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Overspeeds, and how much is too much...

airguy

airguy

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On my 9A I've got an IO360 and Whirlwind RV200 prop with their Jihostroj governor. Over the last couple days doing taxi testing prior to first flight I've been doing some progressive higher power runs, including sudden full-throttles, mostly to see if I could make the engine stumble or miss or do anything else that would concern me if it happened in the air. For this purpose I was being intentionally "ham-handed" with the throttle and throwing it around rather quickly, much more so than would normally be done in a non-emergency scenario. As I said, I was trying to make the engine do something bad.

Well, the engine played nice-nice through it all, and this morning I'm perusing my datalog from the Dynon Skyview, which was set to record all parameters at 1/4 second intervals. One thing that I was looking at was RPM stability versus manifold pressure, and I found that on those occasions where I would absolutely slam open the throttle, then engine would speed up and the governor let the RPM overshoot a bit before pulling it down. In several cases I saw two consecutive datasets (1/2 second) in the mid-upper 2700 rpm range, and 3 times I saw a single dataset (1/4 second) above 2800. The highest I saw was 2850 rpm. In all cases the governor stabilized in the upper 2600's within the span of a full second, usually within 3/4 second. I think it's likely this has been happening to all of us with C/S props for about 60 years, and only recently do we have data collection good enough to see it.

I called Whirlwind and spoke to an engineer there named Greg, discussed this situation, and he expressed surprise that the governor actually captured the target speed as quickly as it did, considering the rapid change that was being asked of it. He advised first off that I not be so hamhanded with the throttle (which I won't normally, this was testing) and second that these minor overspeeds are expected and accepted within some range of RPM. He wouldn't tell me what they considered the upper range of acceptable, but did say they have tested to 3100RPM without problems as long as it is a simple short excursion. They are much more concerned with sustained overspeed in the several-second to minutes range. My governor seems to be quite happy stabilizing at 2650-2670 rpm, and he actually suggested I dial that up just a hair for a target of 2700, but I think I will leave it right where it is for the moment. The power production of the IO360 at that RPM gives an acceleration that I am more than comfortable with and I don't see the advantage of chasing that small detail just yet, during early Phase I.

Anyway - just a datapoint on Whirlwinds view of minor overspeeds - thought I would share that. The whirlwind manual says no action is needed for an overspeed up to 110% of normal redline.
 
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Fine Pitch Stop

airguy said:
On my 9A I've got an IO360 and Whirlwind RV200 prop with their Jihostroj governor. Over the last couple days doing taxi testing prior to first flight I've been doing some progressive higher power runs, including sudden full-throttles, mostly to see if I could make the engine stumble or miss or do anything else that would concern me if it happened in the air. For this purpose I was being intentionally "ham-handed" with the throttle and throwing it around rather quickly, much more so than would normally be done in a non-emergency scenario. As I said, I was trying to make the engine do something bad.

Well, the engine played nice-nice through it all, and this morning I'm perusing my datalog from the Dynon Skyview, which was set to record all parameters at 1/4 second intervals. One thing that I was looking at was RPM stability versus manifold pressure, and I found that on those occasions where I would absolutely slam open the throttle, then engine would speed up and the governor let the RPM overshoot a bit before pulling it down. In several cases I saw two consecutive datasets (1/2 second) in the mid-upper 2700 rpm range, and 3 times I saw a single dataset (1/4 second) above 2800. The highest I saw was 2850 rpm. In all cases the governor stabilized in the upper 2600's within the span of a full second, usually within 3/4 second. I think it's likely this has been happening to all of us with C/S props for about 60 years, and only recently do we have data collection good enough to see it.

I called Whirlwind and spoke to an engineer there named Greg, discussed this situation, and he expressed surprise that the governor actually captured the target speed as quickly as it did, considering the rapid change that was being asked of it. He advised first off that I not be so hamhanded with the throttle (which I won't normally, this was testing) and second that these minor overspeeds are expected and accepted within some range of RPM. He wouldn't tell me what they considered the upper range of acceptable, but did say they have tested to 3100RPM without problems as long as it is a simple short excursion. They are much more concerned with sustained overspeed in the several-second to minutes range. My governor seems to be quite happy stabilizing at 2650-2670 rpm, and he actually suggested I dial that up just a hair for a target of 2700, but I think I will leave it right where it is for the moment. The power production of the IO360 at that RPM gives an acceleration that I am more than comfortable with and I don't see the advantage of chasing that small detail just yet, during early Phase I.

Anyway - just a datapoint on Whirlwinds view of minor overspeeds - thought I would share that. The whirlwind manual says no action is needed for an overspeed up to 110% of normal redline.
Click to expand...

It's good practice to set the propeller's fine pitch stop (not the governor stop) such that during a static run-up the max engine RPM is slightly below the governor max setting. In other words, if your governor is set to 2700 RPM, the prop fine-pitch stop should be adjusted to about 2600 -2650 static. This helps ensure that the engine can't over-rev during rapid throttle application on the ground, and will also limit the amount of lost thrust due to a governor failure. This assumes that Whirlwind's do have a fine pitch stop adjustment like the Hartzell's do.

Also, if you have a counter-weighted crank supposedly rapid engine acceleration can damage the damper-weights.

Skylor
 
If the experience of nearly every Cirrus driver I watch from my hangar is any indication then transient overshoots most not be an issue. Sorry, was that not my inside voice?
In all seriousness, I have seen data like Mark notes in engine books regarding the allowance for transient overspeed. Can't say they all do but many document the tolerance.
 
Greg,
Greg from Whirlwind gives good advise. You don't usually need to 'ham-fist' the throttle, and won't usually see an overspeed with your WW prop. If you do jam the throttle full forward and get an overspeed warning momentarily, it's not a concern unless it lasts for more than a few seconds. I fly a lot of formation and we always wind it up to at least 2000 RPM prior to brake release on a formation takeoff. This allows the engine to stabilize before we release brakes and start to accelerate. This isn't a bad practice for any takeoff - except - don't hold the brakes. Stand the throttle up (RV8) ~2000 RPM, then put the hammer down. It should accelerate to your max set RPM. If it overspeeds for a second, don't worry about it. If that warning bothers you, change your max governor limit a little higher so it doesn't bother you at this critical phase of flight, but monitor your tach reading to make sure you are not exceeding your limitation.
 
Scott,

I did several full power runs with what I would consider a "normal" throttle application, going from near idle to full throttle smoothly over about 3/4 second as I do in the C-172 with FP prop, and it never exceeded 2700 rpm, I think I saw 2690 once in the data this morning. That supports what everyone has been saying and should not cause an overspeed alarm.

I pretty much have a nice warm fuzzy about the prop/governor operation at this point, I was just sharing my experience as a datapoint for the crowd.
 
Greg
Please do not take offense with my comments, but I would not want any one else to consider this type of testing to be necessary. In fact this is probably the absolutely worst thing you can do to ANY type of engine, particularly a fresh unbroken in aircraft engine. Extreme rapid throttle movements are always the enemy to collection of moving parts and "testing" is not an excuse for mismanagement of equipment.
It will be hard for you to not take offense with my comments, and I apologize for that, but holy cow this was a bad idea!
 
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Tom Martin said:
Greg
Please do not take offense with my comments, but I would not want any one else to consider this type of testing to be necessary. In fact this is probably the absolutely worst thing you can do to ANY type of engine, particularly a fresh unbroken in aircraft engine. Extreme rapid throttle movements are always the enemy to collection of moving parts and "testing" is not an excuse for mismanagement of equipment.
It will be hard for you to not take offense with my comments, and I apologize for that, but holy cow this was a bad idea!
Click to expand...

Agreed, it's not good treatment of an engine - but taking off on a first flight without knowing what the engine will do on a rapid throttle application is not good treatment for the airplane and pilot either. My engine has 105 hours on it as well, it's already broken in - still not good treatment though, you are correct.
 
airguy said:
Agreed, it's not good treatment of an engine - but taking off on a first flight without knowing what the engine will do on a rapid throttle application is not good treatment for the airplane and pilot either.
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Are you going to be applying ham-fisted rapid throttle changes to the aircraft on the first flight?

Do a short static run-up to max RPM while monitoring CHTs and other parameters, to make sure you can get sufficient RPMs, then close the throttle and check idle characteristics, then leave it alone.

Not sure what you expected to learn by shoving the throttle forward and admittedly abusing it that you wouldn't learn from a normal advance of the throttle.
 
RV7A Flyer said:
Are you going to be applying ham-fisted rapid throttle changes to the aircraft on the first flight?
Click to expand...

I believe that was answered with my original opening statement.

No, I don't intend to ever intentionally do this, but I recognize that when things go from bad to worse in the cockpit sometimes we do things that we would not normally do. If it comes down to wrinkling an airplane, or not, based on how fast I apply the throttle on a botched landing, you better believe I'm going throw it in as fast as I feel I need to - and I wanted to know that the engine would respond in that case instead of bogging down and quitting. That was the point of the exercise.

I also carry a fire extinguisher in the airplane, though I have zero intention of using it. I carry insurance on the airplane, though I have zero intention of using it. I have life insurance on myself, though I have zero intention of using it. All these things are there "just in case Plan A,B,C do not work", just as the throttle testing was.
 
airguy said:
I believe that was answered with my original opening statement.

No, I don't intend to ever intentionally do this, but I recognize that when things go from bad to worse in the cockpit sometimes we do things that we would not normally do. If it comes down to wrinkling an airplane, or not, based on how fast I apply the throttle on a botched landing, you better believe I'm going throw it in as fast as I feel I need to - and I wanted to know that the engine would respond in that case instead of bogging down and quitting. That was the point of the exercise.

I also carry a fire extinguisher in the airplane, though I have zero intention of using it. I carry insurance on the airplane, though I have zero intention of using it. I have life insurance on myself, though I have zero intention of using it. All these things are there "just in case Plan A,B,C do not work", just as the throttle testing was.
Click to expand...

You don't need to get testy with people who raise questions about what you're doing. It's a fair question...what do you expect to learn from your test, that nobody else seems to have required learning on their aircraft?

I'll grant that you learned one thing, though...that mistreating your engine during testing can make you have to go ask if you've damaged it.
 
On the contrary, what I learned from this thread is that VAF is quickly going the way of POA, with plenty of people jumping at any opportunity to tell other people they are screwing up, regardless of what they are doing.
 
Throttle useage

While I agree that aggressive throttle movement is not good technique, the reality is that four and six cylinder Lycoming's have been surviving very aggressive throttle movement for forty plus years. The Aerobatic engines typically have a TBO of 1400 hours. Teardown of one 1500 hour IO 540 from a Pitts S2B revealed an engine in near perfect condition. It probably would have easily gone to 3000 hours. The overhaul shop said it was one of the best first run engines they had ever seen. This despite the fact that it did not have an oil filter and got 50 hour interval oil changes vs 25 hours recommended without filter.
One aerobatic school operator( 0 360 fixed pitch) always ran below red line rpm and had a very dismal record with not making TBO.
Treat the engine gently until the rings are seated and the temperatures stable, then run it like you stole it.
Once broken in, if the temperatures and oil pressure are good, you cannot hurt a current generation normally aspirated Lycoming.
 
airguy said:
. . . and I wanted to know that the engine would respond in that case instead of bogging down and quitting. That was the point of the exercise.
Click to expand...

Sounds like a valid test to me.

Was the directional control pretty stable? Or standard anticipatory rudder input?


Frankly, damper weights aside, I wonder what parts know the difference. Pistons, rods, cylinder pressure, bearing forces . . . not the worst condition they ever see. The temperatures are short lived. If there is a know specific issue, then lets discuss it. Facts and data.

I like the reminder to set the fine pitch to rated rpm, so the governor does not have to catch up. Actually, I suspect it is oil volume to move the prop not the actual governor response. AND - if you don't test it you won't know!!

I am sure you made progressively faster throttle application, so it was a managed risk.

For a data point, a runaway heavy duty I-6 (16 liter) diesel engine with a governor failure will easily accelerate at 2000 rpm/sec. RUN
 
Nothing unusual on the control inputs other than the expected right rudder input needed.

Going back and looking at my engine data from those high power runs, I remember thinking I moved the throttle pretty quickly but the data shows the manifold pressure rising from 13.1" to 25.9" over a period of 8 datasets, or 2 full seconds. I am pretty certain I made the full throttle movement in one second, maybe the rest of the time is instrument response lag.

I'll have to plot some graphs of the RPM and ground speeds against the manifold pressure and fuel flows to confirm that.
 
airguy said:
...data shows the manifold pressure rising from 13.1" to 25.9" over a period of 8 datasets, or 2 full seconds. I am pretty certain I made the full throttle movement in one second, maybe the rest of the time is instrument response lag.
Click to expand...

I suspect that extra second the time required for the air to accelerate through the throttle. The engine is sucking the air into the cylinders, but the air isn't moving quickly enough through the throttle to replace at a normal rate just yet.

Similar phenomenon to why a carb engine without an accelerator pump sputters and can quit when opening the throttle rapidly.
 
airguy said:
On the contrary, what I learned from this thread is that VAF is quickly going the way of POA. . .
Click to expand...
Oh Please! Do not even make such a statement! There is absolutely no comparison between the two forums. I have not been on POA for many years now specifically for the reason you are suggesting. This site DOES NOT EVEN BEGIN to allow the type of maddening comments that site allows. And, I hope it never will.
 
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