manifold pressure in R22
 

Hovering IGE a beta 2 with me (160 lbs)and little fuel (about 5 US gals) noticed a rather high MAP (23 ish) -refuelled to about 26 US gals and hovering MAP was about 21. Does this make any sense(change of C of G ???) or is the gauge (or pilot) a bit suspect?

carb ice perhaps?

hands_on123,

Please could you explain how having carb heat applied increases demand for power? Thanks.

Its late and I m jet-lagged so may be talking b'loks but won't carb heat reduce the power available due to the difference in air density compared with unheated outside air?

PW

I think what hands_on123 is meaning, is that if you are in a hover for instance, and you apply carb heat, there is a reduction in power, and helo will sink, but if you apply more power to hold height, you will be hovering at a higher power setting to maintain hover height. Please correct me if im wrong, im tired and grumpy, having had 3 weeks of !!!!! weather I havnt flown much.
Also, cholmondeley maybe at the first flight there was not much wind, and then maybe the wind picked up so you benefited from translational lift, so you got an increase in lift for no increase in power, causing helo to climb, so you needed to reduce the power setting to hold height.
Preparing to duck. :D
Hughesy

I think the overall effect of carb heat in the hover is to reduce MAP for a given throttle position. I don't see how it changes the power "demand". The power required is surely exactly the same.

I agree that the wind effect (or recirculation effect) is probably the most likely answer to the original question.

almost there
 

well, yes, carb heat reduces power developed--for a given throttle/collective position.

But it's not that you then have to pull more power, for the airframe itself doesn't see the carb heat, it just needs the same amount of power to beat the air into submission as it did in the first place.

So you have to increase the THROTTLE ("pull power") in order to open it farther and let more, but thinner, heated, combustion air into the engine, in order to produce the SAME power as pre-carb heat.

Throttle open farther will reduce suction in intake manifold, so the MP will show a higher number. Lots higher, most of us "discover" we forgot the carb heat was on when we flinch at the outrageously red-lined MP.

Now, ONWARD! To the methodologies of splitting hairs with only an Exacto knife and a hand lens . . .

Why would anyone want to apply carb heat in the hover ? You only need carb heat below 18".
That's why people get a rough engine during warm up - carb icing occurs even before they think of applying some power for a lift. Happens all the time - student sits on the ground doing slow checks, goes to lift - rough engine.

Robinson Safety Notice 25 says

So "keep out of yellow arc" (and current advice appears to be a more conservative "keep between 10-15 DegC"), unless below 18 inches MAP in which case ignore DegC and apply full carb heat.

RC

104% of what?
 

Hi there - I suppose I should know the answer but I don't. Can anyone give me a simple explanation...

I fly the R22 - normal operating ERPM and RRPM is top of the green at 104% - ok, fair enough, but 104% of what? Where does this come from?

Thanks, in anticipation of any replies.

A very simplistic explanation is "100% of the design RPM".

Most likely, the original A model ran at 100%, and with time and experience, it was found that 104% gave better results with an acceptable increase in wear and tear.

In the S76B, we run at 107% - you may ask why the gauge doesn't get recalibrated to read 100%? Forked if I know, maybe too expensive to have different gauges for different machines.

I have always considered that RRPM in percent is easier to cope with than the raw numbers.

the figures are percentages of 2550rpm (engine) and 510rpm (rotors).

2550rpm is actually 100% rpm. the engine is derated to 131bhp (O-360) at 2652rpm, which is 104% of 2550 :)

hope this helps.

Jim

Carb heat = reduced air density = less oxygen per mass of air = less fuel burned per mass of fuel = less HP.

Less HP = govornor opens throttle butterfly to maintain RRPM = higher MP for same blade pitch (same HPR).

However, in this case higher MP does not = more HP so manifold pressure limits do not apply, as MP limits are designed to keep max continuous engine output <= 124 hp (or <= 131 HP 5-minute takeoff).

Super simple, huh.

While we are on this subject, I know that hovering downwind needs more power ie higher MAP. But why? I ought to know this and probably did once, but I even consulted Shawn's book this week and could not find the answer there either.

Why does the rotor disk care which way the wind is entering it? Because the TR is in the way of the airflow?

Muffin: because hovering downwind is actually flying backwards. 10 kt wind from behind = 10 kt flying backwards to maintain your position relative to the ground. And flying backwards isn't the most aerodynamic thing....

To the rotor disc, it does not matter whether it is 10kts from in front/side/rear - However, like it or not, hovering downwind your feet will be working like mad to keep you straight and wasting all that excess power.

TeeS

I heard that re-certification of the gauges to show 100% was too costly.

Besides, if the needles are in the green they're fine, so we can spend more time worring about the carb heat / MP comparison, where the wind is . . .

Well, at least you realised that there was a question! A little while before my FI test, I asked a couple of f/w PPL friends if I could practise doing a briefing on them. It was all going well, until one asked, just as you did: "104% of what?" It stopped me in my tracks at the time, as I just couldn't think of an answer that made sense and was easy to understand.

"large translational lift effect that occurs at that speed?"

When you wake up and smell the salts, you might want to consider whether there IS a large translational lift at 10 kts.......and you could also dig out the original R22 and R22 Beta POH and take a look at the section on carb heat.

I'm only pedantic when I'm sober, but perhaps you could explain why you think an R22 going backwards at any speed is better than or equal to the aerodynamic efficiency of one going forwards ?

If you can, then copyright your answer immediately before an engineer makes use of this new law.

(And to the original poster who no doubt just wanted a simple answer........welcome to pprune.) :}

When we think "hovering downwind", what we are really doing is hovering in a reduced/zero airspeed state (compared to hovering upwind).

Less airspeed means higher AOA for the same lift, so more drag, more HPR, which equals more torque which equals more T/R thrust which also takes more power.

A helicopter hovering (at zero groundspeed) with a 8-KT direct tailwind may well take less power to hover then the same situation and direct headwind. This because the M/R vortex will be blown away from the T/R. Of course, if the pilot is making significant pedal inputs to hold heading, any such gains will likely be negated.