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.