Engine Red line as a result of Speed Vs Power

SixPapaCharlie

SixPapaCharlie

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I have been spoiled by the magic of Cirrus.

When I fly my Grumman, I notice the RPMs are a lot more dramatically affected by pitch.

Is red line on the RPM gauge a warning that I am overpowering the engine or is it a warning that parts are moving too fast no matter what the cause?

If I cruise at 2600 RPM but pitch down and RPM goes to red line due to descent increasing speed, is that a concern?
 
SixPapaCharlie said:
I have been spoiled by the magic of Cirrus.

When I fly my Grumman, I notice the RPMs are a lot more dramatically affected by pitch.

Is red line on the RPM gauge a warning that I am overpowering the engine or is it a warning that parts are moving too fast no matter what the cause?

If I cruise at 2600 RPM but pitch down and RPM goes to red line due to descent increasing speed, is that a concern?
Click to expand...

Be interesting to hear what an engine designer says. I always heard redline was set to limit loads associated with rotating speed. Then I bought a magic piece of paper along with a few fuel system parts and suddenly the redline moved up a couple hundred RPM.

As far as the airplane pushing up engine speed during descent, not much of a worry on these direct drive engines. Some aircraft (not Rotax powered) have a problem with a gear drive and the aircraft pushing engine speed up.
 
SixPapaCharlie said:
Is red line on the RPM gauge a warning that I am overpowering the engine or is it a warning that parts are moving too fast no matter what the cause?
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Remember a prop is a wing? Pitching nose down and increasing airspeed you reduced the angle of attack on the prop by changing the relative wind, engine RPM increases.

Redline means fastest RPM they should be allowed to turn. Over that is bad.
 
I am pretty sure what causes the limit changes based on engine/plane/prop combinations.
Therefore, sometime you may not damage anything in descent, in other situations, it could be really bad (e.g. the limit is caused by the prop tip speed)

Tim
 
Slow down to go down... yah, you should consider pulling a knob to keep the RPM's below the red line. Perhaps the BLACK one?

[Insert "What's the manual say" here]
 
The red line is to limit RPM not power directly. At sea level on a standard day red line would be max rated power so in that instance the red line is both to limit power and rpm. At 8000 feet redline is probably only 75% rated power. Exceeding red line does not turn the engine into a time bomb but it certainly could shorten the time between overhauls in the long run. Most aerobatic pilots are pushing their 320 and 360 engines up to 3200 rpm or more in some cases for short periods of time.

For instance when the Questair Venture was designed, the factory worked with Continental to develop the IO-550G which is rated for 280 hp compared to the earlier models which were rated 300. The difference was that the G was limited to 2500 rpm and the earlier models were rated for 2700 rpm. The difference in TBO was 1700 for the 300 hp and 2000 for the 280 hp.
 
Clark1961 said:
Be interesting to hear what an engine designer says. I always heard redline was set to limit loads associated with rotating speed. Then I bought a magic piece of paper along with a few fuel system parts and suddenly the redline moved up a couple hundred RPM.
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Redline is the number that some engineer felt comfortable with considering things like the loads on pushrods, valves, bearings, prop, etc. (could be based on calculations, could be based on dyno testing, could be "that's the number we always use). Some other person might feel comfortable pushing things just a bit more - for a fee. Just like STCs that magically increase the gross weight - if the original limits were conservative, someone is willing to make a buck by letting you push those limits.
 
Capt. Geoffrey Thorpe said:
Redline is the number that some engineer felt comfortable with considering things like the loads on pushrods, valves, bearings, prop, etc. (could be based on calculations, could be based on dyno testing, could be "that's the number we always use). Some other person might feel comfortable pushing things just a bit more - for a fee. Just like STCs that magically increase the gross weight - if the original limits were conservative, someone is willing to make a buck by letting you push those limits.
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I'll give a different perspective, the same core engine is rated for several different horsepower levels. The engine manufacturer presented a particular engine to aircraft manufacturer and they used it. A year or two later the aircraft manufacturer said That they wanted a little bump for marketing so the engine manufacturer complied.
 
SixPapaCharlie said:
If I cruise at 2600 RPM but pitch down and RPM goes to red line due to descent increasing speed, is that a concern?
Click to expand...

Some fixed pitch airplanes use flat props to make max rated HP on takeoff and climb yet those props will overspeed in WFO level cruise. It's the operator's job to keep the RPM at or below redline. Above redline with a fixed pitch prop isn't a worthwhile thrust producer in my experience.
 
SixPapaCharlie said:
I have been spoiled by the magic of Cirrus.

When I fly my Grumman, I notice the RPMs are a lot more dramatically affected by pitch.

Is red line on the RPM gauge a warning that I am overpowering the engine or is it a warning that parts are moving too fast no matter what the cause?

If I cruise at 2600 RPM but pitch down and RPM goes to red line due to descent increasing speed, is that a concern?
Click to expand...
Use the throttle to keep the engine within limits. If you want to go fast let the engine speed up but still keep it within limits.

The magic you refer to is not cirrus. It's a constant speed prop thing.
 
If you look at the overspeed concerns of the propeller and engine manufacturers, you'll find the prop manufacturer's limits much lower and their demands for special inspections stricter. The prop is probably the most heavily-loaded part on the whole airplane, with centrifugal forces being the biggest concern. Increasing speed 10% increases those forces 21%. 20% overspeed gives a 44% increase in centrifugal force. That's why they want that prop off and sent for NDT--to look for cracks and other signs of damage.Even at normal RPMs there are many tons of force pulling that blade outward.

It's also why nicks need to be dressed out of prop blades, as they create stress risers that can cause eventual cracking and possible blade failure, and losing a big enough chunk of blade can cause vibration massive enough to tear the engine off the airplane. With several hundred pounds gone off the nose, what happens to the CG? Will he airplane glide? Nope.

Some racers run aircraft engines way beyond redline using short propellers. One told be that he ran his O-200 at 4000 RPM. He also said he had a steel cable around the engine, fastened to the airplanes firewall structure, to hang onto it if it tore loose when the prop failed.
 
There are issues with overspeeding the engine but also with the prop itself. If you overspeed the prop too much it will go supersonic (exceed the speed of sound). That puts stresses on the prop it is not made for. This is why aerobatic planes really need constant speed prop, so you dont overspeed the prop. The Citabria's that have a fixed pitch prop have limitations on maneuvers that involve steep dives that overspeed the prop.

Do any airplanes have rpm governors? Anyone know?
 
A friend of mine routinely runs his aerobatics planes to 3400. I assume he's pitched flat to do that. I don't understand how a CS prop would limit that?

He does favor MT props over Whirl Wind because of his high rpm use.
 
The red line is the speed that if you exceed you run the very real risk of your engine coming from together. Don't do it. Things like connecting rods get overstressed and can let go. You should be frightened if you exceed the red line for any engine.

And yes, someone is going to come here and say, but sometimes an engine in another application has a higher redline, but essentially is the same engine. Maybe so, but those engines are few and far between in the scheme of things. There is no reason to do it, and lots of bad things that will happen if you do.
 
I know several guys running Lycomings on experimentals that intentionally exceed the standard redlines and they're doing fine. Their interest is maximum thrust for STOL ops.
 
Ravioli said:
Slow down to go down... yah, you should consider pulling a knob to keep the RPM's below the red line. Perhaps the BLACK one?

[Insert "What's the manual say" here]
Click to expand...
Keep in mind, Bryan hasn't had much blue knob exposure. The Cirrus magic did that for him.
 
I misread the original, to the redline s/b fine I would think, I was thinking fixed pitch where it is some times possible to exceed redline with power in a descent. But I don't know. I would not EXCEED red line.
 
Stewartb said:
A friend of mine routinely runs his aerobatics planes to 3400. I assume he's pitched flat to do that. I don't understand how a CS prop would limit that?
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Not uncommon for fixed pitch serious acro planes. But you don't understand how a CS prop would limit RPM? A CS prop would probably require a dive to 300mph in order to turn up 3400 RPM once the governor has moved the prop to max course pitch at full power. Within normal airspeeds, a CS prop will limit RPM more than what the fixed pitch guys turn. But I'm sure you know how a CS prop works and maybe I misunderstand your question.

And regularly turning these RPMs for a few seconds on downlines doesn't hurt the engine or the prop. Been done for decades. I don't know anyone who's blown anything up doing this.
 
Don't forget about lubrication. The faster those parts are spinning, the more lubrication you need, even in a decent you may not be getting enough of that slippery stuff everywhere it needs to be if you spin it too fast.
 
SixPapaCharlie said:
I have been spoiled by the magic of Cirrus.

When I fly my Grumman, I notice the RPMs are a lot more dramatically affected by pitch.

Is red line on the RPM gauge a warning that I am overpowering the engine or is it a warning that parts are moving too fast no matter what the cause?

If I cruise at 2600 RPM but pitch down and RPM goes to red line due to descent increasing speed, is that a concern?
Click to expand...
Uh. Don't Cirrus's have a constant speed prop? Don't the Tiger have a fixed pitch? The redline on the Tach is there to protect the engine. The redline on the Airspeed indicator is there to protect the airplane. More or less, I suppose if over revving the engine caused enough vibration it could hurt the airplane.
 
Formula One air racing engines are O-200s that turn about 4,400 rpm. They're also highly balanced and probably have a run life of about ten hours.
In the automotive world, the 1.6liter Toyota motor that made 110 hp in an MR2 for 100K miles made about 240-260hp in a Formula Atlantic. It needed rebuilding every 14 hours. (Yes, some different parts.)
It's all a tradeoff.
 
My friend must have his governor dialed up that far. I know he cracked his WW prop hub doing it. My Cessna's governor was dialed up to 2900+ in combination with a bad tach, so I didn't recognize it. My prop balancer discovered it and it must have been that way for a long time. I sure had good takeoff performance, but dang it was noisy.

I ran an 82-42 on my old Cub. I had it re-pitched to 40 to see how it'd work. It was good at takeoff but in cruise it would overspeed easily with zero increase in airspeed. It didn't make any difference for air braking, either. That surprised me. I bought a new 82-42 to replace it. New was a huge improvement over the old toothpick at 42 or 40. Engines turn props. Props make thrust. I'm picky about my props.
 
Stewartb said:
A friend of mine routinely runs his aerobatics planes to 3400. I assume he's pitched flat to do that. I don't understand how a CS prop would limit that?
Click to expand...

The engine is not powering the prop much, the prop is gets its rpm power from the air. Turn the prop blade 90 degrees to the wind (we all know thats not possible, just a thought experiment) and there isn't any turning force coming from the wind. So the prop wont turn from the wind. Well thats what the governor on a constant speed prop does. It turns the prop into the wind to keep rpm from getting too high as well as too low. If the prop is going too fast, pull the prop control out. At least thats the way I understand it. It was explained to me this way by an aerobatic pilot. The Decathalon had the constant speed prop that allowed steeper dives, the Citabria GCBC did not.
 
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RoscoeT said:
Joke. A Lycoming turned to 2970 RPM requires a teardown inspection LOL.
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Yep, and if he spun it to just 2835 he has to -

1. Identify and correct the cause of the overspeed.

2. Complete the “Cylinder Overspeed Inspection” procedure in this Service Bulletin.

3. Drain the lubricating system.

4. Remove oil screens and filters.

5.Examine all screens and filters in the lubrication system for metal contamination. If
any unexplained metal accumulation is found, identify and correct the cause before
returning the engine to service.

6. If the engine has magnetos, complete the “Magneto Overspeed Inspection” procedure
in this Service Bulletin.

7.Complete the “Valve Train Overspeed Inspection” procedure in this Service Bulletin.
 
luvflyin said:
Uh. Don't Cirrus's have a constant speed prop? Don't the Tiger have a fixed pitch? The redline on the Tach is there to protect the engine. The redline on the Airspeed indicator is there to protect the airplane. More or less, I suppose if over revving the engine caused enough vibration it could hurt the airplane.
Click to expand...

Yes, the Cirrus has CS however, no blue knob. That's the magic part. You just set your RPM and forget it.
The Grumman is Fixed and RPM changes if speed is increased or decreased. No magic to offset that.
 
SixPapaCharlie said:
Yes, the Cirrus has CS however, no blue knob. That's the magic part. You just set your RPM and forget it.
The Grumman is Fixed and RPM changes if speed is increased or decreased. No magic to offset that.
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How do you do that? Punch in numbers on a keypad?
 
SixPapaCharlie said:
Yes, the Cirrus has CS however, no blue knob. That's the magic part. You just set your RPM and forget it.
The Grumman is Fixed and RPM changes if speed is increased or decreased. No magic to offset that.
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I think "The Grumman is Fixed and RPM changes if speed or throttle is increased or decreased." is more accurate.
 
Just talked to my buddy. I never stop learning. My prop uses oil pressure to move the prop off the fine pitch stops. Aerobatic props use oil pressure to move the prop off the coarse pitch stops. That explains why aerobatic planes bog in maneuvers that compromise oil pressure. On the fine pitch side it's all about the governor setting. Interesting stuff.
 
Stewartb said:
Just talked to my buddy. I never stop learning. My prop uses oil pressure to move the prop off the fine pitch stops. Aerobatic props use oil pressure to move the prop off the coarse pitch stops. That explains why aerobatic planes bog in maneuvers that compromise oil pressure. On the fine pitch side it's all about the governor setting. Interesting stuff.
Click to expand...
Separate question regarding this... Wouldn't we want all of our props to be like aerobatic planes? That way the prop would auto-feather with a loss of oil pressure.
 
Mtns2Skies said:
Separate question regarding this... Wouldn't we want all of our props to be like aerobatic planes? That way the prop would auto-feather with a loss of oil pressure.
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Does loss of oil pressure to the governor equal loss of all oil pressure?
 
I want my prop to go to the fine stops in a pressure loss. It'll act like a fixed pitch prop at that point, and being flat it'll work better with a compromised engine. Besides, I like pushing the prop flat to slow me down for approach and landing.
 
Clark1961 said:
Does loss of oil pressure to the governor equal loss of all oil pressure?
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According to this diagram... I'm too dumb to know that answer ;)
CSU%20Hub%20On%20Speed.jpg

CSU%20On%20Speed.jpg
 
Stewartb said:
I want my prop to go to the fine stops in a pressure loss. It'll act like a fixed pitch prop at that point, and being flat it'll work better with a compromised engine. Besides, I like pushing the prop flat to slow me down for approach and landing.
Click to expand...
What about stretching glide?
 
Too lazy to quote, but IIRC, in single engine aircraft, the prop is supposed to go to fine pitch in the event of oil pressure loss. In a multi-engine aircraft, the prop is supposed to go to feather.
 
Anymouse said:
Too lazy to quote, but IIRC, in single engine aircraft, the prop is supposed to go to fine pitch in the event of oil pressure loss. In a multi-engine aircraft, the prop is supposed to go to feather.
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Okay.... why? (on single engine)
 
Mtns2Skies said:
Okay.... why? (on single engine)
Click to expand...

It has to do with how the propellers are designed. Single engine typically use pressure to increase pitch props. So by default they will go to find pitch. It ends up being simpler and lighter weight because the prop will centrifugally want to go to fine pitch anyway (thus not requiring counterweights), and the default you want is to have some thrust if you lose oil pressure so you can still fly.

On twins, you want a failed prop to feather so you can still fly. So in order to do that, you need it by default to go to a coarse pitch. As such, they use pressure to decrease pitch props. When you shut down an engine you have 0 oil pressure, so you need it to go to feather by default for this to work. These props have counterweights to help them centrifugally want to go coarse in addition to the internal springs. They also have fine pitch stops in them to keep the blades at fine pitch when you shut down from an idle RPM so as to prevent them from feathering during normal shutdowns, which would make starting harder.

Some singles will use pressure to decrease pitch props (but non-feathering) to improve glide range in the event of an engine failure. These are rarer cases, but they do occur.
 
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