I know rev limiters exist, not sure what engines have them and exactly how they work. Anyone know? Does the typical Cessna 172 have a rev limiter? If so, how does it work?
How do aircraft engines limit revs?
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Stewartb
Final Approach
Constant speed props use prop governors to adjust pitch to a redline rpm limit. Fixed pitch props are usual selected by length/pitch to provide high horsepower at takeoff while not exceeding redline in level cruise. In my own experience with fixed pitch the aircraft TCDS specified a static rpm range. You have to make that rpm on the ground. A pilot may need to limit the high revs during flight.
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No rev limiters in the magneto?
Capt. Geoffrey Thorpe
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You have a mechanical governor with most constant speed props - that will tend to limit the speed.
Most fix pitch prop aircraft don't have one that I am aware of, but I'm sure there is some exception somewhere. But not a 172.
One is not likely to miss a shift on an aircraft so there is not much demand...
But, obviously, I need to learn to type faster.
Most fix pitch prop aircraft don't have one that I am aware of, but I'm sure there is some exception somewhere. But not a 172.
One is not likely to miss a shift on an aircraft so there is not much demand...
But, obviously, I need to learn to type faster.
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Tom-D
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there is nothing in a Mag that will limit speed of rotation.
At a very high RPM the points will start to float.
kgruber
Final Approach
The most famous was the runaway prop of PanAM Flight 943, a Statocruiser that ditched at Ocean Station November in 1956.
https://www.youtube.com/watch?v=XvagZxur7sU
The Kahalui, Maui Airport is named in commemoration of Capt OGG, the captain of flight 943. PHOG. The VOR is OGG.
https://www.youtube.com/watch?v=XvagZxur7sU
The Kahalui, Maui Airport is named in commemoration of Capt OGG, the captain of flight 943. PHOG. The VOR is OGG.
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Dan Thomas
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Fixed-pitch props on certified airplanes are normally sized and pitched to give redline RPM in full-throttle, level flight near sea level. As you climb, the air gets thinner, reducing the load on the prop but also reducing the power available from the engine, so it's pretty much a saw-off for most. I have never been able to exceed redline in level flight with either fixed pitch or constant speed, but put it into a dive with full throttle and it will go past redline, even on a constant-speed, since there are pitch stops in the prop.
The engine and propeller manufacturers both have instructions for dealing with overspeeds. The propeller is the most critical; it is the most highly-loaded item in most airplanes and a 10% overspeed means a 21% increase in centrifugal forces that can start cracks. 20% over and you put 44% more force on that prop.
Some owners might install a "climb" fixed-pitch prop that has a lower pitch to get more RPM and therefore HP for takeoff and climb, but that pilot will have to watch the tach in cruise; it will easily run past redline.
The TCDS for a certified airplane list the propellers that are legal for, and have been tested on, that airframe/engine combination.
The engine and propeller manufacturers both have instructions for dealing with overspeeds. The propeller is the most critical; it is the most highly-loaded item in most airplanes and a 10% overspeed means a 21% increase in centrifugal forces that can start cracks. 20% over and you put 44% more force on that prop.
Some owners might install a "climb" fixed-pitch prop that has a lower pitch to get more RPM and therefore HP for takeoff and climb, but that pilot will have to watch the tach in cruise; it will easily run past redline.
The TCDS for a certified airplane list the propellers that are legal for, and have been tested on, that airframe/engine combination.
![brian]](/community/data/avatars/m/21/21096.jpg?1456258959){kind=avatar}
brian]
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Unfortunately, I've had to learn a lot about props recently as I have an interesting example.
The beech electric prop has an 88" diameter and is used on the early 165, 185, and 225 hp continental engines. Rpm control is limited by the following (copying from the above a bit).
- a pitch setting on the prop based on the engine the prop is installed on.
- this limits the pitch range you have control over from the pilots seat.
- (optional electric control which I don't have.)
- you go up, w/o a turbo the engine has less ability to make full rpm
Generally speaking, a bunch of guys with sliderules figured out the physics such that the engine / prop combination inherently limits the rpm- outside a prop govenor (electric or hydraulic).
In any case, you could point the nose of the airplane at the ground in course pitch and eventually overspeed the prop. . . .
The beech electric prop has an 88" diameter and is used on the early 165, 185, and 225 hp continental engines. Rpm control is limited by the following (copying from the above a bit).
- a pitch setting on the prop based on the engine the prop is installed on.
- this limits the pitch range you have control over from the pilots seat.
- (optional electric control which I don't have.)
- you go up, w/o a turbo the engine has less ability to make full rpm
Generally speaking, a bunch of guys with sliderules figured out the physics such that the engine / prop combination inherently limits the rpm- outside a prop govenor (electric or hydraulic).
In any case, you could point the nose of the airplane at the ground in course pitch and eventually overspeed the prop. . . .
So lets say you hold the brakes and create 50mph wind with the prop and plane stopped. Now you are flying at 100mph. Is there a total of 150mph of wind?
For every speed of the airplane and speed of the propeller, there is an optimum design of twist rate. Even a constant speed prop cant change its twist rate. How does that work? A compromise? If so, for what rpm and airspeed is it optimized?
Is there ever a constant speed prop that is actually causes higher airspeed of the plane at lower rpm than max rpm?
For every speed of the airplane and speed of the propeller, there is an optimum design of twist rate. Even a constant speed prop cant change its twist rate. How does that work? A compromise? If so, for what rpm and airspeed is it optimized?
Is there ever a constant speed prop that is actually causes higher airspeed of the plane at lower rpm than max rpm?
brian]
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It's just thrust and each engine/propeller combination produce a set of thrusts based on rpm, power setting, altitude, pitch, etc. Remember the thrust vector is opposite your velocity- they don't add and eventually you stop going faster...
I think (I'm too young and my wallet is too thin) that the p51 was authorized for a combat takeoff where the rpm was reduced producing a higher manifold pressure and more power. it would be interesting for the turboprop drivers to comment ...
I think (I'm too young and my wallet is too thin) that the p51 was authorized for a combat takeoff where the rpm was reduced producing a higher manifold pressure and more power. it would be interesting for the turboprop drivers to comment ...
Stewartb
Final Approach
Let's use my old Cub for an example. I used a MacCauley 82/42 "Borer" prop on that plane. It would spin about 2450rpm static. That's important since the aircraft TCDS requires a minimum static rpm. My prop exceeded that requirement. So most of us know the prop diameter is 82". What's the 42 mean? It's the distance in inches that the prop will travel in one revolution of the prop with no drag. While I made 2450 static the actual RPM on takeoff would spool up to 2600 and it would remain there on climb out. Level cruise netted 2700, the engine redline. If I was trying for absolute max performance I'd want 2700 on takeoff and climb out to reach the max hp rpm, but for overall utility of the airplane 42 worked best. If I take a calculator and multiply 42" by 2700 rpm I'd get the theoretical speed for that plane, but with draggy Cub gear, big tires, etc I topped out at about 85mph, not the closer to 115mph the prop calculation will determine.
Now take my 180. If I run it up with the brakes locked I get very near my 2700 rpm redline. I may reach it but I can't recall. No matter, on takeoff roll and climb out it's at 2700 and will remain there as long as I leave the throttle full in. Once I clear obstacles and am comfortable I'll reduce throttle, the rpm remains at 2700, and then roll back the prop as conditions require. The prop blades definitely change the "twist" during the change of power. On takeoff the prop is very flat and it becomes increasingly course as I get to cruise altitude and level off, or as the work load decreases and the power application is converted to rpm. Airspeed and drag aren't important to that prop. I tell it what speed to spin and for the most part it'll do it, regardless of drag or load. Is rpm limited by power? No, it's determined by the prop governor, which is ground adjustable for high limits. The prop does have pitch stops but those are prop shop items and are specified by the prop mfg. the advantage of the CS prop is that I can select to apply the full 275hp whenever I choose because the prop will adjust itself to allow the engine to spin at full rpm.
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Dan Thomas
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I think the questioner was asking about pitch distribution as the blade rotates in the hub. The CS prop is designed for some compromise, probably around the middle of its range, in terms of pitch distribution. The ideal (and probably impossible) CS propeller would have its inboard blade area increase and decrease pitch more than the outer areas.
For example: If we increase the blade pitch by five degrees on an 80" propeller, we're increasing its bite by about 22" per revolution at the tip (40" from the crank axis) but only about 13" per rev at a point 24" out from the crank axis. You can see that the working area is going to shift outward as pitch increases and inward as it decreases. The inner areas might even drag some at highest pitch.
You can see it maybe a little clearer if you were to move the blade to a lower pitch so that the tip had no bite at all; the inner areas would still have lots of pitch.
It would take some special, flexible yet rigid material to create the ideal propeller. By the time we have such stuff, propellers will be long obsolete.
Dan
poadeleted20
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My experience suggests otherwise. Every fixed pitch prop installation I've ever flown with will run well over redline at full throttle at sea level in level flight, some as much as 300 over.
I'd have to ask what aircraft/engine/prop combination this was, because your experience is grossly different from mine in nearly every fixed prop single made by Beech, Piper, Cessna, or Grumman.
Absolutely true.
Also correct.
poadeleted20
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Not sure what you mean by that.Some are optimized for cruise, and some for climb -- like the 57 and 59 pitch props available for the Grumman Traveler/Cheetah. The cruise prop is optimized to allow 75% power cruise over the widest range of altitudes, and the climb prop is optimized to give the most power possible at Vy without exceeding redline in cruise over too wide a range of conditions.
No doubt the prop will be slightly more efficient at one RPM than another in cruise flight, but I doubt the difference would be significant. One could test this by operating at several different RPM's and setting the throttle as required for 75% power at each, and the seeing which gives the best airspeed. I doubt you'd see more than 2 knots difference, and likely not even that much.
Well the wind from the prop at static runup is 50mph. You are flying 100mph. Is there now 150mph relative wind over the airplane (in the area of the prop wash). Not sure I can explain it any better than that.
poadeleted20
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Ah -- I see. No, it's not directly additive. Only way to measure that would be with some sort of pitot tube in the prop wash. BTW, how did you determine the airflow from the prop was 50 mph during static run-up?
The numbers are made up. But it is some number of mph. And for understanding, it the actual numbers don't matter.
It seems to me that they would add, vector addition. The two are slightly different angles of attack, but it would be close to direct adding.
It seems to me that they would add, vector addition. The two are slightly different angles of attack, but it would be close to direct adding.
warthog1984
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Check your frame of reference.
brian]
Cleared for Takeoff
Yea, what warthog said ...
Maybe you are looking at this like a rocket (maybe not). If we assume the air particles are leaving the prop at a given velocity, then the airframe can not go any faster than that velocity. Now when the aircraft is going fast, the newly accelerated air particles start at zero velocity and are accelerated by the prop to the same velocity as before. (That is, each "bite" of air the prop takes starts at zero and is accelerated to pretty much the same speed as when you have the breaks set on the ground.)
At least for me, this way of thinking is good for a rainy day, but not useful in the aircraft. Best to think of the prop / engine as just a mechanism for generating thrust and the POH usually provides a table of settings producing different thrusts typically in HP.
Maybe you are looking at this like a rocket (maybe not). If we assume the air particles are leaving the prop at a given velocity, then the airframe can not go any faster than that velocity. Now when the aircraft is going fast, the newly accelerated air particles start at zero velocity and are accelerated by the prop to the same velocity as before. (That is, each "bite" of air the prop takes starts at zero and is accelerated to pretty much the same speed as when you have the breaks set on the ground.)
At least for me, this way of thinking is good for a rainy day, but not useful in the aircraft. Best to think of the prop / engine as just a mechanism for generating thrust and the POH usually provides a table of settings producing different thrusts typically in HP.
poadeleted20
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Might seem that way, but it isn't actually that way. Lots of fluid dynamics involved.
No "rev limiter" as you are used to with a modern car. Basically it is limited by load (the prop) and the maximum amount of fuel and air it can flow. With a CS prop the governor just increases the pitch to limit RPM to fuel flow. With a fixed pitch prop in a dive, the limiting factor is the supersonic drag increase on the prop tips, however this is typically beyond the redline of the engine, so you have to be the 'rev limiter'. Outside of that, it's just restrictions in the induction matched to the drag profile of the prop at max rated power.
Tom-D
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actually it is the size of the carburetor Venturi.
when it can only suck so much air it can only go so fast.
poadeleted20
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If you want to put it that way, you could say it's the structure of the airplane, i.e., the speed at which the wings rip off, since you can otherwise get the prop to turn faster by nosing over more and letting speed build even if your putting as much fuel/air through the venturi as possible. But I think that's a bit beyond what the OP was really asking.
Ted
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Negative. I could block off the carb entirely, but wwith enough power I could still easily overspend the engine.
Either way, there's plenty of airflow to allow the engine to overspeed in various phases of flight if not properly watched.
Vance Breese
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In my opinion it is the throttle position that controls the speed of the engine, not the size of the "carburetor Venturi".
There is a difference between 'control' and 'limit'. You cant get any more air through than the venturi will allow. The throttle plate can restrict flow from there, but it can't add more. The answer is the same as "What limits a plane's speed?", "Drag"
Tom-D
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Vance Breese
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In a descent with my Lycoming IO-320 B1A I use the throttle to limit the RPM.
It appears to me my engine would exceed the maximum recommended RPM in a dive if I did not retard the throttle.
Do you feel I am not operating the engine correctly?
How would I limit the RPM with the venturi?
warthog1984
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Vance Breese
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Apparently I am confused.
I thought the venturi was to accelerate the air to create a low pressure to draw fuel from the float bowl.
I thought the throttle was to limit the amount of air that enters the engine.
It appears to me that my IO-320 B1A doesn’t have a venturi.
It appears to me my Rotax 914 has a variable venturi and a throttle.
Tom-D
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You Injection control has a volume limiting device, as well as a device that measures the amount of air that passes.
Vance Breese
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warthog1984
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The throttle does (at heart) nothing more than move a butterfly valve to restrict flow through the throat of the carb venturi. You could manually move the valve plate and achieve the same effect. In fact, if you have a choke, this is what you are doing.
For fuel injection, the venturi / air inlet is regulated the same way, but fuel is separately squirted into the cylinders instead of mixing at the carb.
Vance Breese
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In my opinion in a carbureted engine the throttle limits the amount of air the engine can draw in by restricting the inlet tract.
I feel that the location upstream of the venturi does not change the function of the throttle to limit the speed of the engine.
It appears to me a venturi is shaped to speed up the air to create a low pressure to draw fuel from the float bowl in a carbureted engine.
In my Lycoming IO-320 B1A fuel injection the fuel appears to me to be injected into the ports rather than the cylinders.
That is YOU limiting the RPM, and you are operating it properly doing so, that is not the ENGINE limiting RPM. That is the difference. The engine limit is a natural limit, and yes in a dive with a fixed pitch prop, the engine limit may be in a destructive operating range before the prop tip drag limits the RPM.
The throttle always limits the amount of air through the venturi. Most carbs use a butterfly plate at the base of the venturi downstream from the main jet. A variable venturi carb does it in an aerodynamically cleaner fashion by changing the size of the venturi itself.
Almost. The Throttle Plate on a carburetor limits the output flow, the Choke plate limits the input flow, the difference is the amount of vacuum registered at the main jet when the restriction is applied. The throttle will limit both fuel and air at the same mixture ratio, while the choke will simultaneously limit air and INCREASE fuel flow changing the mixture.
A Fuel Injection system does indeed spray a continuous flow of fuel at the intake valve from a pressurized system rather than have vacuum pull it through the jet in the carb.
Vance Breese
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The constant velocity carburetors on my Rotax 914 appear to me to have a throttle and the carburetor measures the pressure differential caused by the throttle opening to adjust the slide (variable venturi) to give a stronger signal to the main jet.It also has a tapered needle to better manage the mixture that goes up and down with the slide.
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