Constant Speed Props

[burninggoats]

Yes. I'm being sarcastic. I always taught my students to lean on the ground, when using carb heat, and not to go full lean when descending. Not only that, but usually I did my job right and they also knew the reason why they did these things, other than the fact that I said so (which is usually the reason people give for being at full rich when they shouldn't).

[photofly]

I hope your just being sarcastic, i've been in carb'd aircraft numerous times in the last couple months and we get a mag drop of ~ 200. Cause - fouled plugs from NOT leaning on the ground. I always lean on the ground and never have a problem, wish students (that arn't mine) would do the same

Of course if you do a mag check with the mixture leaned, a 200rpm mag drop might be expected.

[Shiny Side Up]

You only want to lean the mixture on the ground to save fuel and engine costs to maximize profit.... because you are part of the greedy FTU scam rampant across Canada.

SHAME ON YOU!

Drat! Foiled again!

[aholenstein]

Is low MP with High RPM damaging to the engine? Let's say i am doing a power on decent from 12K feet. I keep my MP at 20in and RPM at 2450. This decend slowly cools the engine. No risk if shock cooling. As the CHT lowers to around 280 degrees, can i reduce MP to idle (10MP or less) with 2500RPM and do a steep decent on a long final? Basically descending at 3000fpm? I can't find any info about this anywhere to know if i potentially damage the engine that way...

[digits_]

Is low MP with High RPM damaging to the engine? Let's say i am doing a power on decent from 12K feet. I keep my MP at 20in and RPM at 2450. This decend slowly cools the engine. No risk if shock cooling. As the CHT lowers to around 280 degrees, can i reduce MP to idle (10MP or less) with 2500RPM and do a steep decent on a long final? Basically descending at 3000fpm? I can't find any info about this anywhere to know if i potentially damage the engine that way...

Why would you want to?

As long as you don't exceed a limitation in the POH, you shouldn't be damaging anything. But generally higher rpm means more wear and tear on the engine parts. It's usually also accompanied with more drag, and thus a higher fuel burn.

Why not descend with cruise RPM?

[PilotDAR]

It concerns me a little that a 3000 FPM descent might be necessary as a part of a planned flight profile. To accomplish that descent rate in a piston GA single, you'll be being hard on something about the airplane, and passengers for sure if you have any.

That being said, being alert to the possible damage associated with shock cooling is excellent. The engine does not mind what its CHT is while running, but rather the rate at which the CHT reduces, so everything with smooth and easy going power changes.

When you need to descend, that will obviously require power reduction. When you reduce the power to much less than cruise power, the prop won't be constant speed any more, it'll be on the fine pitch stops anyway. Using the propeller for braking is not really much of a thing for most piston GA planes. That those have this characteristic have other factors which need to be understood. It is possible to change propeller pitch with power to produce some braking, but this would be abusive to the engine and prop.

I suggest that the scenario you present for standard operations is much more easily handled with better descent planning, and not trying to exploit non standard engine handling techniques. If you're flying jumpers, that's a whole other discussion, which should first be had with the jump plane owner/experienced jump pilot.

[aholenstein]

My question was more hypothetical... if i had to decent quickly and shock cooling, airspeed isn't a factor, am i putting unintended stress on the aircraft that i am not aware of? Basically, can i decent with full fine prop with low MP, basically using the prop as a speed break would result in a high rate of decent without exceeding airspeed limitations...

[PilotDAR]

You can descend with the propeller in full fine pitch, and the power at idle, up to the airpseed limitations of the airplane (considering the "yellow zone", flap limiting speeds, and abiding the red line), in calm air, without damaging the airplane, if shock cooling is also considered. You should operate in accordance with the flight manual. Different propellers will produce different results in drag.

There is much more to this, but it is outside the scope of a hypothetical question on an internet forum, operating the airplane within the flight manual limitations is always the right answer.

[Bede]

Basically, can i decent with full fine prop with low MP, basically using the prop as a speed break would result in a high rate of decent without exceeding airspeed limitations...

Yes, that's how pretty much every turboprop does it when they need to descend at a high rate. However, I can't see how you could possibly do 3000 fpm in a piston airplane.

[TeePeeCreeper]

Basically, can i decent with full fine prop with low MP, basically using the prop as a speed break would result in a high rate of decent without exceeding airspeed limitations...

Yes, that's how pretty much every turboprop does it when they need to descend at a high rate. However, I can't see how you could possibly do 3000 fpm in a piston airplane.

Piston or not Bede, I can guarantee you that a mere moral with lackadaisical skill could pull way more than 3000 f a minute as a rate of decent. (Without due care to a piston engine admittedly so)

TPC

[PilotDAR]

I can guarantee you that a mere moral with lackadaisical skill could pull way more than 3000 f a minute as a rate of decent.

And land afterwards?

When I flew jumpers in the 185, using modest, but applied skill, I would struggle to achieve such a rate....

[Big Pistons Forever]

As a general comment high RPM with very low MP should be avoided where possible. The problem is that the cylinder pressure is low and there is danger of the piston rings not being held firmly against the cylinder walls and scuffing the cylinder liners. A fellow I knew badly damaged his cylinders flying at cruise at high altitudes in a normally aspirated engine. He was running 16 inches and 2700 Rpm.

With respect to a 3000 fpm descent rate I am struggling to see how you could do that within the POH limitations in most piston singles.

[rookiepilot]

Heavily oversquare or undersquare, neither seems particularly wise to me, until final when the prop goes to full fine pitch anyway……

16 inches and 2700 RPM…weird.

23 / 2150 is nice…..bit over is ok.

[‘Bob’]

As a general comment high RPM with very low MP should be avoided where possible. The problem is that the cylinder pressure is low and there is danger of the piston rings not being held firmly against the cylinder walls and scuffing the cylinder liners. A fellow I knew badly damaged his cylinders flying at cruise at high altitudes in a normally aspirated engine. He was running 16 inches and 2700 Rpm.

With respect to a 3000 fpm descent rate I am struggling to see how you could do that within the POH limitations in most piston singles.

Pull G

[Big Pistons Forever]

16 inches and 2700 RPM…weird.

The 16 inches was at full throttle because of the altitude. That combination will give you more power then selecting a lower RPM but is very hard on the engine.

[rookiepilot]

16 inches and 2700 RPM…weird.

The 16 inches was at full throttle because of the altitude. That combination will give you more power then selecting a lower RPM but is very hard on the engine.

How high was he? 16 inches….

[digits_]

As a general comment high RPM with very low MP should be avoided where possible. The problem is that the cylinder pressure is low and there is danger of the piston rings not being held firmly against the cylinder walls and scuffing the cylinder liners. A fellow I knew badly damaged his cylinders flying at cruise at high altitudes in a normally aspirated engine. He was running 16 inches and 2700 Rpm.

Was he still within POH limits during that flight?

[NotDirty!]

16 inches and 2700 RPM…weird.

The 16 inches was at full throttle because of the altitude. That combination will give you more power then selecting a lower RPM but is very hard on the engine.

How high was he? 16 inches….

I would guess significantly higher than that! Tens of thousands of inches, likely!

[rookiepilot]

The 16 inches was at full throttle because of the altitude. That combination will give you more power then selecting a lower RPM but is very hard on the engine.

How high was he? 16 inches….

I would guess significantly higher than that! Tens of thousands of inches, likely!

[Big Pistons Forever]

As a general comment high RPM with very low MP should be avoided where possible. The problem is that the cylinder pressure is low and there is danger of the piston rings not being held firmly against the cylinder walls and scuffing the cylinder liners. A fellow I knew badly damaged his cylinders flying at cruise at high altitudes in a normally aspirated engine. He was running 16 inches and 2700 Rpm.

Was he still within POH limits during that flight?

Good question. He was regularly flying a V35 Bonanza at 15000 or 16000ft and the POH that I looked at doesn't expressly prohibit that MP and RPM combination but the cruise power charts only give cruise power settings for 2100, 2300, and 2500 RPM.

[rookiepilot]

As a general comment high RPM with very low MP should be avoided where possible. The problem is that the cylinder pressure is low and there is danger of the piston rings not being held firmly against the cylinder walls and scuffing the cylinder liners. A fellow I knew badly damaged his cylinders flying at cruise at high altitudes in a normally aspirated engine. He was running 16 inches and 2700 Rpm.

Was he still within POH limits during that flight?

Good question. He was regularly flying a V35 Bonanza at 15000 or 16000ft and the POH that I looked at doesn't expressly prohibit that MP and RPM combination but the cruise power charts only give cruise power settings for 2100, 2300, and 2500 RPM.

Mmmmm.

https://www.aopa.org/go-fly/aircraft-an ... ft-bonanza

[PilotDAR]

Thread drift, but it's probably time anyway....

While doing mogas testing in a normally aspirated, carburetted O-520, in a 185, the objective was to find the absolute ceiling. I got it to 21,050 feet, but could not hold it there. I held it at 20,800 feet, stall horn tweeting, 75 MIAS, 12 in MP (full power) and 2500 RPM (full fine). Interesting, but I did not stay long.

Yes, when I had dumped the jumpers from the 185, I pulled some G most of the way the way down (no G meter though, so I was careful), but still not 3000 FPM descent. Another pilot eventually wrinkled the wings doing something dumb trying to get down quick, and they wrote the plane off (It was not a cream puff). Cessnas are tough though....

[pelmet]

Thread drift, but it's probably time anyway....

Yes, when I had dumped the jumpers from the 185, I pulled some G most of the way the way down (no G meter though, so I was careful), but still not 3000 FPM descent. Another pilot eventually wrinkled the wings doing something dumb trying to get down quick, and they wrote the plane off (It was not a cream puff). Cessnas are tough though....

Interesting. Reminds me of when I was skydiving. One of the weaker pilots went into a cloud and lost control but recovered. I showed up later to jump and did so....out of that aircraft. Plane had looked OK. Turned out that there was major damage at the struts. Perhaps exaggerated but I was told the bolts were nearly pulled out.

[PilotDAR]

The wing damaged jump 185 to which I refer was in the Ottawa area, probably close to thirty years ago. I heard of the event through the jump organization I knew, and it was verified later, as a friend of mine bought the damaged wings and had them completely rebuilt for his amateur built 185 project.