Rotax at a 1400 rpm idle? Mine will hardly stay running that low. I have it adjusted so I can get it down there, but it's not very stable at 1400 ... it takes about 1950 to smooth out.

5800 at Take Off? I don't think I can get mine to 5000 at Take Off.

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Re: Trim for speed? Pitch for speed? Power for altitude

Postby chavycha » Tue Jun 16, 2015 8:48 pm

The Skycatcher's electric trim is really coarse. Oftentimes there isn't a correct setting which relieves all control pressure in cruise. You then have to adjust power slightly to get a stable altitude hold.

I prefer the manual wheel trim that Cessna used on its other light singles. Much more intuitive and infinitely adjustable.
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FastEddieB
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby FastEddieB » Tue Jun 16, 2015 8:56 pm

BrianL99 wrote:5800 at Take Off? I don't think I can get mine to 5000 at Take Off.


I repitched the prop on my Sky Arrow to get about 5,200 rpm on takeoff. Before I did I was not quite hitting 5,000 rpm.

Much better performance on takeoff and climb. Maybe a slight hit in fuel economy at cruise.

I can freely experiment, since I'm Experimental. Others may be limited by what the manufacturer specifies for prop pitch.

Best to remember that a ROTAX makes 100 hp, but only at 5,800 rpm. The farther from that you are, the less power you actually get from your engine.

Just a thought - a fixed pitch prop is always a compromise.
Fast Eddie B.
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MrMorden
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby MrMorden » Wed Jun 17, 2015 7:12 am

BrianL99 wrote:I've never really seen much discussion of the stability of an LSA and quite honestly, they're damn unstable.

It's sort of the "dirty little secret" of LSA's ... they can be a handful to fly.


I don't think it's a "secret" that very light weight airplanes with low wing loading are less stable in turbulence and more susceptible to wind effects than heavier ones.

But yes, they *can* be a handful. That's one of the reasons there is a higher incidence of LSA accidents with pilots that transition from heavier airplanes, than pilots who have only flown LSA. They require somewhat different techniques from planes with more inertia and higher wing loading, and many with the "it's just like any other airplane, they all fly the same" attitude have learned that lesson the hard way.
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MrMorden
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby MrMorden » Wed Jun 17, 2015 7:14 am

chavycha wrote:The Skycatcher's electric trim is really coarse. Oftentimes there isn't a correct setting which relieves all control pressure in cruise. You then have to adjust power slightly to get a stable altitude hold.

I prefer the manual wheel trim that Cessna used on its other light singles. Much more intuitive and infinitely adjustable.


I like my manual trim. Sometimes I just barely move it just to fine tune, doing that is a bear with electric.
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

Nomore767
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby Nomore767 » Wed Jun 17, 2015 8:51 am

"Does anyone run their Rotax at a 1400 rpm idle? Mine will hardly stay running that low. I have it adjusted so I can get it down there, but it's not very stable at 1400 ... it takes about 1950 to smooth out."

I would say 1400 rpm is too low.

I recently had the carbs rebalanced in my 912 ULS. Afterwards I noticed the idle was too low. Of course if you yank the power back quickly it can go lower but even a gradual move to idle in the flare would illicit an "Engine Speed" aural warning and the idle rpm would be in the yellow. It bothered me that it could stop on approach. I get the "engine speed" warning normally as the engine stops.

On shutdown I always stabilize at idle and then switch off each ignition module slowly, one by one. Normal idle rpm would show a drop but the engine would still run. Recently mine would shut down at this point.

Yesterday I had my idle speed adjusted to approx 1800 rpm and it's much better.

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MrMorden
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby MrMorden » Wed Jun 17, 2015 9:49 am

Nomore767 wrote:"Does anyone run their Rotax at a 1400 rpm idle? Mine will hardly stay running that low. I have it adjusted so I can get it down there, but it's not very stable at 1400 ... it takes about 1950 to smooth out."

I would say 1400 rpm is too low.

I recently had the carbs rebalanced in my 912 ULS. Afterwards I noticed the idle was too low. Of course if you yank the power back quickly it can go lower but even a gradual move to idle in the flare would illicit an "Engine Speed" aural warning and the idle rpm would be in the yellow. It bothered me that it could stop on approach. I get the "engine speed" warning normally as the engine stops.

On shutdown I always stabilize at idle and then switch off each ignition module slowly, one by one. Normal idle rpm would show a drop but the engine would still run. Recently mine would shut down at this point.

Yesterday I had my idle speed adjusted to approx 1800 rpm and it's much better.


Agreed. I think 1400rpm is the *minimum* idle speed. Mine has been 1600rpm to 1800rpm. 1600rpm takes more throttle management on the ground to avoid gearbox chatter, above 1800rpm the airplane doesn't want to come down as well. I can't image running 1400rpm.

I have a friend who runs a Rotax 582 in his airplane...his idle is 2700rpm. :shock:
Andy Walker
Athens, GA
Sport Pilot ASEL, LSRI
2007 Flight Design CTSW E-LSA

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dstclair
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby dstclair » Wed Jun 17, 2015 10:15 am

The generally agreed idle RPM for fixed-pitch Rotax 912 is ~1800 rpm (do a search on this forum, rotaxowner and ctflier).

Brian -- I think I'm at 1750 on my Sting which works pretty well. I've had it as high as 2000 which seemed to generate more speed than I wanted when landing on shorter strips. I'm one of those who likes to run the pattern (from abeam the numbers) at idle. YMMV.
dave

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azsportpilot
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby azsportpilot » Wed Jun 17, 2015 1:57 pm

BrianL99 wrote:
HAPPYDAN wrote:All very interesting - and I thought all those fluctuations I have been experiencing were just the Skycatcher or, more likely, my inexperienced handling. It's somewhat comforting to know it's just a characteristic of LSAs in general.


That's another reason I brought up the subject. I've never really seen much discussion of the stability of an LSA and quite honestly, they're damn unstable.

It's sort of the "dirty little secret" of LSA's ... they can be a handful to fly.


Agreed..... the public perception is that they are easier to fly and this could not be farther from the truth

many pilots transitioning from 172's and such are shocked to find they lack the skills, finesse and precision to safely operate even the most benign LSA's..... CT and the like
John Sarra CFI-S
1946 Aeronca Chief

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Re: Trim for speed? Pitch for speed? Power for altitude

Postby SportPilot » Wed Jun 17, 2015 2:05 pm

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Last edited by SportPilot on Wed Aug 19, 2015 4:08 pm, edited 1 time in total.

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FastEddieB
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby FastEddieB » Wed Jun 17, 2015 2:23 pm

SportPilot wrote:I don't think LSA is any easier or harder to fly, just different.


Agreed.

I don't think they're markedly different from typical 2-seat trainers - C-150, Tomahawk, Skipper, or Cubs, Champs and the like.

Light weight and light wing loading makes them bounce around a bit more. Then again, with less weight they can be more responsive since there's less inertia.

Like SportPilot says, just different.
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3Dreaming
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby 3Dreaming » Wed Jun 17, 2015 4:39 pm

I think Fast Eddie and Sport pilot are correct in that they are different. The biggest difference when transitioning is the fact that many LSA have more adverse yaw than many of the airplanes people have been flying. At least that is the biggest problem area I see people having when trying to transition. A quiet smooth stick makes a happy airplane.

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FastEddieB
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Re: Trim for speed? Pitch for speed? Power for altitude

Postby FastEddieB » Wed Jun 17, 2015 6:01 pm

The other thing that can bite a pilot "stepping up" to Light Sport is the increased effect of any given crosswind.

We get accustomed to thinking "we" can easily handle a 12G20 crosswind, let's say, in a Cirrus or Bonanza or Mooney.

But then we find that same wind is a much larger factor in a plane that lands more slowly.

I know I have to periodically remind myself of that fact when making go/no go decisions.
Fast Eddie B.
Sky Arrow 600 E-LSA • N467SA
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