Engine RPM
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Engine RPM
Following a mag drop problem (cured by leaning), I opened the throttle fully, but could only achieve 2,200 rpm. The engine is red lined at 2,500, and supposed to produce 180bhp at 2,700. A quick burst down the runway felt ok, so we returned to the end of the runway and took off without a problem. Levelling off at 2,500', and leaving the throttle fully open still only achieved 2,350 rpm.
I suspect that max rpm would probably only be seen in a dive, but is 2,200 acceptable on the ground? Is it just that the propellor limits the engine speed when the aircraft is stationary? The engine is new - only just over 100 hours - and is fitted to a Cherokee Archer III.
I suspect that max rpm would probably only be seen in a dive, but is 2,200 acceptable on the ground? Is it just that the propellor limits the engine speed when the aircraft is stationary? The engine is new - only just over 100 hours - and is fitted to a Cherokee Archer III.
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It's not normal
I have flown the Archer II a lot (same engine) and I usually saw 2400 rpm at full power runup (2300 at least). 2200 is totally unaceptable. Even at a high hot field, I think the rpm would be the same, as the propeller drag decreases at a rate close to engine power loss.
The mixture may be too rich (more likely) or too poor.
In flight, I have always retarded the throttle to avoid exceding 2500 rpm. Your plane is definitely out of shape.
The mixture may be too rich (more likely) or too poor.
In flight, I have always retarded the throttle to avoid exceding 2500 rpm. Your plane is definitely out of shape.
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This is summat which has always intrigued me.
Our checklists tell us what revs at which to perform power checks, how many revs we need to lose on carb heat application, and max drop and max difference for the mags.
Why not a static rpm check in the power checks?
It would seem to be a simple performance indicator - 'if my prop won't turn at at least 2350 (or whatever) rpm when I give it full throttle at the hold, I won't fly'.
I can't believe that windspeed at the hold point, ambient pressure and temp, airfield elevation etc could effect the max rpm overmuch. (Leaving aside hot'n'high airports, where presumably one leans for takeoff and does other odd things).
In my aircraft I perform power checks at 1800, but once the high rev checks are complete, and before I retard the throttle to check idle, I give it a burst of full throttle and check that it *will* go flat out for take off.
Steve R
Our checklists tell us what revs at which to perform power checks, how many revs we need to lose on carb heat application, and max drop and max difference for the mags.
Why not a static rpm check in the power checks?
It would seem to be a simple performance indicator - 'if my prop won't turn at at least 2350 (or whatever) rpm when I give it full throttle at the hold, I won't fly'.
I can't believe that windspeed at the hold point, ambient pressure and temp, airfield elevation etc could effect the max rpm overmuch. (Leaving aside hot'n'high airports, where presumably one leans for takeoff and does other odd things).
In my aircraft I perform power checks at 1800, but once the high rev checks are complete, and before I retard the throttle to check idle, I give it a burst of full throttle and check that it *will* go flat out for take off.
Steve R
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I had a similar situation arise on the Aztec (on one side only) and the cause was found to be two oiled plugs in the same cylinder, meaning that the aircraft was producing no more than (and probably considerably less than) five/sixths power. Of course in a four banger, this would be three-quarters. If both plugs have oiled (i) it won't show in a mag check and (ii) no amount of running at a lean mixture will clean them.
If static RPM is down you must abandon the flight. If you don't, why bother with power checks, or any other checks, for that matter?
Timothy
If static RPM is down you must abandon the flight. If you don't, why bother with power checks, or any other checks, for that matter?
Timothy
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Hold on there, don't scrap the aircraft just yet!
Most Lycomings driving a fixed pitch prop with a max rpm of 2700 will only pull 2200-2300 static.
Engines turning variable pitch props (a la Aztec) should reach red line when static.
Whilst a poorly engine may well result in a lower static rpm achieved, a knackered prop may also cause a discrepancy between two similar aircraft. An old prop with the tips clipped to the minimum allowable diameter normally revs higher (on the ground and in the air) than a new thick and long one.
Remember that with mechanically driven tacho, at 2300 rpm, a 10% error (often seen on car speedos) will mean a 230 rpm mis-read!
On the CofA flight test the static rpm is carried out (and should be recorded by any decent engineer on a ground run after any servicing). This is normally carried out crosswind to prevent the wind effecting the results.
As for checking it, pre-flight, why not check it as you begin the take off run and still have time to abort.
At 2500ft you are unlikely to see max rpm unless the prop is a low drag one - clipped and thin through age normally. The POH should tell you what power setting/altitude will give what rpm.
Hope that helps.
Most Lycomings driving a fixed pitch prop with a max rpm of 2700 will only pull 2200-2300 static.
Engines turning variable pitch props (a la Aztec) should reach red line when static.
Whilst a poorly engine may well result in a lower static rpm achieved, a knackered prop may also cause a discrepancy between two similar aircraft. An old prop with the tips clipped to the minimum allowable diameter normally revs higher (on the ground and in the air) than a new thick and long one.
Remember that with mechanically driven tacho, at 2300 rpm, a 10% error (often seen on car speedos) will mean a 230 rpm mis-read!
On the CofA flight test the static rpm is carried out (and should be recorded by any decent engineer on a ground run after any servicing). This is normally carried out crosswind to prevent the wind effecting the results.
As for checking it, pre-flight, why not check it as you begin the take off run and still have time to abort.
At 2500ft you are unlikely to see max rpm unless the prop is a low drag one - clipped and thin through age normally. The POH should tell you what power setting/altitude will give what rpm.
Hope that helps.
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Don't know anything specific about the Archer but in section 2 of the AFM for the C-172:
"The static RPM range at full throttle is 2065 - 2165 RPM."
Max is 2400 RPM. So a 300 RPM difference between static and redline doesn't seem crazy.
"The static RPM range at full throttle is 2065 - 2165 RPM."
Max is 2400 RPM. So a 300 RPM difference between static and redline doesn't seem crazy.
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Wind direction and RPM
Most of you that are flying fixed pitch aircraft will have a "cruise " prop fitted , these props have the pitch ( or A of A) set to give the best performance in the cruise airspeed range go above that cruse TAS and the A of A of the blade decreases and therefore the "lift" that the prop blade gives falls so the aircraft wont go any faster.
At the other end of the airspeed range IE at rest the prop blade is fully stalled and is no more that a wind shovel !, this is a very high drag situation so the engine RPM will not be as high as when the prop is working at its best TAS.
Doing the engine run up into wind will get result accordind to the wind speed after all a 15kt headwind is more than 10% 0f the speed range of most fixed pitch light aircraft and will more than likely unstall the prop giving an over optimistic static RPM , with a tail wind the oposite would happen.
It will come as no surprize to to you all that when doing a C of A airtest the static RPM check is done crosswind so that the wind speed will not effect the static RPM result.
At the other end of the airspeed range IE at rest the prop blade is fully stalled and is no more that a wind shovel !, this is a very high drag situation so the engine RPM will not be as high as when the prop is working at its best TAS.
Doing the engine run up into wind will get result accordind to the wind speed after all a 15kt headwind is more than 10% 0f the speed range of most fixed pitch light aircraft and will more than likely unstall the prop giving an over optimistic static RPM , with a tail wind the oposite would happen.
It will come as no surprize to to you all that when doing a C of A airtest the static RPM check is done crosswind so that the wind speed will not effect the static RPM result.
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The engine is red lined at 2,500, and supposed to produce 180bhp at 2,700.
PA 28's aren't my thing, but perhaps this engine has been 'derated' by fitting a coarse prop thus limiting RPM - Anyone...?
Kingy
(1) Are you sure that the red line is painted in the right place?, people do get these things wrong occasionally.
(2) Borrow an optical tacho and check the prop RPM against the cockpit gauge, they can be badly innacurate on some aircraft. (A friend of mine who test flies light aircraft for CAA takes an optical tacho with him and tapes it to the coaming for air tests.)
G
(2) Borrow an optical tacho and check the prop RPM against the cockpit gauge, they can be badly innacurate on some aircraft. (A friend of mine who test flies light aircraft for CAA takes an optical tacho with him and tapes it to the coaming for air tests.)
G
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Thanks to all who commented on this issue. The aircraft had just had its annual and this was its first flight and also the first time that a mag drop had been experienced, hence the concern! The company that did the check have looked at it again this week. The static ground run produced just below 2200 rpm which, according to them, is correct. I think an optcal rev. counter was used, as they quoted a resolution better than 100 rpm The performance in the air, i.e., airspeed /rpm relationship was as previously experienced, about 100 knots at 2225rpm at 2-3000'.
Hopefully all is well....
Hopefully all is well....
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Good thread.
When I read this:
I thought "D'oh! Of course, it's a ground roll check, so there's no point in it being written down, you were probably taught that in lesson #2"
Then I read:
..and, after supplying my own punctuation , I realised the sense of doing pre-flight static tests crosswind.
A cruise prop is presumably changing from air shovel, through semi-airshovel, to very-draggy lift producer during the first 20-30 knots of takeoff roll, so full revs can't be expected until a time when eyes need to be outside the cockpit.
So obviously Timothy is right, you should never fly without full revs, but my question stands.
Why isn't a max static rev range, in a crosswind position, printed on written checklists? It seems an easy check with a definable expected result.
Steve R
When I read this:
As for checking it, pre-flight, why not check it as you begin the take off run and still have time to abort.
Then I read:
Doing the engine run up into wind will get result accordind to the wind speed after all a 15kt headwind is more than 10% 0f the speed range of most fixed pitch light aircraft and will more than likely unstall the prop giving an over optimistic static RPM , with a tail wind the oposite would happen.
, I realised the sense of doing pre-flight static tests crosswind.A cruise prop is presumably changing from air shovel, through semi-airshovel, to very-draggy lift producer during the first 20-30 knots of takeoff roll, so full revs can't be expected until a time when eyes need to be outside the cockpit.
So obviously Timothy is right, you should never fly without full revs, but my question stands.
Why isn't a max static rev range, in a crosswind position, printed on written checklists? It seems an easy check with a definable expected result.
Steve R
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Probably because as noted above, it appears in the POH. A document that not many people seem to want to read anymore. Once you know what the static allowances are, specifically the lower one, why put it on the checklist too? It isn't going to change unless the prop or engine is modified, and then there will be a requirement for it to become part of the POH anyway.
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I don't believe you have a problem. My Cherokee 180, with a Lycoming 0-360-A4A, gave 2200 this morning at the begining of the take-off. The temperature was around -12C, wind calm and the altimeter 3040in. and an altitude of 703ft.
That is quite normal for a 180 Cherokee with a cruise prop. I have a Sensenich 76EM855-0-60.
The engine should give the static RPM when being run cross wind (or calm wind). There is no case for a pilot to thrash the engine on the run-up pad. The checks call for a mag check at 1700rpm and a Carb' heat check. The max RPM should be noted at the begining of the take-off run and if the static number is not met then it is time to abort and as this is in the very early stage of the take-off is not a problem.
Treat your engine correctly and it will look after you.
Speedbird48
That is quite normal for a 180 Cherokee with a cruise prop. I have a Sensenich 76EM855-0-60.
The engine should give the static RPM when being run cross wind (or calm wind). There is no case for a pilot to thrash the engine on the run-up pad. The checks call for a mag check at 1700rpm and a Carb' heat check. The max RPM should be noted at the begining of the take-off run and if the static number is not met then it is time to abort and as this is in the very early stage of the take-off is not a problem.
Treat your engine correctly and it will look after you.
Speedbird48
