R/W Stage-1 Check Ride Prep: Limit Manifold Pressure

CARoss

CARoss

Filing Flight Plan
Joined
Sep 24, 2007
Messages
22
Location
Portland, OR
Display Name
Display name:
Chris
Hey all...

My buddy, RotaryWingBob, suggested that I sign up here at PoA - I'm in the starting phases of my VFR141 R/W Private Pilot Cert. Really enjoying it so far.

Now it's time for me to take advantage of the collective brain power out there... and I have a question that I'm hoping the R/W crew can help with.

My CFI and I are running through some test questions and, while working one, I came across something I really don't understand. She's really tricky, so this is a rather complex question, so forgive the run up:

Givens:
Altitude in question: 5066ft MSL
Temp @ SL: 15º C
Pressure @ SL: 28.92"
Helicopter: Robinson R22 Beta
Weight: Max Gross Weight of R22 Beta

Questions:
1) Determine the Pressure Altitude (PA).
2) Determine the Density Altitude (DA)
3) Can we IGE hover at altitude in question?
4) Can we OGE hover at altitude in question?
5) What is our Vne?
6) What is our Max Takeoff Pressure (MTP)?
7) What is our Max Continuous Pressure (MCP)?
8) Would I go here to land?

Answers:
1) PA?:
29.92 - 28.82 = 1.00
1.00 * 1000 = 1000
5066ft MSL + 1000 = 6066ft PA

2) DA?:
Referencing the temperature lapse rate of -2º / 1000ft, the known SL temperature of 15ºC and the known altitude of our potential landing spot of 5066ft MSL, we know the temperature at the landing spot should be 5ºC.

Using the R22 POH 5-3 with 6066ft PA and 5ºC we can interpolate:
DA = 6066ft DA
(Which makes sense since DA is PA corrected for non-standard temp and altitude.)

3) IGE?
Referencing POH 5-5, yes. Our IGE hover ceiling @ 1370lbs is ~6750ft.

4) OGE??
Referencing POH 5-9, no. Our OGE hover ceiling @ 1370lbs is ~5250ft.

5) Vne?
Referencing POH 2-1, we know that 102KIAS is not applicable since we are above 3000ft DA, we are at 6066ft, so need to reference POH 2-11 instead.
94KIAS

6) MTP?
Referencing POH 2-10. I locate 6066ft PA, go up to 5ºC and hit the "Full Throttle" line and stop at 22.5.

7) MCP?
Now, here is where I get confused... for MCP, I'm supposed to subtract 1 inch from MTP... so 21.5. But, that's wrong.
In this case, I'm told to do a normal reading for MTP, ignoring the "Full Throttle" line for MTP and get 23. Then, subtract the 1" to get 22... which is the correct answer.

Problem I'm having is that I don't understand why I re-read and get 23 rather than using the reading I just calculated of 22.5 for MTP and result in a MCP of 21.5.

Can someone explain the logic behind this? It is not that I can't memorize the rule, it is just that I have this need to know why... it just helps me remember.

8) Would I go there?
No, just not safe enough of a buffer zone.

So, sorry for the long winded question... it is just that I need to make sure I understand all the calculated data first before I get to that last question.

ANY thoughts?

Thanks,

-Chris
 
Chris I think there are at least two folks who can help answer that. Bob and Laurie both are fling wingers. Welcome to the board.
 
CARoss said:
7) MCP?
Now, here is where I get confused... for MCP, I'm supposed to subtract 1 inch from MTP... so 21.5. But, that's wrong.
In this case, I'm told to do a normal reading for MTP, ignoring the "Full Throttle" line for MTP and get 23. Then, subtract the 1" to get 22... which is the correct answer.

Problem I'm having is that I don't understand why I re-read and get 23 rather than using the reading I just calculated of 22.5 for MTP and result in a MCP of 21.5.

Can someone explain the logic behind this? It is not that I can't memorize the rule, it is just that I have this need to know why... it just helps me remember.

8) Would I go there?
No, just not safe enough of a buffer zone.

So, sorry for the long winded question... it is just that I need to make sure I understand all the calculated data first before I get to that last question.

ANY thoughts?

Thanks,

-Chris
Click to expand...


I don't have a Robby POH and I'm not a RW pilot, but I think I understand what's going on here. Typically the reason that there's a limit on continuous HP is that the engine isn't designed for that much stress for more than a short period. It sounds like you would normally limit the MCP to 1 inHg less than MTP but since the engine's output at high DA is limited to less than the real MCP, there's no need to subtract the inch.

The "Full Throttle" line represents a physical limitation, because the air is thinner, you simply cannot get the engine's MP higher than that line. But it appears that you still are supposed to find the theoretical MTP (which can't actually be reached as indicated by it being above the "full throttle line") and subtract 1" for the continuous limit. And at even higher DA's you won't be able to even reach the MCP limit.
 
lancefisher said:
The "Full Throttle" line represents a physical limitation, because the air is thinner, you simply cannot get the engine's MP higher than that line. But it appears that you still are supposed to find the theoretical MTP (which can't actually be reached as indicated by it being above the "full throttle line") and subtract 1" for the continuous limit. And at even higher DA's you won't be able to even reach the MCP limit.
Click to expand...

Thanks for the input... this is interesting.

Hmm my understanding, and believe me - I may be totally off here, of the "full throttle line" was that it meant the throttle had been twisted all the way till it hit the stop. While the engine could take more... the "pedal was all the way to the floor."
 
CARoss said:
Thanks for the input... this is interesting.

Hmm my understanding, and believe me - I may be totally off here, of the "full throttle line" was that it meant the throttle had been twisted all the way till it hit the stop. While the engine could take more... the "pedal was all the way to the floor."
Click to expand...

Well the engine could "take more" if there was more to take, but "full throttle" simply means that the throttle butterfly is aligned with the airflow through the carb or throttle body and more importantly, the manifold pressure is as high as it can be for the RPM (RPM has a small effect on MP, for a given throttle position, the higher the RPM the lower the MP).
 
CARoss said:
6) MTP?
Referencing POH 2-10. I locate 6066ft PA, go up to 5ºC and hit the "Full Throttle" line and stop at 22.5.

7) MCP?
Now, here is where I get confused... for MCP, I'm supposed to subtract 1 inch from MTP... so 21.5. But, that's wrong.
In this case, I'm told to do a normal reading for MTP, ignoring the "Full Throttle" line for MTP and get 23. Then, subtract the 1" to get 22... which is the correct answer.

Problem I'm having is that I don't understand why I re-read and get 23 rather than using the reading I just calculated of 22.5 for MTP and result in a MCP of 21.5.

Can someone explain the logic behind this? It is not that I can't memorize the rule, it is just that I have this need to know why... it just helps me remember.
Click to expand...

OK Chris, I see the dilemma...

I believe the logic goes like this. The full throttle line isn't truly a limitation -- it simply says that you run out of throttle when you hit it. I think the way they're teaching the use of R22 Beta chart on 2-10 is incorrect. I don't agree that the answer for problem 6 is correct. The answer should be 23" MP. The full throttle line simply says that you'll never be able to reach more than 22.5" MP.

Here's what this is all about. Robinson uses MP has a proxy for power output. The engines on newer ships are all derated. For instance, the O360-J2A engine in a Beta II is fully capable of producing 180 HP without damage to the engine. However the drive and rotor systems cannot accept that much torque without long-term damage. Robinson limits MP to avoid damage to these systems (thus derating the Beta II and the Beta engine to 131 HP continuous for T/O).

The derating serves several purposes. As I said before, it protects the drive and rotor systems. It also babies the engine which is how Robinson gets a 2200 hour TBO. Third, it provides reserve power at high DA. Finally (and I'm repeating what Robinson says in the Pilot Safety course) it allows operation at high DA without requiring leaning of the mixture.

My guess is that most pilots wish that Robinson would rip the MP gauge out and replace it with a torque meter like turbine ships have. That would read % power and would provide an absolute and easy limitation to use. They'll have to go this way in the R66 because MP doesn't have any meaning with a turbine, but I'd sure like to it migrate down to the piston ships as well.

Hope that helps!
 
lancefisher said:
Well the engine could "take more" if there was more to take, but "full throttle" simply means that the throttle butterfly is aligned with the airflow through the carb or throttle body and more importantly, the manifold pressure is as high as it can be for the RPM (RPM has a small effect on MP, for a given throttle position, the higher the RPM the lower the MP).
Click to expand...

It's a little different in most helicopters Lance, because, unlike a constant speed prop, RPM is fixed by the governor. Full throttle means what you said, but it's a dicey area -- if the pilot attempts to pull more pitch beyond that point, the additional drag will cause RPM to decay. Normal operating RPM in Robbies is a very tight range -- 101-104% in the R22s Chris flies, and even tighter in an R44 - 101-102%. And the only time you should be out of that range is either in an autorotation or at ground idle.
 
RotaryWingBob said:
It's a little different in most helicopters Lance, because, unlike a constant speed prop, RPM is fixed by the governor. Full throttle means what you said, but it's a dicey area -- if the pilot attempts to pull more pitch beyond that point, the additional drag will cause RPM to decay. Normal operating RPM in Robbies is a very tight range -- 101-104% in the R22s Chris flies, and even tighter in an R44 - 101-102%. And the only time you should be out of that range is either in an autorotation or at ground idle.
Click to expand...

Bob,

how difficult is it to maintain those limits? does the governor take care of it under most normal flight circumstances? it seems silly that you cant have the RPM below 100% but I digress. Do the ranges of available RPMs typically get wider with larger helicopters? How wide of a range is that 101-104 or 101-102, is it very sensitive or can you change the actual RPM a fair amount?
 
CARoss said:
Just an update, with all the studying over the past few weeks, and the great answer from this crew... looks like I'm on target for my "Autorotation Lesson 1" next week.
Click to expand...

ah falling with style. :D My friend who is training the R-22 said that he has something like 1.5 seconds to respond to an engine failure before the disc loses the momentum required to auto? crazy! once he gets his license and his 10,000 hrs required by SFAR whatever to take passengers i want to see it though
 
From my basic / early understanding the R22 is the Volkswagen of the helicopter world. (Relatively) inexpensive, light and reasonably easy to maintain. With that, obviously, comes some negatives... one of which is the low mass main rotor. The low mass lets you do some interesting things with engine power, etc, but also means that the main rotor slows down very quickly in an emergency. I've also been told that this is one of the bigger reasons there is SFAR specifically for Robinsons... they are common and tricky if you don't know the nature of the beast.

I'm told this is why many choose to learn on the R22. If you can learn to fly a Robbie... you should be able to fly anything. Kind of like learning to drive . You learn to drive stick on your buddy's 1983 Volkswagen Diesel Rabbit. Once you master that... the BMW M6 is quick jump. :D

tonycondon said:
ah falling with style. My friend who is training the R-22 said that he has something like 1.5 seconds to respond to an engine failure before the disc loses the momentum required to auto? crazy! once he gets his license and his 10,000 hrs required by SFAR whatever to take passengers i want to see it though
Click to expand...
 
tonycondon said:
Bob,

how difficult is it to maintain those limits? does the governor take care of it under most normal flight circumstances? it seems silly that you cant have the RPM below 100% but I digress. Do the ranges of available RPMs typically get wider with larger helicopters? How wide of a range is that 101-104 or 101-102, is it very sensitive or can you change the actual RPM a fair amount?
Click to expand...
Tony, with a properly functioning governor, the RPM should never get out of that very narrow green zone. The exception is at high DA when the governor fully opens the throttle but the ship needs more torque to maintain RPM. Most helicopters (at least the ones I've flow) have an allowable RPM range that is maybe 5% or less.

In Robinsons there are actually two backup systems, which are actually the way things used to be before governors.

At the end of the collective there is a twist grip throttle. Twist it away from you, you get more power (the opposite of a motorcycle!). The throttle is normally operated by a servo controlled by the governor, and you can actually feel it make adjustments in flight when it makes minute twists (another reason that a death grip on the controls is a bad idea).

Now normally, the pilot flips on the governor, and lets it take the throttle up to give you flight RPM. In the absence of a malfunction, the pilot doesn't tweak the throttle until he's back on the ground and wants to reduce to idle RPM -- flip off the governor, twist the throttle towards you.

The throttle is also controlled by a correlator which twists the throttle away from you when you raise collective, and towards you when you lower it. The governor normally overrides the correlator. If the governor fails, the drill is to switch it off and then let the correlator do its thing. So long as the pilot doesn't make any abrupt control movements, the correlator does a remarkably good job of maintaining RPM, though not as good a job as the governor does.

If all else fails, the pilot can directly control RPM with the throttle and override the correlator.

RPM becomes a major issue below 100%. In recurrent training, we practice flying a pattern or two with the governor off and RPM at 90%. The lower it goes short of 90%, particularly at high DA, is when thing become unglued. Let it get low enough and the rotor system goes into a low RPM rotor blade stall. That is an unrecoverable situation, and you are going to drop like a rock.
 
tonycondon said:
ah falling with style. :D My friend who is training the R-22 said that he has something like 1.5 seconds to respond to an engine failure before the disc loses the momentum required to auto? crazy! once he gets his license and his 10,000 hrs required by SFAR whatever to take passengers i want to see it though
Click to expand...
Actually, practice autos are kind of fun, Tony.

The factory puts out the 1.5 second number, but there's been lots of discussion about it. Many pilots believe that you've got 5 or more seconds to floor the collective before the RPM gets so low you can't recover. If you have airspeed, it can always be traded for RPM in an auto -- pull back on the stick or make a turn and the RPM will go up.
 
Not to answer Bob's question, but... I'm going to see if I know the answer here. I'm all about practice these days.

The governor takes full control at 80% and does a pretty good job maintaining RPM. I've been warned that it is so good that carb ice is very hard to detect. As the engine loses power from carb ice, the governor tries to fix things on its own. Then, all of a sudden, if you are not paying attention to your carb heat gauge, you are in a heap of trouble.

tonycondon said:
Bob,

how difficult is it to maintain those limits? does the governor take care of it under most normal flight circumstances? it seems silly that you cant have the RPM below 100% but I digress. Do the ranges of available RPMs typically get wider with larger helicopters? How wide of a range is that 101-104 or 101-102, is it very sensitive or can you change the actual RPM a fair amount?
Click to expand...
 
Rotorheads,

Thanks for the great insights! Excellent point chris about the carb ice. I suppose the only indication you would have of that would be feeling the governor increase throttle but not seeing any change in the RPM indications? I knew about the correlator device, pretty clever stuff.
 
CARoss said:
Not to answer Bob's question, but... I'm going to see if I know the answer here. I'm all about practice these days.

The governor takes full control at 80% and does a pretty good job maintaining RPM. I've been warned that it is so good that carb ice is very hard to detect. As the engine loses power from carb ice, the governor tries to fix things on its own. Then, all of a sudden, if you are not paying attention to your carb heat gauge, you are in a heap of trouble.
Click to expand...

More correctly, the carburetor air temperature gauge. But your point is true. Not only will the governor possibly mask a carb ice problem, but may mask other power plant related problems as well.

The 80% is a Robinsonism (if that's a real word) and is arbitrary. Robinson changed the check list a few years ago and now tells you to switch on the governor during the startup sequence.

Everybody I know including the DPE who did my initial rating and some of my R44 transition thinks that's a really bad idea. The way I was taught (and that I used on my PP-RH checkride) is to complete the mag check, carb heat check (if not a fuel injected engine), and sprag clutch check, then while still at warmup RPM, switch the governor on and roll the throttle up to 80% and let the governor start to take over. I then hold the throttle back and let the governor take the RPM up gently, and if it stops where it should, all is happy. The reason is to avoid an overspeed if you have a runaway governor.
 
ok now i have another question. the original post was about manifold pressure limits. is this also indirectly controlled by the governor? it seems that through the discussion that throttle is used to control rotor RPM? seems backwards of a constant speed prop where throttle sets manifold pressure and prop control (collective-ish?) sets RPM. Please un-confuse me.
 
tonycondon said:
ok now i have another question. the original post was about manifold pressure limits. is this also indirectly controlled by the governor? it seems that through the discussion that throttle is used to control rotor RPM? seems backwards of a constant speed prop where throttle sets manifold pressure and prop control (collective-ish?) sets RPM. Please un-confuse me.
Click to expand...

You've got it right, Tony!

With a constant speed prop, the pilot manipulates engine RPM with the prop control, and power with the throttle, and the job of the governor is to maintain RPM by varying prop pitch.

In a helicopter, the pilot manipulates pitch with the collective, and the governor's job is to maintain constant RPM by tweaking the throttle thru a servo.

So it is kind of backwards. The pilot in a fixed-wing doesn't control prop pitch.

The pilot of a helicopter doesn't control RPM (if there is a governor, anyway), except in an autorotation (when the governor doesn't help).
 
so lets say you are flying in a non governed situation (oh that sounds exciting!), which i assume you practice, do you use throttle or collective to control RPM?
 
tonycondon said:
so lets say you are flying in a non governed situation (oh that sounds exciting!), which i assume you practice, do you use throttle or collective to control RPM?
Click to expand...

If the governor bites the dust (rare but does happen) then the pilot uses the collective as he normally would. There is a throttle correlator which adds throttle when he raises the collective, and backs it off when he lowers it. Most of the time, unless he does abrupt maneuvering, the correlator will do a pretty good job of maintaining RPM, but the pilot can tweak the throttle if the RPM starts to get too low or high.

An autorotation, however, is a whole new ballgame! Basically the drill is to keep the helicopter straight with the antitorque pedals, floor the collective, and pull back on the stick a bit because the nose will want to drop. If the ship is heavy, the pilot will need to bring in a little collective to avoid overspeeding the rotor system (which is freewheeling and unconnected from the engine). What you're looking for in a Robbie (and this varies from helicopter to helicopter) is 65-70 KIAS and RPM maybe a tad under 100%. You play with the collective and cyclic to get these numbers as a steady state up until it's time to flare.
 
great insights bob, thanks again.

now i just need to find about 10 grand laying around and i can put all this knowledge to the test :D:D
 
tonycondon said:
great insights bob, thanks again.

now i just need to find about 10 grand laying around and i can put all this knowledge to the test :D:D
Click to expand...

My pleasure Tony! There's a huge knowledge gap between fixed-wingers and us lobotomized flingwingers (it was Laurie who suggested we are lobotomized... she may be right!). But there's also a huge gap between powered pilots and glider pilots too ...
 
tonycondon said:
so lets say you are flying in a non governed situation (oh that sounds exciting!), which i assume you practice, do you use throttle or collective to control RPM?
Click to expand...

I did most of my training in a Hughes 296 which doesn't have a governor. The newer models produced by Schweizer have a low rotor RPM wrning horn.

While Bob is right (as usual), in the H296 we are tough to use either or both as the situation calls for. If you have high power and low RPM lower the collective and reduce drag (and lift), low power low RPM add power, high power high RPM reduce power or raise collective or both...

As Bob says normally the correlator does a real good job and the governor (automatic or pilot twisting throttle) only makes minor corrections.

Joe
 
Areeda said:
As Bob says normally the correlator does a real good job and the governor (automatic or pilot twisting throttle) only makes minor corrections.

Joe
Click to expand...

Joe, somewhere in the back of that swamp I call a mind, is the recollection that a number of early helicopters didn't have correlators. I even seem to recall, but can't confirm, that the very early Bell 47's didn't have a correlator either. Now, that's got to increase the pilot's work load!
 
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