How to explain a CS prop to an A&P?

BigBadLou

BigBadLou

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Lou
Trying to get a pre-buy done on an airplane and when I asked the A&P about the tachometer showing only 2500rpm with the throttle and prop control knobs full fwd, he tried lecturing me on density altitude and how heat affects the maximum power available to an engine.

How can I politely explain to this A&P that a CS prop should maintain given RPM even with lower manifold pressure at higher altitude/DA?

(yes, I checked the POH and it says 2700rpm at full prop control fwd)
 
Well, except that if you don't have the power to drive 2700 RPM even at minimum pitch, it will top out where it tops out.
 
Ok Lou, so at least now I know the aircraft you're looking at is a CS prop plane, 2700 rpm limit...that limits things down a bit...;P

Cessna 182/Piper Arrow perhaps?
 
Easy, find a new AP.


You're putting a heap of faith and risking quite a bit of money, a prebuy is no place for a novice wrench, at least when the money for buying a turd would be coming out of your pocket.
 
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BigBadLou said:
Trying to get a pre-buy done on an airplane and when I asked the A&P about the tachometer showing only 2500rpm with the throttle and prop control knobs full fwd, he tried lecturing me on density altitude and how heat affects the maximum power available to an engine.

How can I politely explain to this A&P that a CS prop should maintain given RPM even with lower manifold pressure at higher altitude/DA?

(yes, I checked the POH and it says 2700rpm at full prop control fwd)
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If the book says 2700 at your DA and it's only producing 2500, just show him the book. I don't even see a need to talk about a CS prop. It's not producing the power that's published. Period.
 
BigBadLou said:
How can I politely explain to this A&P that a CS prop should maintain given RPM even with lower manifold pressure at higher altitude/DA?
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I hope he's a young mechanic who just hasn't quite learned about CS props yet. If he's older and more experienced and still doesn't know he should turn in his A&P.
 
The low pitch stop should be rigged to a position, or very close to it, where the governor will NOT change pitch AND static engine RPM (on the ground brakes locked etc) will be under redline.

I've seen static ground runs where RPM hits redline and the governor is active. That's wrong for a typical SEP airplane.

For instance:

Static run-up of 1970 and later year cardinals is 2670 +/-50 per the manual. Redline is 2700. You must flight test/ground roll the airplane when adjusting the max RPM stop on the governor.

Tachs are horribly inaccurate and YMMV
 
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"""""""Static Run-Up - What is Normal?

There has been some confusion in the field concerning propeller low blade angle setting, the governor setting and how it relates to static run-up and take-off RPM. As a general rule, engine redline RPM cannot be reached during a full power static run-up. Contrary to popular belief, the governor is not controlling the propeller at this time, the propeller is against its low pitch stop. Attempting to increase propeller static run-up RPM by adjusting the governor high RPM screw will have no effect and will probably result in a propeller overspeed during the take-off roll. On a single engine aircraft several considerations determine both the low and high blade angle settings. Normally 25 to 100 RPM below rated take-off RPM is desirable and acceptable during a static run-up. McCauley's practice is to set the low blade angle so that rated take-off RPM is not reached until the aircraft has reached some significant groundspeed during roll out. At this time, and only this time, the propeller is controlled by the governor. There are two advantages to this practice. First, the maximum static RPM can be used as a check on developed horsepower as with a fixed pitch propeller. Any loss of maximum power over time is readily apparent during a preflight check. Second, if the propeller remains in a flat pitch after take-off due to some malfunction, the higher angle low pitch setting will permit more thrust to be developed to fly the aircraft without overspeeding the engine."""""""

http://www.mccauley.textron.com/pga_governors.html

That, and as bny8t said, mechanical tacks are horribly inaccurate.
 
Great answers and explanations, thank you, gentlemen.
It turns out that the first mechanic did not end up working on the plane and the second guy knew his way around.
The conclusion is that the static prop rpm is within specs.
 
are you sure that is the redline on the engine. on a 360-A1A the Lycoming redline is 2900rpm. The Type Data Sheet redline is 2700 for the Mooney. The tips on the 74" prop don't go supersonic till 3000 RPM
Some People use a $15.00 Radio Control world optical tach to verify. Or their phone.
 
Or their phone.
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Whoa. I've been looking for such an app for years now. I'm sure I asked here about 2 years ago. Could not find a decent one. One exists now?
 
Put a streetlight on the backside of the prop.
A two blade prop will appear to stop twice at 1800rpm as I recall.

If it stops at say, 1900rpm that is an error.
 
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Norman said:
I hope he's a young mechanic who just hasn't quite learned about CS props yet. If he's older and more experienced and still doesn't know he should turn in his A&P.
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Nonsense. Maybe someone's been lucky and only worked on jets. I started out in the Air Force working on F4 and T-38, then got my A&P, never really had an interest in working propeller driven aircraft. But I did. Worked on G-1's over four years. Now there's a prop.
 
Constant speed prop airplanes show an rpm decrease with leaning the mixture, pretty much the same drop as fixed pitch props. Explain that.
 
No, all the time. That is the common ones. There may be some constant speed props that dont, but all the modern Hartzels and McCauleys and MT's and their clones do it. Go out and try it if you dont believe me. Go to 2000rpm (or whatever the runup RPM is) lean and watch the RPM DROP!!

They have to, otherwise how would you ever lean it based on RPM drop?
 
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Sounds like a variable vs constant speed prop.

Unless the prop is below the speed range and at the low pitch stops..... The drop will be in MP not speed.
 
coloradobluesky said:
Go to 2000rpm (or whatever the runup RPM is) lean and watch the RPM DROP!!
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Isn't that below the constant speed range?

Just thinking about how they work, if in the range, when leaning would not the weights in the governor move in as soon as the rpm started to drop, flattening the pitch to return to the set rpm?

It's been a while since I've messed with one in practice, so if I'm wrong, so be it.
 
This is the way it works with a Constant Speed Prop. Set the RPM to 2000RPM (or the recommended RPM drop), and lean the mixture until you get an RPM drop. Then push the mixture in a bit. You are now leaned.

Leaning in cruise at higher power and rpms works pretty much the same way. Lean to RPM drop. Standard way of leaning. It does work a LITTLE bit different in cruise because it goes down in rpm a bit, then goes back up about to half the amount it came down, or so, trying to get to where it was.

If you don't believe me, go out and try it. I do it every time I fly, both at runup and at cruise with a Lycoming 0360 and a Hartzel constant speed prop.
 
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now try it at a higher RPM......rinse and repeat.....then report back. :D
 
I DO IT EVERY TIME I FLY MY AIRPLANE!

RPM drop will happen when you lean. Less at more power, but still there. See, you can LEAN at any rpm or throttle setting using rpm drop. Good thing too, otherwise we couldnt lean based on rpm drop. Would HAVE to lean on EGT's or something else.

I did this with the ATP that I bought my plane from. His comment was, "huh, PIC has it wrong". (I think he may have consulted for them)

A lot of pilots have this wrong because we are TAUGHT that it wont happen. But it does...a little. Enough to lean even at full throttle and rpm. But I am suprised so many have not noticed it. I am at 5280' altitude here in Colorado and leaning is done at every runup and in every flight, so I notice it more. My students used to comment with "why does the rpm drop, I thought it was a constant speed prop"? Well, it just does.

Don't know why more pilots don't notice the incongruity. There are a LOT of constant speed props around. I think they size that oil valve so it works this way on purpose, so you can lean. It may be that it is just a natural consequence of the control system. All control systems have natural lag and on and off zones, othewise they would oscillate rapidly. Im not sure about that. But I am CERTAIN you get a noticable rpm drop when you lean, or even when you just pull the power back a a bit.

If you don't believe me, try it and you will see.
 
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coloradobluesky said:
RPM drop will happen when you lean. Less at more power, but still there. See, you can LEAN at any rpm or throttle setting using rpm drop.
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Why doesn't the governor bring it back up again?
 
At high power it tries to, and almost does. RPM goes down a bit, and then comes back up, but not quite all the way back to where it was. Not sure why. Engine isnt making as much power, maybe a little lower oil pressure too. Like I say, not SURE why.

You get a small rpm drop when you reduce the throttle too BTW.

The "Constant Speed" aspect isnt perfect. Its a bit like cruise control on a car, set it at 52 and it oscillates between 51 and 53. All control systems oscillate a bit. Although with all the digital stuff who knows, maybe some dont.

But all I can say is, when you lean, you get an rpm drop. And its suprises many pilots that Constant Speed Props work that way. Good thing they do though, so we can lean based on rpm....So maybe they build it that way on purpose, maybe not and its just a natural thing that control system does. Im not sure about THAT.

Try it and observe carefully what happens. You will see it Im pretty sure they almost all do it. All the ones Ive flown do.
 
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coloradobluesky said:
...

The "Constant Speed" aspect isnt perfect. Its a bit like cruise control on a car, set it at 52 and it oscillates between 51 and 53. All control systems oscillate a bit. Although with all the digital stuff who knows, maybe some dont.

...
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IDK about that. If you look at how a synchrophase system works on a multi engine, it would seem that mechanical governors are crazy precise. Not only does a synchrophase system matches rpm between two engines, it matches the exact position of the prop on each rotation.

Edit - bit more research on my part, looks like synchrophase has an electronic component to it, not sure about straight synchronizer systems.
 
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coloradobluesky said:
Try it and observe carefully what happens. You will see it Im pretty sure they almost all do it. All the ones Ive flown do.
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My Cirrus definitely did not. Standard prop governor on an IO550N, albeit linked to the throttle.

I could set the RPM, 2600 let's say, then do extensive leaning from ROP through peak to well LOP and the RPM would not budge. Extensive experience with constant-speed props over many decades showed similar behavior.

Pretty sure you're quite wrong about this, based on the mechanics of prop governors, but I don't feel like arguing further, so that's that! :yes:
 
FastEddieB said:
My Cirrus definitely did not. Standard prop governor on an IO550N, albeit linked to the throttle.

I could set the RPM, 2600 let's say, then do extensive leaning from ROP through peak to well LOP and the RPM would not budge. Extensive experience with constant-speed props over many decades showed similar behavior.

Pretty sure you're quite wrong about this, based on the mechanics of prop governors, but I don't feel like arguing further, so that's that! :yes:
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++2....Agreed. :yes:
 
OK. Different props exhibit different behavior. My airplane is an Aviat Husky with Lycoming 0360 and Hartzel prop. Since the Cirrus has the instumentation to lean using CHT and EGTs etc, this might indicate that they make it do this on purpose on the Husky by fiddling with the valve or some such, so you can lean via rpm drop. Good to know.
 
One thing people may not be aware of. When I set the throttle from full power back to 18" of manifold and bring the prop rpm back to 2000 rpm, the manifold pressure goes UP From 18" to 20" as the rpm goes down.

This gives me my 20/20 and 5.5gph that I putput around on around here for fun. Pretty sure that one is universal with all manifold pressures.
 
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coloradobluesky said:
One thing people may not be aware of. When I set the throttle from full power back to 18" of manifold and bring the prop rpm back to 2000 rpm, the manifold pressure goes UP From 18" to 20" as the rpm goes down.

This gives me my 20/20 and 5.5gph that I putput around on around here for fun. Pretty sure that one is universal with all manifold pressures.
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Manfold pressure is affected by two things: the position of the throttle plate, and the volume of air going past it. When you reduce RPM, the volume of air goes down since the cylinders are taking fewer cubic feet per minute of it. The decreasing volume means decreasing velocity and the drag of the plate has less restriction on the airflow, so the pressure rises in the manifold. Stop the prop altogether and watch the pressure rise to ambient.
 
The only time the RPM drops when leaning during cruise in my mooney is when you lean so far that the engine stops running. Prop governors are mechanical, and they are not connected to the mixture knob. It has nothing to do with avionics or type, it has to do with spinning weights. This really should have been taught to you before getting that endorsement.
 
It's not what I was taught. It is what I have observed when I do it. Yours works differently.
I never said the mixture was connected to the prop govenor. I agree it isn't. I don't really know how it works. I don't have to. I just have to know how to work it.
 
coloradobluesky said:
At high power it tries to, and almost does. RPM goes down a bit, and then comes back up, but not quite all the way back to where it was. Not sure why. Engine isnt making as much power, maybe a little lower oil pressure too. Like I say, not SURE why.

You get a small rpm drop when you reduce the throttle too BTW.

The "Constant Speed" aspect isnt perfect. Its a bit like cruise control on a car, set it at 52 and it oscillates between 51 and 53. All control systems oscillate a bit. Although with all the digital stuff who knows, maybe some dont.

But all I can say is, when you lean, you get an rpm drop. And its suprises many pilots that Constant Speed Props work that way. Good thing they do though, so we can lean based on rpm....So maybe they build it that way on purpose, maybe not and its just a natural thing that control system does. Im not sure about THAT.

Try it and observe carefully what happens. You will see it Im pretty sure they almost all do it. All the ones Ive flown do.
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I think you should review the prop system a little. Mixture wouldn't have any impact on rpm unless you lean to the point the engine is barely running. That would be way past lop ops.
 
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