R44 MAP Gauge over-reading?

[Hot and Hi]

I am puzzled by the following behaviour of the manifold pressure (MAP) gauge in our Robinson R44 Raven II: At our elevations and temperatures (hot & high, did I mention this?) the 5-min take-off rating is always higher than what the engine can deliver. At least according to the limitations chart in the Pilot's Operating Handbook (POH).

That is to say that - while the 5-min take-off rating for the balance of the aircraft (gearbox, rotor head, blades, etc.) would allow higher power from the engine (e.g., equivalent to 24.8 in Hg of MAP), as per limitations placard in the POH, available engine power is supposedly reached before that (namely when the throttle is fully open, e.g., at a MAP of 23.5 in. Hg). We would say that the max avail engine power at this moment is "throttle limited".

In the before said example, the ambient pressure would be around 25.0 to 25.2 in. Hg, and this is also what the MAP gauge shows with engine off (both before, and after the flight, somebody asked me this, so it might be worth mentioning).

Now, in actual flight I seem to consistently be able to pull more MAP than what the limitations placard says: Using again above example, I can without exception pull 24.5, even 24.8 in. MAP without any droop in RRPM. It feels as if I could still pull more, but don't dare trying as at > 24.8 in. Hg MAP one would exceed the take-off power rating of the R44.

Is this normal? How is this possible? I thought that the full-throttle MAP reading in a normally aspirated engine is mostly dictated by ambient pressure, and not subject to the variation between individual engines that there may be.

Could the MAP gauge be over-reading? But how could one then explain that the MAP reading with engine off (which must be equal to ambient pressure) is again correct?

[tu154]

Ok I'll bite, I think you are missing the entire point of the gauge here. Of course you can pull more power than the limit, and the gauge does not always tell you when you are about to run out of power, although it can... :E

The limit is about protecting drive train and engine over the long term, not what you can pull on any given day.

[mack44]

G'day Hot and Hi,
Check if there is a leak in the MAP gauge line coming from engine to gauge. I also once had a discrepancy with Map gauge and altimeter reading different ambient, caused by some solder left in gauge at time of manufacture.
Cheers Mack.

[SARWannabe]

G'day Hot and Hi,
Check if there is a leak in the MAP gauge line coming from engine to gauge. I also once had a discrepancy with Map gauge and altimeter reading different ambient, caused by some solder left in gauge at time of manufacture.
Cheers Mack.

I second this, we had a Raven II, which at 102% before takeoff, collective fully down, was reading 18"MAP when I should have been reading 14" at our density altitudes. Therefore artificially limiting the power available. It was a problem to do with one of the pressure lines to the MAP gauge.

[TwinHueyMan]

This sounds pretty normal to me... at the factory, they mentioned that the theoretical max MAP available is station pressure (or what the MAP gauge says with engine off) minus 1.0-1.5" for induction system inefficiencies. In practice, I've seen up to 0.5 below station pressure before the droop, every bird is different.

As for determining where that will be on a chart, the line graph form of the MAP limit chart that is available on the RHC checklist (available from the factory) seems to do better than interpolating the block chart. It's hard to find the max available MAP with solid accuracy on that chart. The other quick way is 29.92" (or 30.00" for rough calcs) minus an inch per 1000ft PA and then take off the 1.0-1.5" for the induction loss.

Or maybe I'm misunderstanding the issue...

Mike

[Hot and Hi]

Thank you for the responses received so far!

Both potential explanations offered (a leak in the vacuum line to the MAP gauge, and different degrees of inefficiencies in the induction system) explain why a MAP gauge reading station (ambient) pressure correctly while then higher MAP values with the engine running.

- In the case of a leak in the vacuum lines, the MAP gauge would indeed be over-reading (actually under-reading the vacuum in the manifold), thereby artificially limiting power allowed.

- In the case of differing levels of inefficiencies of the induction system (variation from one engine to the other), the gauge would read correctly. Effectively that means that with lesser induction loss you have a stronger engine, and the pilot would need to adhere to the limits shown in the MAP chart, in order not to overstress drivetrain or engine.

Would it be plausible though that the induction losses vary by such a large degree from one R44 installation to another?

[Helicodger Pilot]

I'm no R44 expert, but in general the condition of your induction air filter can cause a significant loss of MP. Likewise the routing of any scat hose between the filter and the throttle body. Sorry if that's obvious...

[Aluminium Mallard]

It's normal for the engine to be able to make power above the MAP limit on the placard without drooping rpm.

The torque produced from a "standard" engine varies with density altitude (pressure and temperature).
The MAP gauge/OAT/placard combination is a poor mans torque gauge. Limiting the torque ensures the components (engine / gearboxes) go the full 2200 hours.

Pulling past the limits on the placard is applying more torque to the rotor drive system than what is approved... its bad airmanship and not necessary with good planning. The engine and gearboxes will cope with the increased torque but may not last the full 2200 hours.

Not all engines are created equal however... they do not make the same power (torque) at the same indicated MAP. An indication of the health of the engine is the MAP when you are on the ground at 102% with the pedals neutral. Where I am (usually ISA +25) a good engine will indicate 12-13, a bad 15+. It's a measure of much power its taking to overcome the drag and friction of the rotor system which is fairly constant between machines. Learn what a healthy engine produces for your conditions and check your MAP every time you wind up if you want to avoid surprises when taking off.

There are so many potential issues that make an engine a "Bad engine" e.g. low compression, loose plugs, bad leads, bad fuel, magneto, blocked air filter mixture... if its drooping at less than the placarded limit (with the throttle fully open) I would suspect you have one of those.

If its drooping and the throttle isn't fully open then it might be the governor actuator is a bit tired.

[Hot and Hi]

Thank you for all your advice.

As a reminder, the question was not whether one can pull more power than allowed by the limit chart.

But rather whether it is to be expected that one can pull more inches Hg MAP over and above the value that the manufacturer says one cannot exceed because the throttle is already fully open. And whether observing such higher indicated MAP value would suggest either a faulty MAP gauge or a leak in the vacuum line to the MAP gauge.

Maybe this picture from the Internet can help. In this example:

- MCP is limited 22.7 inHg
- Take-off Power should be 24.3 inHg
- However, at 23.3 inHg the throttle is already fully open, and the book suggests that you cannot pull higher MAP

If one pulled more collective, MAP is supposed to stay at that full throttle value, but RRPM droops due to higher angle of attack/drag.

Some of us however report here that this is not necessarily the case.

https://static-s.aa-cdn.net/img/ios/549114077/44173b092f068e56c75c2fe561ab0330

[Aucky]

If one pulled more collective, MAP is supposed to stay at that full throttle value, but RRPM droops due to higher angle of attack/drag.

Some of us however report here that this is not necessarily the case.


'The book' actually says very little to explain the MAP placard, but what's important to note is that at low density altitudes the engine is capable of producing more torque than the drive train is rated for so you can easily pull more than the MAP chart says but in doing so you are over torquing the gearbox.

As density altitude rises the engine cannot deliver the same power and you reach a point where the MAP chart is giving you a theoretical maximum for full throttle, which may be less than the torque the gearbox is rated for, and may not actually be achieveable if for some reason the engine is underperforming. Keep pulling collective here past the MAP placarded value and the RPM will droop. You see this at approximately 6-7000ft altitude (ISA temps) in a Raven II. You'll also feel the governor making large throttle adjustments on the twist grip as it starts to operate near full throttle, and if you lower the lever too quickly from a high MAP the RPM may get high as the governor is slow to close the throttle.

However, back to the first issue which you started this thread with. If the are seeing values much above 15" at 102% with the collective fully down anywhere near sea level I'd suggest it may well be the line to the MAP gauge leaking, or a failed MAG (be this would easily be identified in the mag checks).

[puntosaurus]

That whizzy Tim Tucker app is for the normally aspirated Raven, not the fuel injected Raven II that you are flying. The Full Throttle line is displaced to the right and less steep in the Raven II. That was the point of the IO540 in the Raven II, to get better hot and high performance.

HTH

PS. Here's a video of the Raven II version of the app.
video

[Hot and Hi]

That whizzy Tim Tucker app is for the normally aspirated Raven, not the fuel injected Raven II that you are flying. I appreciate this. I was showing the chart only as an example for clarification, as some Ppruners here didn't see the point. (I am actually using the Tim Tucker app for the R44 II, and can recommend it all round.)

what's important to note is that at low density altitudes the engine is capable of producing more torque than the drive train is rated for so you can easily pull more than the MAP chart says but in doing so you are over torquing the gearbox.That is agreed, but not the point of this threat.

MAP chart is giving you a theoretical maximum for full throttle ... Keep pulling collective here past the MAP placarded value and the RPM will droop.This is what this threat is about. Actually, about what could be wrong if the RRPM does not droop despite the placard saying it will droop.

Interestingly enough, two Ppruners suggested that maybe nothing is wrong. That it could be a sign of a 'stronger' engine, in that our R44 Raven II possibly just has a more efficient induction system (e.g., cleaner air filter), allowing the full-throttle MAP to come almost equal to station (ambient) pressure.

But we will also check out a possible leak in the vacuum line to the MAP gauge. Many thanks again to those who have applied their mind to the problem presented.

[puntosaurus]

Ah. So you appreciate then that whilst you had the correct data in front of you on your ipad that would perfectly illustrate your issue, you chose to post some irrelevant data from the internet as clarification.

Then you might also be open to the possibility that your original presentation of the problem was less than totally clear, and hence why it took some helpful PPRuners some time and effort to divine the real issue.

:ugh:

[Hot and Hi]

I second this, we had a Raven II, which at 102% before takeoff, collective fully down ... should have been reading 14" at our density altitudes. ... It was a problem to do with one of the pressure lines to the MAP gauge.
Makes sense. I'd like to test this. Where do I find the expected MAP reading (102%; collective down) for a given density altitude?

[Aucky]

This is what this threat is about. Actually, about what could be wrong if the RRPM does not droop despite the placard saying it will droop.

As eluded to. You would never expect the RPM to drop at the placarded manifold pressure limit (or even a few inches MAP over the limit) unless you're operating around 6000ft DA or higher in a Raven II, because you are not throttle limited, but instead drivetrain torque limited. So this is totally normal!

In response to your other question it doesn't say anywhere the MAP you'd expect to see at 102% lever down. But st sea level it's around 14"MAP. And he higher the altitude the lower this reading as atmospheric pressure decreases. So around 5000ft it's approximately 11-12"MAP. If you ever see greater than 15" MAP at 102% lever fully down in a Raven II I'd suggest something is worth investigating.

When my manifold pressure line had a problem it was reading 18" MAP on the ground at 102%.

[rotorfossil]

It was my experience that manifold gauges are not the most accurate instruments having observed differences of up to 1” MAP in static readings between individual aircraft in the fleet. This partially accounting for some being “good” performers and others not so “good”.

[Hot and Hi]

... at sea level it's around 14"MAP. And he higher the altitude the lower this reading as atmospheric pressure decreases. So around 5000ft it's approximately 11-12"MAP.
Tested today: 11.5 inHG MAP (102% RRPM, collective fully down)

Elev: 5,100 FT
OAT: 30 deg C
QNH: 1021 mb
Density ALT: around 8,100 FT

So with the low and high extremes being indicated correctly by the MAP gauge (both station pressure with engine off, and MAP reading with collective down, 102% RRPM), where to look now?

Can't be a "bad engine", misreading gauge or a leak in the vacuum line anymore. The only explanation fitting all data points I can see is "more efficient induction system" / cleaner air filter, as was mentioned by some Ppruners. Sounds also almost too good to be true :hmm:

In particular, this result doesn't suggest I was actually using less-than-MCP when the gauge says that I am using MCP. (I was secretly hoping I could open more throttle without exceeding MCP, thereby coming closer to VNE in straight & level flight...).