5 minute power limits R44
 

got into a bit of a debate during my BA flight review: On the R44, is the limiting factor for 5 minute max power the gearbox power handling capacity, or engine longevity?

Main Rotor gearbox

Ersa seems to know it so well, he or she don't need to elaborate the answer. I'm however not that confident.. that he or she is correct. The 5 minute rule was first introduced (in Robinson family) in the r22 hp, if I recall correctly. The key part was the oil cooler, which leads me to believe the limiting factor is the engine and it's temperature.

I never heard any mention of the MGB being the limiting factor on a Robinson.

Actually Semi confident that the MCP and 5 min rating are set to prolong the life of the engine so it can achieve its 2200 hr life and also so there is power available at altitude..

A test pilot once told me that the ratings are there to be used for 5 mins once per flight, and that will give you the advertised engine life between overhauls.

You could use it for 5 minutes every 5 minutes if you felt the need, but have your credit card handy at the next service.

What is the difference between using the 0320 in a cessna or a R22 ?

The 5 min limit is for TBO (Time between overhauls ??)
I have seen an IO540 fitted to a fixed wing (can't recall the model) and the turbo allowed the MP to go to 33", the TBO wasn't 2200 though.

There will be drive train weak spots; belts, MRGB, etc.

ooookkkkk. These 6 replies mimic the debate between my flight examiner and me. He has heard it is due to MRG temp rise due to inadequate lubrication, therefore if you are in a hurry you can fly takeoff power for 5 minutes, back off for a minute to "get the oil back between the gear teeth", then jack the power back up for another 5 minutes, rinse and repeat.


I'm pretty sure its due to derated HP for 2200 TBO. And if they'd meant "intermittent speed enhancement power" they'd have said that in the POH.


If I can get somebody at Robinson to answer the phone, I'll post their answer.

gator2:
I’m stretching some brain cells here, but the 5 min limit is neither one nor the other but all of the above.

Most helicopters are derated for overall performance requirements. If you dig into Part 27 and Part 33 you will see lists of limiting factors used, to include drivetrains, fluid temps, system RPMs and component life.

While ersa made a valid point between fixed and rotor wing apps, this is an apples to oranges comparison by the sheer fact that airplanes generate lift through airspeed (via prop rpms) and helicopters directly through engine rpms. A helicopter engine operated at fixed wing limits would run out of steam at altitude. Not to mention require the drivetrains to be built much stronger.

The 5 min limit is one of many standard certification items to ensure an aircraft can perform throughout its flight envelope, type design,and mfg’r requirements. Ever wonder why a twin engine helicopter has an aircraft based 5 min limit with both engines on line, but when OEI the limit drops to 2 ½ minutes per engine? Me too.

All said and done, when Robbie certified the R44 I would bet all design items were considered on the 5 min limit and not just 2 of them.

And just to throw another wrench into it so to speak, if your 44 is operated Part 91 in the States there is no FAA requirement to overhaul the engine at 2200 hours. So that kind of “limits” the engine theory.

But as with any operational limits, abuse them at your peril.

W1

Gator2

I would say your examiner is on the money , with his response .

The fact that the MCP and “5 min limit” are derived from the graph to allow the same power into the transmission at different altitudes 205hp cont/225hp for TO (from distant memory - R44 R1) would indicate it’s mainly a gearbox saving limitation.
Once you reach an altitude where the engine can only produce 205hp (full throttle height) you are allowed to run the engine wide open all day long with no TBO penalty.

Let me preface by saying I'm not qualled in the R44, I have a short amount of time in the R22. So why am I posting here? I currently own and fly a fixed wing plane with an engine which is time limited for max power.

The E-225 engine in my plane is able to produce 225HP at 2650 for 2 minutes. Max continuous operation is WOT at 2300.

For any installation, this can be limited even more by the airframe mfg, but it cannot be exceeded due to the engine type cert. What is certain is that heat kills. The hottest part of any IC Otto cycle engine is the exhaust valve stem, seat, and valve guide. The next area of concern is the top of the piston, which is a hypereutectic alloy of Al, Si, and other goodies.

Extracting heat from a Helicopter engine is always a challenge. No matter how well designed, the cooling flow, and air path will always create hot spots. The TBO is a function of the best case of the hottest part of the engine. For the big bore Conti, the main bearings, and the cam bearings are usually good for > 4000 hours. However, the cylinders, valves, pistons and rings are much more highly stressed, which is supported by the number of cylinders replaced, and the 'top overhauls' made on many engines.

I will opine that there is a combination of systems that limit the maximum HP to 5 min or less. However I know that most of the restriction is going to be related to heat generation in the engine.

On the factory course we were told that after 5 minutes you could lower collective, check Ts &Ps then pull straight back into it again. Seemed kind of pointless.

A cautionary tale for those in favour of the 'rinse and repeat' concept.

Many moons ago I was paired with a pilot, day on/day off, doing gravity survey and I was unable to match his performance every morning where he managed the longer distances in rising OAT than I could achieve. Operating from Tom Price, Western Australia in ~36C days in a Bell 206BIII.

It turned out that he was pulling 2.5 minute power limits, backing off below the limiting TOT for a few seconds and then straight back into the higher limit.

Someone was looking after me as I was scheduled to fly on Saturday, but he had broken a skid tube on Friday and the aluminium welder was in the pub by the time he returned. The ginger beer decided to call off Saturday's flying and do the skid repairs plus a case half change on the Saturday.

When the case halves were pulled, most of the stators had failed and were bending into the path of the compressor blades. It wouldn't have lasted 30 minutes flying had I flown that day over tiger country with nowhere to go in a forced landing.

I realise that the R44 is a piston and the reason for the R44 5 minute limit is the topic of this thread, but the idea of pulling to a higher limit then backing off only to pull back to the higher rating is one that I cannot and will not agree with, purely from turbine experience.

John , completely agree, the Robinson flight manual states take off 5 mins

That‘s one of the big secrets for me even in twins. Wich T/O needs 5mins or 2,5 in twins.
My college flies powerline patrol with Bell206.
He‘s doing the same thing. Pulling up power for 5 min until 100%torque than lowering pitch for very Short time and than he starts the same.......
So what is the meaning of 5min t/o power????

Your point is escaping me a little bit there. Twin engine 5 minute takeoff limit is so you don't rip the guts out of the helicopter MGB/TGB/IGB and driveshafts. The OEI limits are so you don't rip the guts out of the engine.

And this talk about reason for 5 minute take-off limit in R44, or whatever helicopter. I mean, who cares whether it's for engine warranty or engine TBO or MGB or anything else? It's not something the pilot needs to know. The examiner might ask you, but he can't fail you for not knowing...he might fail you for not knowing the said limit as published in the RFM, but he can't fail you for not knowing why or how or whatever speculation not published in the RFM.

The 5 minute take-off power limit is just that. From the hover, commencing takeoff pull up to that power limit, 5 minutes starts, once clear of obstacles, set MCP or climb or cruise power. If after 5 minutes you still aren't clear of obstacles - and I'd be wondering why that might be the case - you keep pulling that 5 minute power for as long as you need to. And if that amount of power 'aint enough to avoid bumping into something in the take off path, you use all the power the engine can give you, for as long as you need it. And at the end of the flight make an appropriate entry in the technical log.

You can't be using 5 minute take off power in the cruise just to go fast, or to climb fast, back it off at the 5 minute mark, and then grab another 5 minutes worth of take-off power to go fast. That is not what 5 minute take-off power is all about...and besides, some helicopters have power limits based on airspeed.

That is just daft. If you need more than 90% TQ in a B206 to hover OGE then you are too heavy for the prevailing conditions.

gulli:
We're drifting off topic, but the point was only to the time frame of the limit (2.5min) not the reasons behind it. This is one of those questions I would ask product support people to keep them on their toes. Nothing more. But I never really did get a good answer.


Why is a twin OEI limited of 2.5 mins exactly half the aircraft 5 min limit? Why not 3 mins or 1.5 mins?


If a twin OEI limit is higher, why not apply the same limit to the same core engine installed in a single engine aircraft?


And... if the aircraft had 3 engines would the OEI limit equal the aircraft limit divided by 3? Or 1.66 mins?

I"m not a metalurgist, my eng background is nuclear but I recall that most all the metals we deal with have a molecular structure which is damaged by cumulative heat cycles. This is most true when those metals are operating just below their plastic or melting point. Each time a metal is subject to exceptional heat, the molecular structure is deformed, or weakened just a small amount. We're talking about EGTs of 1500-1600F flowing out an exhaust port made of siliconized Al. The continuous temp of that exh passage is working very close to the melting point of the alloy. Keep doing that, and the bits that make up the valve train/piston are going to start deforming.

And as this applies to catastrophic failure, I would say that the limits only counting toward TBO are inaccurate. Exceeding the time/power limit could definitely result in immediate failure. A hot spot in the cylinder, suffered as a result of exceeding the conservative limits could lead to detonation, and very rapid temp rise, which will see the cylinder leave the aircraft, or the rod detaching from the crank in some spectacular display. Next time I'm at the hangar, I will provide some photo evidence of the problem. I have one on my bench with a large hole, where a large hold does not belong.

I think its a perfectly plausible question to ask a pilot in a flight review

Yeah we are drifting, but for the benefit of those who might not be familiar with twin engine helicopter limitations:

Let's say for example, in twin engine helicopter the 5 minute take-off power limit is 100% TQ. At that power setting the MGB sees 2x100%TQ = 200% TQ. The limiting factor is due to the MGB. Now let's say you are OEI in that same helicopter and you are operating at the OEI limit of 127% TQ. The MGB sees 127% TQ: remember, it can do 200% TQ for 5 minutes. Easy, no sweat at all for the MGB at any OEI limit. But that engine sure is getting hot pumping out 127% TQ for (up to) 5 minutes.

Which is why I said earlier, the 5 minute take-off limit is stop you ripping the guts out of the MGB/IGB/TGB and driveshafts, the OEI limits are to stop you ripping the guts out of the engines.

This is a simple explanation, the actual situation is slightly more complex because in some twins there are things such as 30 second OEI power limit, dual engine soft limiting, and dual engine blow away power, and combined TQ limits when one engine is better than the other, blah blah blah, but the principal is the same.

And finally, an angle to your question is why is the 2.5 min OEI limit half the 5 minute twin engine minute limit? It is only half in time, not half (actually double) in power. Why it is half in time I don't know, and I don't care, I'm only the pilot and I only know what is written in the RFM. And if that examiner asked me the same question that was raised at the opening of this topic, my answer would be "don't know, don't care". Sure, he is entitled to ask the question, but I don't need to know the answer.

gulli:
Good explanation. It almost mirrors one I received from a Pratt rep a number of years ago. We were going through a learning curve on our new EC135Ps where they were flying around OEI for various "reasons."

We changed the 1st two engines but on the 3rd they sent a rep and boroscope. A discussion started on the 2.5/30 min OEI which you can see in the downloads.

Long story short... if engine can hang for more than 2.5 why keep the limit. He like me think the 2.5 is a certification "check box" item. But cheaper than wringing out the rest of the system. He mentioned just look at the 139 if you get a little excited on the collective, you change half the M/R limit times.

When I asked the 3 engine divided by 5 mins question he offered to buy the 1st beer. Oh well, maybe next time. Thanks again. W1

And we now return you to your regularly scheduled program on the R44...

In which case I have nothing further to say as I know nothing about the R44.

Except, and this applies to R44. #1 Knowing the limitations of your aircraft is a good starting point. #2 Knowing why or how those limitations came into existence is not important, but if you care to find out, no harm in doing so. If you take a check-ride with me, expect questions on #1, there will be no questions from me on #2. I'll be perfectly happy if the student knows and applies all limitations.

Still on twins OEI, when only one engine is pumping power big-time into the MGB, it is getting it asymmetrically, instead of one from each side. This puts stresses on the gear teeth, bearings, MGB supports, and the little people inside the box putting coal onto the fire.

The exception of course is the B212 / 412 where the engines feed to a combining box before it goes to the MGB by a single shaft.

Regrettably a lot of that sort of thing used to go on, and still does...more and more newer design helicopters now have black boxes that record all sorts of numbers and any errant pilots can be identified by way of laptop computer and be retrained by way of Chief Pilot.

Yeah, but if you're burning up OEI engine limit time the engine gets changed, not the MGB. The MGB is designed for one engine pumping power big-time into it from one side. Just as the CBOX in 212/412 is designed for one engine pumping power big-time into it from one side.

Unfortunately, in my case #2 is all I have to go on when you pilot types show up with the odd light on and want to know why. But I digress again... W1

I hear you. Just be greatful those pilot types told you about the odd light. It's the pilot types that don't tell you about the odd light that are of concern.

Well, boys, I had a chance to talk to the tech guys at Robinson. Its the engine, not the MRG.


Proof: The gearbox in the Raven II is exactly the same as in the Raven I.


And that is why I believe it IS important to understand HOW the limits came to be as well as WHAT they are. If I'm in a crap situation, which I intend never to be as I do this just for fun (in fact I have a rule: at the first close call of any kind my Heli flying is done), I know I can pull takeoff power as long as I need it, and the result will be wearing the engine out sooner, NOT falling out of the sky due to a catastrophic failure of the gearbox.


If you want to go down the "just follow the rules" path, why bother to teach aerodynamics at all to student pilots? Just tell them "As you are slowing down, once it starts to shake around 20 kts you will need more power until you are really close to the ground"

OK then, I'd like to know how the Vne limit came to be in R44.

p.s. helicopter shaking as you slow down to 20 knots, that's a Robinson thing is it? The helicopters I fly don't shake, and if they do, it gets grounded for rectification.

I think he is loosely referring to the vibration you feel as you pass out of translational lift.

Many people know that the Vne in the R22 is due to forward cyclic position. At Vne (and most rear cg), the stick is forward enough that you are at the certification limits for controllability - the regs require you have enough push left to overcome a gust.

In the R44 I was somewhat convinced it might be retreating blade stall, but I talked with Pat Cox @ Robinson a couple years ago and he said no, that the Vne is just to control stress on the dynamic components and make sure components all reach their design lifetime.

Well, there you go, gulliBell. Now, if for some reason you need to exceed Vne in a R44, you'll know its possible. Something might break before its supposed to after you do it, but if you need to do it to save a life, go for it.


Can't really think of a legit life saving reason to exceed Vne though.....passenger in cardiac arrest, need to get on ground now? But, I guess in that case I'd just enter a vortex ring state, get down to about 50 ft agl, employ Vuichard, exit VRS at 10 ft, land, save the passenger.

Fantastic! We haven't had a good Vuichard punch up on here for at least a couple of weeks. Of course if Vuichard didn't work for the guy before you (smoking hole in ground etc) there's always autorotation as a gentler alternative...

Actually in an R44 you'd have to be trying pretty hard to exceed the sea level Vne, as in nose pointing distinctly downward. I don't think full 5-minute power would get you there in level flight.

The Vne for an S76 is 155kt, I managed to get it up to 180 briefly when I was travelling quickly with a 35kt tailwind and ran into a front with around 40kt headwind. No engineering problems or aspects to be looked at, the machine can cope with such things.

If you are not pulling its guts out with power, there should be no worries in an R44

Apart from this bit if you exceed Vne, you have reduced the life on components - you have no idea by how much or what damage you may have done.

It is simple - don't exceed the limits but of you do, inadvertently, have the aircraft checked throughly.

Do you want to be the one flying when a pitch change rod or control horn gives up early in its life due to overstress?

Well, you know Crab, that's the scary part.


"......aircraft checked thoroughly."


In the examples you chose, say a control horn has started a microscopic crack due to overstress in the Vne incident. Is the mechanic going to fluoroscope every part? Of course not. Could the crack propogate and fracture between 100 hrs? Hell yes.


Your advice is right on, but........its not actually that good to be an engineer and fly other peoples machines. I try not to think about it.

Gator - agreed but at least if it is reported, it might get checked more thoroughly each flight and possibly changed early just in case.:ok:

When we flew display manoeuvres in the Lynx, they were all documented and each cycle of each manoeuvre deducted a certain life from specific components.

Similarly on the Sea King, an overtorque, even if relatively minor, would reduce the life of the gearbox by a calculated amount dependent on the severity.

If you pretend these things don't happen and don't own up, you are building problems for yourself or others further down the line.

I own up. It's the other guys flying the thing I worry about, but can't do anything about, so I try not to think about.