R22 Corner

[Whirlybird]

Governor on or off for autos? Just a matter of choice I think. I've done both with instructors; each instructor seems to have a preference. If I'm practising them alone with a power recovery, I prefer to have the governor on; it's one less thing to think about. YOU have to join the needles anyway; but then the governor takes over - no big deal.

NigD2,
I hated autos for ages. But after practising them to the ground for what seemed like hours and hours during my CPL course, I suddenly realised they could be fun, and told my instructor that. He reminded me of this during my recent LPC, when I got nervous at the idea of doing autos to the ground, since I'm a bit rusty again. Do enough of 'em, and you might come to love 'em!

[headsethair]

"regular governor-off refresher training sounds like a very good idea, and perhaps should be part of the annual LPC"

It is where I fly. We also do a new thing on R22 and R44 - the testers pull the tacho cb as a surprise. This is related to a recent incident where a 44 was flying along happily and the tacho dropped to zero. So, the pilot decided to immediately auto. He landed hard and chopped the tail off.

Turned out there was nothing wrong with the engine. The tacho needle/spindle had failed...........

So - might be a good idea to check all pilots for hearing........generally speaking, if the cylinders sound intact, the rotor rpm OK. the airspeed is OK and the VSI isn't diving - you'd think anyone with even half a brai...........sorry. I'll stop. Don't want to get banned.

[Whirlybird]

headsethair,

Yes, from the comfort of a desk, or probably even flying with a helluva lot of rotary hours behind you, you would think that anyone with half a.... But up there when you haven't had that much experience and something goes wrong, and you know you have to do something but you're scared and your brain's slowed down as a result...it's all too easy to make the wrong decision. I know; I've been there.

[Helinut]

System failures like tachos are not peculiar to R22s of course. In fact, the inbuilt engineering redundancy in R22 RRPM control aids is very high so you will be unlucky not to get some help in RRPM control - (there are 5 separate aircraft systems).

When I did the Bell 206 factory course (which is unbeatable if you fly their aircraft) they made mention of a number of "engine failures" where a perfectly serviceable helicoper was flown into the ground because of a tacho generator failure giving a spurious indication. One of my colleagues had a torque indication failure with other failure indications in an AS355N just a couple of days ago. It all ended uneventfully, fortunately.

Experience can certainly help in these cases, but experience is expensive and difficult to obtain. However, a small part of all that time we spend on the ground waiting to go flying can be usefully spent with the AFM and technical manuals getting to understand how these things work. When something does fail you are in much better shape to intelligently assess the problem. And best of all, you can gain such knowledge at NO COST!

[Irlandés]

R22 Performance Chart Interpretation

OK, this one's a little complicated to explain so please bear with me. Any text book I've ever read has said that every degree of temperature deviation off a standard ISA day (15C at sealevel, temperature lapse rate 1.98C for every 1000ft) is equivalent to 120 feet. So on an ISA +10 day (ignoring humidity) a pressure altitude of 0ft should equate to 1200ft of DA. This is easily confirmed by checking a PA/DA conversion chart such as this one from the R22 manual. Pretty standard stuff and up to here no problem (I hope! ) .

Now, if you want to calculate the IGE ceiling for an R22 Beta (at say 1300lbs for the sake of argument) using this IGE Ceiling chart (from the same manual) you will see that your ceiling is 8,500ft of density altitude (point of intersection of ISA line 'Standard Day' with vertical to 1300 lbs). Now at this stage we could stop using the graph and work out the rest in our heads: DA = 8,500ft, ISA +10 day, PA=(8,500-(10x120))=7,300ft also confirmed by aforementioned PA/DA conversion chart.

If however you decide to use the IGE Ceiling chart to calculate your PA, it will give you an answer of 8,000ft PA (intersection of interpolated ISA +10 line (not OAT +10 line!!) with vertical to 1300lbs). So for our 10 degrees of deviation from ISA we have paid a price of only 500ft which equates to 50ft for every degree and not 120ft.

Can anyone explain this discrepancy??? Or have I lost the plot and am incapable of reading a perfomance chart?

If at this stage you are totally lost and wish to brush up on DA and PA calculations, I recommend going to my previous post 'Want something to read?' If you are not totally lost, please enlighten me! Now, where did I leave those Tylenol??

Thanks!
Irlandés

[[email protected]]

Irlandes, I think it is because you have tried to interpolate from the standard day line - I understand why but since there is only one ISA line on the graph you are only guessing as to where the ISA +10 line would be.
The gradient would be parallel to the standard day line but you can only best guess the spacing using the spacing on the OAT lines.
This may or may not be reasonable but I suspect that Robinson would argue that they did not include such a line on the graph, therefore you shouldn't make one up. If there had been a series of ISA+temp lines then you could actually 'interpolate'.
To be honest the lapse rate is likely to change so much in the real world that assuming it is ISA+10 at altitude just because it is ISA+10 at sea level is asking for trouble. You are better off getting an accurate OAT from either Met reports (balloon ascents) or from the OAT gauge when you get up there.
The NZ handbook on performance is a very good guide and every operator should be made to read it. As a Mil pilot I would have to have a very good reason to operate without OGE performance plus a 5% Thrust margin. I know the light single operators will tell me they could never get the job done if they had to have this much spare performance but the accident statistics speak for themselves.
Planning to land in the mountains with only IGE hover capability is like russian roulette - you only have to get it wrong once!

[Irlandés]

Hey Crab,
thanks for your reply and for taking the time to wade through my post. On a practical level (the one that really counts!) you are of course totally right and I couldn't agree more.

On a purely theoretical level however (please indulge me here), I would suggest that I'm not really guessing where the ISA +10 line is. If you consider that the ISA line intersects the 1300 lb vertical at approx -2 degrees OAT (pretty clear on the graph) which exactly corresponds to the ISA temp at 8,500ft (15-(8.5x2)=-2) then it seems logical that the ISA +10 line must intersect the same 1300 lb vertical at + 8 degrees OAT (-2+10=+8) which on this graph corresponds to 8000 ft PA and not the 7,300 ft I believe it should be. Do you see what I mean? My worry is that the graph tells you that your ceiling is higher than it actually is for ISA + values, i.e. if there is an error, then it's not erring on the side of safety. Obviously the reverse holds true for a 'minus' ISA day where performance improves. Also the same holds true for the R22's OGE graphs. The way I see it, on a 'hot and heavy' day, the graphs are not penalising you as much as they should for non-standard temperatures. Maybe I need glasses...

Ah well, back to the real world...

Irlandés

[[email protected]]

Irlandes, I fully understand your concern and think it is worth highlighting this anomaly on the graph. I would be very interested to see what experienced R22 operators have to say. You could always email it to Frank Robinson and ask, just don't copy it to Lu Z or there will be another conspiracy born!

[Nick Lappos]

Irlandes,

I think the problem is that you are expecting the hover performance of the aircraft to be driven only by the density altitude. For any helicopter, the hover performance is based upon the altitude, temperature and engine power available, as well as the ancilliary limits, such as tail rotor effectiveness.
The gain in hover IGE weight with reduction in altitude is a product of the both rotor efficiency, (which is almost entirely driven by density altitude), and the engine power available, which gains power as altitude is reduced, and also as temperature is reduced.

I believe the hover performance is more strongly influenced by engine power than by rotor efficiency, and you have deduced that from the charts.

As an experiment, look at several constant density altitude conditions, where the temperature is increased considerably, and see if the weight to hover is a constant. I believe the lower temperature conditions should have higher HIGE performance.

[Irlandés]

Well Nick I've had to rethink my original reply to your post and subsequently deleted it (I was in a dazed and confused state! ). I did what you suggested using the line of constant 12,600 ft DA on the graph and here are the results...

12,000ft PA, OAT -4 degrees = DA 12,600ft, MAUW = 1100 lbs

10,000ft PA, OAT +17 degrees = DA 12,600ft, MAUW = 1150 lbs

8,000ft Pa, OAT +40 Degrees = DA 12,600ft, MAUW = 1200 lbs

You said that you believed the lower temperature conditions should have higher HIGE performance. But it seems judging by these figures, that DA being equal, performance is increased at the lower altitudes/higher temperatures. This is reflected by the increase in maximum allowed all-up weight. Does that make sense?? Is the piston engine performance therefore more adversely affected by higher altitudes than by higher temperatures for a constant DA? That's the only way it seems to make sense to me. Am I still in a dazed and confused state?

That apart, thanks for throwing light on the error of my assumptions. As usual, it all seems so obvious in hindsight. I always took for granted that an aircraft would perform identically irrespective of the pressure altitude/temperature combination as long as the density altitude didn't change. If only life were so simple! I guess this assumption is only correct for non-powered flight, gliders etc. This for me is a real eye opener and something I thought I understood well needs a bit of reevaluation.

Thanks!
Irlandés

[HeloTeacher]

Just a quick shot at a short answer.

Because of the carburator and its associated temperature drop on the engine inlet air, I would not be surprised to learn that the engine gets a little bit of a temperature-related performance benefit. Becuase the R22 is not turbo equipped, there is no associated benefit to offset lower ambient pressure.

This could explain the performance numbers.

[Figure Of Merit]

Well- never thought I'd say this but I think Nick Lappos is wrong (there I've stuck my neck out now!)

The engine is de-rated which means that, having set the limit manifold pressure, the engine power output is the same regardless of the Density altitude. This is 124 BHP or 131 BHP depending on the limit (MCP or 5min T/O power) we are talking about. Either way the function of the limit manifold pressure chart is to set the max power the drive train can handle for a wide range of altitude/temp combinations.

This works up to an altitude/temp combination where the throttle is fully open- Robinson call this the "critical altitude". Hotter and higher than here and, yes Nick I agree, the engine cannot put out the Horse Power that it has been derated to. Below or colder than this though the power output remains steady despite changes in DA. You can see this effect best on the OGE hover charts for the R22HP and Alpha - the throttle being fully open is the cause of the "knee" in the curve and the nosediving performance.

If you take two points of equal DA on the flatter bit of the curve (use for example the OGE HP & Alpha chart, 8500' pa and -2 degrees C and 7500'pa and +8.3 degrees C) and calculate the Max weight you STILL come up with a discrepancy- despite the engine giving the same power output of 124BHP. So I don't think Nick's explanation holds.

The concept of derating is the same as a torque limit in a SE turbine. If you do the same for the Jetranger over its "Torque Limited" part of the OGE curve (constant engine power output despite different altitude/temp combinations) the performance obeys the DA calculations perfectly.

After a bit of research it seems the most likely explanation for the R22 anomalies is either-

1. The Manifold pressure charts don't accurately hold the engine power output constant or

2. Tip effects which spoil the rotor behaving according to theory. These tip effects are due to the airflow at the blade tips running into compressibility at high Mach numbers. This results in the usual density calculations no longer holding good. A quick dimensional analysis of the equations I could find predicting airfoil pressure from PA and Temp where compressibility has to be taken into account shows that while Height is raised to the power of 2.6 the temperature term is raised only to the power of 1.75. Thus changing height has a far greater effect on performance than changing temperature for the same Density altitude- this agrees with what we can tell from the charts. (I'm prepared to have my maths criticized by an aerodynamicist- but I think the basic problem of compressibilty still holds out).

It is interesting that the JetRanger charts don't show any compressibility effects (despite the 206 having a 11% higher rotor tip speed than the R22). This is either due to differing blade geometry or due to the test pilot technique of "referring the parameters" used in the 206 certification. This is when, having found a few points on the graph, the rest is predicted from theory. It may be that in the old days tip effects weren't understood or did not have to be taken into account when the performance charts were produced.

Hope this is understandable.

FoM

[[email protected]]

Or it could be a problem with assuming everything falls off as linearly as temperature in the ICAO model.
Pressure and density reduce at a greater rate at lower levels - compare the reduction from 0-5000' of relative density (2.76% per 1000') with the reduction between 5000-10000'(2.48% per 1000') whilst the temperature rigidly declines at 1.98 deg C throughout.
Therefore the effect on density of purely a change in temperature cannot be the same at all levels.
As Nick has said before on this thread - rules of thumb are not generally infallible.

[Irlandés]

You can see the R22 OGE chart (HP, Alpha and Beta) that Figure of Merit refers to here...

I'll have to think this one through...

[[email protected]]

Irlandes, I have come to the conclusion that the Robinson graphs are just crap! If DA is meant to be the yardstick by which aircraft performance is measured then constant DA must mean constant performance. I have looked through our Sea King Operating Data Manual and can find no such anomalies - maybe it is because the graphs all have PA, OAT and DA lines on them, whereas the R22 graph just has the 12600 DA line. This makes it easy to miss the very thing that you have spotted - ie that the performance claimed by the graph is at odds with all common sense and practice.
There may be, as FOM says, problems with MAP management and compressibility errors, especially the latter since Mach no is governed primarily by temperature and the increase in AoA required at altitude will affect the Mcrit of the blade.
But, as he also points out, the 206 has no such compensation - so is it us not understanding some crucial aerodynamic phenomenon or is it just a poorly constructed (whether by accident or design) graph?

[Nick Lappos]

Dear Figure of Merit,

Unlike then Pope, I don't even pretend to be infallable! Sometime it seems that way because I have seen the exact issue, measured it and experienced it, but in this case, I am as much a novice as anyone out there in cyberspace. This thread is another really good one, because from a simple question, real understanding can be pried out. Thanks to Irlandes, who provided the suspect chart, we can do some more examination. In every case, I implore Ppruners to challenge (the ground effect crowd had no such problem!) each of us, so we all learn more.

I certainly agree with you about my fallability, but (with some quibblingness) my original post was a speculation, based on what the limiting factors for rotor hover performance are, and a request for more data to seek the real answer. While the Robinson is probably derated up to the Knee of the WAT curve (the slope change at 5000 to 7000 feet), it is power limited above that. The two slopes of the curve reflect two of the several possible limiting factors:

1) Constant power section - The lower less steep slope from sea level up to about 5000 feet is the fall-off of performance of the rotor due to density altitude, while the power is a constant (this is the area where you are quite correct, the derated engine puts out constant power). The performance loss of the rotor is about 1 lb of lost hover weight per 100 feet of altitude increase (about 50 lbs per 6,000 feet, actually). This is purely the affect of the density being reduced, so the blade must be operated at an increased angle of attack, so the induced power is higher, and some performance is lost.

2) Reduced power section - The slope above the knee is the result of the performance loss due to engine power loss, PLUS the lost rotor efficiency due to density altitude. That slope is about 250 lbs per 5,000 feet, about 5 lbs lost per 100 feet of altitude gain. This is the sum of the 1 lb per 100 feet of the rotor efficiency and one would guess 4 lb per 100 feet due to other effects such as engine power decay and or loss of tail rotor thrust.

If this were not a family holiday in the States I would spend more time with the chart and try to deduce more, but that will have to wait, since a turkey waits for me!

[Figure Of Merit]

Ooops- Ignore my last post above.

I had got the bits of the curve the wrong way around.

The correct way around is-- the portion to the LEFT of the "knee" is where the throttle is fully open. The "steep" bit on the curve is the constant power portion.

Rechecking a few figures shows that where the performance of the machine is unlimited by engine power (to the RIGHT of the knee in the R22 HP and Alpha OGE curve) the weight is constant at constant DA

For example 1000 feet at +30 is equivalent to 3000 feet (ish) at ISA and 5000 feet at -10. All have a max weight of 1340 lbs.

God-- How embarrassing!!

Off to eat Turkey (american visitors staying) and humble pie

[[email protected]]

Irlandes, further to my last post I have been very boring and used your three examples of constant DA and checked using the gas constant, temperatures in Kelvin and accurate pressures from ISA and the Density in all 3 is within 0.0001 kg/m3. So there is no mysterious change in DA caused by altitude vs temp and I can only conclude again that the Robinson graphs are crap. DA is DA is DA so the answer to why you get 3 different AUM for 3 identical DAs can only be answered by Uncle Frank.

[Figure Of Merit]

Crab- I agree that DA is DA as far as the rotor is concerned (compressibility effects ignored) . But as the engine sees things, though, DA is not just DA.

On the bit of the graph that Irlandes was looking at the engine's throttle is fully open. The power output of the piston engine is not solely dependent on density altitude, but all kinds of thermodynamic boffin-equations. So for example- although 9000 feet and +10 degrees is the same DA as 10,000 feet at 0 (more or less) the engine is putting out less power in the latter case so the rotor can do less work.

You'll see the same thing with any helicopter once the engine starts to be the limit- in turbines it'll eventually be the TOT or N1 that limits you hot and high. If the engine can't put out the torque the rotor can't do the same work for the same density.

There's no mystery to it (despite my earlier, misguided attempts to muddy the waters) and we can't conclude crappiness at all!!

[Irlandés]

FOM,
I must admit to have being totally confused after your second-last post. Female intuition (although I am not female! ) seemed to tell me that something was not quite right in the state of Denmark although I couldn't quite put my finger on it. I think however, that thanks to Nick and yourself I've been rescued from the void of confusion and this graph suddenly seems perfectly logical. It seems that there were more things between heaven and earth than were dreamt of in my philosophy.

I walk away a slightly wiser man!

Thanks!
Irlandés

P.S. Any humble pie left? I'm kinda hungry.
P.P.S. OK, ok, sorry for overdosing on the Shakespeare.