turns in an auto. what to expect

[Pogue]

The last two lines of the SN-27 are the closest to what I'm talking about (in a roundabout way).

I love these questions, even if I never get them right... OK, if I recall correctly Robinson was looking into why practice autos would sometimes turn into real ones with an engine failure when the recovery was attempted. Turns out there was a carb icing problem even though the carb temp gauge indicated fine. (Sensor not in the right location.) They did some further research trying to find out why this wasn't happening to fixed wing. Turns our that it was, but because of the prop acting as a flywheel the only thing they noticed was a cough or miss when power was applied. Since there's very little flywheel effect in the Robinson installation they came up with the

[joker]

Pogue,

 

It's not a carb issue!

 

However, what you are saying about the carb icing is correct (and useful to someone I'm sure).

 

'Mixture' is along the right lines I suppose...getting warmer (scuse the pun!)

 

I don't know whether this 3750' limition exists with the CBi, as I don't have an RFM. I don't think there is any difference between the 300CB and 300CBi, with respect to what I am talking about. In other words, I think this limitation DOES exist for the 300CBi.

 

All the issues you have stated so far are true of ANY normally aspirated engine, R22 or 300CB. So they can't the the reason why one RFM has an altitude limitation and the other doesn't. Get your mind thinking to what is different between the two aircraft.

 

Right, its my bedtime now, so you'll have to wait until tomorrow...that'll also give me more time to think through my answer!

 

Joker

Edited April 17, 2007 by joker

[me shakes fist]

This is just a guess, but I'd say that the engine has a possibility of quiting from a throttle chop since I think that can cause a rich mixture for brief period of time. A higher DA means thinner air and more chance for that rich mixture to get to a point where it's too rich to keep the engine running, hence no throttle chops above 3750ft DA. As far as the Robbie, SN-27 says to avoid throttle chops completely so it would be redundant to have an altitude restriction as well (since the safety notice covers all conditions).

Edited April 17, 2007 by me shakes fist

[joker]

meshakesfist,

 

I like the way you're thinking, but its still not what I was looking for.

 

I think we need the help of some Schweizer pilots who have the benefit of know what is in and not in the Schweizer RFM.

 

Maybe someone could mention another time 3750' is mentioned in the Schweizer RFM.

 

In the meantime R22 pilots could have a look at the AD97-25-05, and see what their RFM 'Normal Procedures' say on that subject.

 

Joker

Edited April 17, 2007 by joker

[67november]

I don't know whether this 3750' limition exists with the CBi, as I don't have an RFM. I don't think there is any difference between the 300CB and 300CBi, with respect to what I am talking about. In other words, I think this limitation DOES exist for the 300CBi.

 

Right, its my bedtime now, so you'll have to wait until tomorrow...that'll also give me more time to think through my answer!

 

Joker

 

ok here from my RFM (training version)

Emergency Procedures

 

3-10. ENGINE IDLE AT ALTITUDE

 

Engine idle speeds at high density altitudes may be less than those at sea level conditions.

 

WARNING

 

AVIOD THROLLE CHOPS TO FULL IDLE ABOVE 3750 FEET DENSITY ALTITUDE, TO AVOID POSSIBILITY OF ENGINE STOPPAGE.

 

3-11 AIR RESTART

 

1. Establish 52 kts (60 mph) autorotation

 

2. pick out landing spot. if less than 2000 feet above terrrain, proceed with autorotation landing.

 

3. If altitude permits (helicopters with carburated engine - HO-360-CIA);

a. mixture - FULL RICH

b. Throttle - CRACK APPROXIMATELY 1/2 INCH

c. Starter - ENGAGE

 

4 If altitude permits (helicopters with fuel injected engine - HIO-360GIA)

a. mixture - IDLE CUTOFF

b. Throttle - CRACK APPROXIMATELY 1/2 INCH

c. Starter - ENGAGE

d. Mixture push to FULL RICH when engine fires

 

Note: If fuel pump was on at the time of engine stoppage, a flooded condition may have resulted necessitating additional use of starter.

 

 

from this info I'm inclined to think that BOTH the CB and CBi fall into that no chop catagory.

[Goldy]

Why this limitation? Why not the same in the R22?

 

 

No one has answered this question yet.

 

I would, but since I am 269/300/300cbi challenged,( read..dumb...inexperienced in this aircraft, etc) I have absolutely no clue where joker is headed with this....but i might just take a look at his hints..

 

Goldy

[Pogue]

In the meantime R22 pilots could have a look at the AD97-25-05, and see what their RFM 'Normal Procedures' say on that subject.

 

Joker

Well, you've got me good and confused... The AD is about leaning in flight, which the Robinson POH says not to... "CAUTION Inflight leaning with engine mixture control is not recommended. Engine stoppage may result as there is no propeller to keep the engine turning should overleaning occur." It looks like the MA-4-5 carburator & carb temp gauge required inflight leaning and were changed out as a result of the AD. I feel like I'm being led by the nose here since I haven't managed to get any 300 time yet, but is sounds like inflight leaning is done on the 300? If so is the 3750' DA the point at which a throttle chop leans it to shutdown?

[joker]

OK, where am I going with this?!

 

Well, I must admit, I am struggling to work that out myself! I have been wrestling with this all night and all day, scouring the internet to find information and evidence. I still am not happy with everything, but here is a the general idea.

 

It comes down to the new carburettor that was fitted to the R22 0-360-j2a engines. This carburettor is basically set to meter fuel for all altitudes when the 'mixture' knob at full-rich. OK, some power is lost when at altitude, but its a derated engine anyway. The main thing is it eliminates the need for inflight leaning which was identified as the cause of a number of accidents. Also I believe the idle circuit is improved to ensure that at all operating altitudes the engine will idle when the throttle is closed. (It is the exact description of this new carb that I have struggled to find.)

 

Thus, the R22 asks for the mixture knob to be at 'Full Rich' during all phases of flight. It does not recommend 'in flight' leaning.

 

The Schwiezer, on the other hand does not have this type of carburettor. Thus, when at high density altitudes (apparantly 3750' DA) the mixture must be leaned to ensure the power is per the performance charts. This is why you have charts for 'leaned' and 'full rich' in the S300CB RFM. This is also why you have a 'leaning procedure' in the Normal procedures section.

 

Note: In flight leaning is not recommended in the S300 either.

 

 

Inflight Leaning: A Side Warning

 

The problem with inflight leaning is that a pilot can inadvertantly 'over' lean the mixture thus causing the engine to be starved of fuel, and a subsequent stoppage. In fixed wing, this is not a major issue, as the propellor (acting as a flywheel) will restart the engine once the mixture control is set back to a good mix. In a helicopter due to the freewheeling clutch (need to allow for autorotations) if the engine stops, there is no 'instant' restart.

 

So the next issue is this.

 

High Density Altitude and Effect on Mixture:

 

As you ascend, the lower density of the air has the effect of enriching the mixture. To compensate, pilots often 'lean' the mixture (reduce the fuel) to compensate for the reduced air density. This is done usually by adjusting a needle valve from the cockpit, forming a crude method of regulating fuel flow.

 

Now,... the suction of fuel is caused by the weight of the air passing through the venturi. At high DA what have you got? Light air. That reduces fuel flow. If you have leaned then you mechanically reduce fuel flow. So the problem now is that if you fully close the throttle you could actually reduce the fuel flow even more and possibly sufficiently that there is not enough to fire the pistons (engine stoppage).

 

Carburettor Simulator

You can simulate this by going to the carburettor simulator in the link above. Set the mixture setting to lean (as if you had ascended and leaned). Then close the throttle! With the right conditions, you can get the fuel to zero flow.

 

Hence the reason why Schweizer say not to close the throttle full at high DA. The density is too low for fully closed throttle. R22 doesn't need this limitation due the fact that you are full rich. (Again, use the simulator to see this.)

 

Well, that's my best answer at the moment. As I said, I am still struggling to find a good description of the R22 carb and how it differs to the 300CB. Also, now I think about it, the 300CBi should not have this problem as it is an injected engine.

 

Maybe some others can chip in, and we can ge to the bottom of this.

 

My first thought was that this was a correlator / govenor issue (as in the attached leaflet), so I'm sorry if I misled you there.hs_may_jun92_1_.pdf

However, I rejected that because that didn't explain the difference in the two aircraft (R22 has a correlator too). That's why I was a little slow to think it through properly.

 

Lastly, this leaflet has a pretty good explanation on the carburettor. It has a good explanation of the 'idle circuit' which can be seen in the simulator. Accessory_AMT_1_.pdf

 

Joker

Edited April 18, 2007 by joker

[Pogue]

So does the 300 C use the MA4-5 carburator?

[joker]

Dunno! That too is what I'm still trying to find out.

 

But it's not so much about the differences in carbs. Its more about the fact that the Robbie is flown at 'Full Rich'. So no chance of stoppage during throttle closure.

 

The point about the change in carburttor was that until the newer ones were used in Robbies, leaning was a normal procedure. Now its 'not recommended'.

 

Joker

[Pogue]

Dunno! That too is what I'm still trying to find out.

 

But it's not so much about the differences in carbs. Its more about the fact that the Robbie is flown at 'Full Rich'. So no chance of stoppage during throttle closure.

 

The point about the change in carburttor was that until the newer ones were used in Robbies, leaning was a normal procedure. Now its 'not recommended'.

 

Joker

Hmmm, I have an old R22 POH circa 1988 (don't ask) and it has almost word for word the same caution. Flying the old Beta (pre governor with the O-320) you always ran full rich. Looking at the AD you referenced apparently the Beta 2's with O-360's with the MA4-5 carbs did have a procedure for inflight leaning. It appears this applied to only 50 Beta 2/Mariners and with the change to the MA4SPA carbs they went back to running full rich all the time. The stated concern with the AD is inadvertently shutting off the mixture in flight. I can't find any mention of leaning for performance in the R22 POH at all, although the Rotorcraft Flying Handbook talks about leaning on the ground at higher density altitudes. So does the 300C have you lean the engine on the ground? I don't see how that would cause a problem with a throttle chop at 3750 - it seems like the more altitude you have the richer the mixture is going to be unless you're messing with it in flight.

[Goldy]

Personally, I still don't like the fact that technology has had fuel injected engines for DECADES, that we know many accidents are attributed to pilot error due to carb icing, that the temp sensor is in the WRONG place in the 22 to give you an accurate reading...and we don't fix it !!

 

I know Frank doesnt want to do any major upgrades to the R22....the focus was on the Raven II and now the R66...but come on...either fix these issues with a new model or stop selling the 22 altogether!

 

 

Sorry for the ranting...pet peeve of mine..let's get a fuel injected R22 on the market already !

 

Goldy

[joker]

So does the 300C have you lean the engine on the ground? I don't see how that would cause a problem with a throttle chop at 3750 - it seems like the more altitude you have the richer the mixture is going to be unless you're messing with it in flight.

 

Pogue,

 

Yes, the 300CB has leaning procedures and recommends leaning above 3750' on the ground. It has performance charts based on a leaned mixture and a full-rich mixture.

 

You are right that altitude will richen your mixture. If you have taken off full-rich, and climbed to 4000' DA, then you shoudl be still at full rich. The chance of engine stoppage is lower.

 

However there are two things at play here. First due to the less dense air the, although you have a richer mixture, less of that mixture reaches the cylinders...i.e there is less fuel being drawn into the combustion chambers. Second, if you have (as per RFM) leaned the mixture above 3750' this further reduces the fuel quantity that reaches the cylinders. A throttle chop now further reduces the fuel flow, by reducing the MP (flow of air). This combination is deadly.

 

It is this scenario that I presume they are protecting against, by prohibiting throttle chops above 3750' DA.

 

Joker

Edited April 19, 2007 by joker

[Guest pokey]

YIKES ! ! More mixture questions?

 

My 269B is fuel injected ( bendix rsa-5,,i believe) and has an automatic mixture contol---i dont ever recall seeing anything in the flight manual about power chops at altitude.

 

the C models have a bit different F.I. set up ( bendix rsa-7,, i believe) dont remember anything in it's RFM either.

 

A carbureted 269A ( bendix pressure carb) model that we fly quite a bit does have a cruise leaning procedure,,, nothing about throttle chops tho.

 

 

As for the 22 & 44? i guess they have your std. float carburetor.

 

We used to pull the mixture to cut-off position in the C150 at altitude, dont remember the exact numbers, but? the prop would windmill until you slowed it up quite a bit. When we finally could get the prop to stop in flight? took a pretty steep/fast dive to get it to windmill on its own w/ out help from the starter. Cant duplicate this in helicopters tho, because of the overrunning clutch