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Old 15th Apr 2005, 16:58   #1 (permalink)
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Question Manifold pressure dropping during manouvers

This has puzzled me for a while now. On the 44 I fly the Man pressure drops by a couple of inches in steep turns. This without adjusting power at all. Rolling out of the turn the pressure comes right back up to the pre-turn level.

Am I missing something basic here??
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Old 15th Apr 2005, 17:19   #2 (permalink)
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The load factor increases in the turn which tends to push up RPM so governor shuts throttle, lowering MP to maintain RPM. Reverse happens as you roll out of the turn.

Try it in an Enstrom and you have to do it manually to control the RPM.
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Old 15th Apr 2005, 18:15   #3 (permalink)
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First time I tried turns (and not very steep turns) in a Robbo I was too busy to look at the gauge, but I could feel the governor moving the throttle.
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Old 16th Apr 2005, 11:27   #4 (permalink)
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I hope "a while" isn't too long in terms of being puzzled...

Yes, you are missing something basic and important. I strongly suggest you get back the books out and learn why this magic happens.

If you are already licenced, shame on your tutors, your testing officer and/or your self discipline.

I don't think you should be flying with such a poor understanding of helicopter flight and its influences. How do you think you might effectively manage RRPM after an engine failure, for example, without a good understanding of the aerodynamics and physics involved?

If you just learn by memory rather than understanding, sooner or later you will forget it.

Last edited by the coyote; 16th Apr 2005 at 11:39.
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Old 16th Apr 2005, 11:45   #5 (permalink)

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2 basic facts of life:

1. Aerodynamic loading effects in turns in a helicopter. This one's already been explained.

2. What's not so easy to understand is what happens when someone asks a simple question on PPRuNe i.e. almost every time, someone with a holier-than-thou attitude jumps out to attack the poor bloke. The reason for this are more complex.

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Old 16th Apr 2005, 12:59   #6 (permalink)
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H*ly Cr*p....
someone got out of bed with the wrong foot first.

I will not sink down to your level by even commenting on your answer coyote.

To the more grown up people here...thanks. That was what I suspected all along.
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Old 16th Apr 2005, 16:37   #7 (permalink)
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good question

I dont think the books cover this topic much. As a new Enstrom student, I find the question is relavant to my learning. Your post has inspired me to check and see what Shawn Coyle has to say in his book.....
Just found in Shawns book Cyclic and Collective "load factor plays an important effect on Nr".

Interesting. So I bet the rpm and power change only occurs during the roll into the turn and the roll out. A steady turn would require more power I think.

I have heard that a cessna 150 can only do about 20 circles before running short on power to maintain altitude. Have not tried it.
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Old 16th Apr 2005, 17:00   #8 (permalink)
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The MP changes itself because you are no longer trimmed when you enter the turn. To trim, you must reset all controls to achieve a steady-state capability to hold the maneuver until the cows come home.

If you trimmed in the turn, you would find that the mp has to rise in the turn, because the rotor is carrying more weight (load factor). The mp doesn't rise by itself, you will raise it when you lift the collective, right after the vertical speed ticks downward.

The initial mp drop is because the load factor makes a slight coning increase, which raises the rotor rpm and drops the mp.
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Old 16th Apr 2005, 17:07   #9 (permalink)

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Incidently TIMTS, whats the Police 44 like to fly? Any noticable differences from the standard Raven with the extra weight and scope on the front?

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Old 16th Apr 2005, 21:13   #10 (permalink)
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Thanks guys.

I knew it had to be related to coning and coriolis, but I would not have thought the effect would be that pronounced.

The police model flies pretty much like the normal Raven. A bit slower of course..and a lot easier to get to descend for approaches and such. Looses airspeed quicker in the auto. Basically just heavier and draggier. if thats a word.
Nice ship with half tanks or less and two guys..more than that and it gets power limited.
Oh..and we only have 3 seats
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Old 17th Apr 2005, 00:27   #11 (permalink)
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As you are learning to fly an Enstrom (good choice), bear in mind that you can control the RPM as you enter the turn without changing the MP by just adding a little collective.
You are right, you will find that you will descend if you don't add a little throttle or aft cyclic as the power requirement increases with the increase in load factor.

Remember to do the opposite as you roll out.

Without disrespect to Robbie pilots who only fly Robbies,(I was one once) I have found several a bit hazy on the principles of fine RPM control when they have got in my Enstrom. Some are even frightened by it at first. I think that is a result of Robinsons having an excellent governor. Coyote does have a point about emergencies (but could have phrased it a little less critically!) so a bit of flying with the governor off next time you are with a CFI may not be a bad idea.
Its great fun too. Enjoy.
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Old 17th Apr 2005, 06:04   #12 (permalink)
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thanks Gaseous...

I do see coyotes point..and I take the critique to heart...will study up. Its one of those things where I knew the answer, just didn't connect the dots. Just didn't think it would have that much effect....

As for RPM control...well...I have over 900hrs in Schweizers...so RPM control is actually a piece of cake. Although you wouldn't think so from my question!! Probably asked it the wrong way!!!

oh well.

And by the way....I take offense to being called a Robbie pilot.
Just stuck in one for now!!
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Old 17th Apr 2005, 06:06   #13 (permalink)
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It's all because of you that I am looking for an Enstrom. Thanks.
I found an Enstrom for an intro lesson, just a 800 mile drive to San Francisco is all.
The instructor insists that I should buy a turbo Enstrom. He said a new pilot in a F28A will surely run out of power and crash. I called another Enstrom school in Texas and was told the F28A is better for a new owner and will work just fine even with three seats filled. And the engine will last longer.

I know you are happy with your F28A. It is interesting how different pilots have such opposite opinions.

I am going to the factory maintenance school in Michigan in June and will decide which model is better afterward. Probably will look for a "A".
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Old 17th Apr 2005, 10:10   #14 (permalink)
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The Texas guy is a bit nearer the mark.

If you follow the book you will not run out of power.

The problem the other guy refers to is this.

If due to inattention, you find yourself very low on rotor RPM, you may lose tail rotor effectiveness as you open the throttle to recover. This is the one area the 28A is not forgiving at all, but keep RPM in the ball park and tail rotor authority is not a problem. Enstrom put wider blades on the later model to help with this. This is often confused with running out of power but is really pilot error.

To avoid hijacking this thread even more I will start another in a day or two along the lines of 'Owning an old helicopter' as lots of people have asked me about it.

900 hours in a Schweizer!I bet you could teach me a thing or two about RPM control. Sorry. I assumed you were a low timer. Theres no stigma about being a Robbie pilot round here.
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Old 17th Apr 2005, 12:12   #15 (permalink)
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My apologies for being a grumpy pr1ck at times.....If you knew me you would know that I am not like that by nature.

However I do stand by my point that I think it is a serious deficiency in your basic knowledge as a professional pilot, and wonder how it could happen.

Maybe I should have just said that, maybe I was in shock....

But if you asked the guy flying you and your family on the 747 what the flaps do and he says "I really dunno, it just helps you land and take off" you'd have a few concerns wouldn't you?
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Old 17th Apr 2005, 15:40   #16 (permalink)
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Well Mr. Coyote.

I understnad your concern...but I assure you that I do know about the law of conservation of angular momentum and all that.
In just did not think it would have that much effect. The man pressure drops by 3-4inches, and I figured it might be some of that "magic" you are talking about having a go as well.
As we all know, the aerodynamics of helicopters is a bit more complex than the flaps of a 747. I find this topic very interesting and fascinating, and strive to learn as much as possible.

I apologize for frasing the question so bad that it could be understood the way you did.

I, however, do stand by the fact that there is no stupid questions..only stupid answers. If you are a CFI, I am sure glad I am not your student!

Fly safe
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Old 17th Apr 2005, 19:26   #17 (permalink)
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Nick L:

'The initial mp drop is because the load factor makes a slight coning increase, which raises the rotor rpm and drops the mp.'

I'm, a 58 hour R22 pilot with the ink still wet on the PPL so I gladly admit I know next to nothing.

I can appreciate the coning raising the rotor rpm but why does the mp drop? Genuine Q, not a query.
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Old 17th Apr 2005, 20:20   #18 (permalink)
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queries accepted, nobody is perfect, especially me!

When the rotor blade cones, it flaps or bends upward. When the whole set does, the center of gravity of each blade is closer, so the rotor speeds up a bit (ice skater brings arms in) since the rotor's angular momentum cannot change, so the rpm increases to match the total energy stored in the system. When the rpm speeds up, the governor backs down a bit (or the pilot rolls off a bit of throttle.)

This is temporary effect, and does point out how hard it is to report any factor or answer any question. The turn is actually about 7 maneuvers done in sequence:

1) the entry cyclic from level, which results in an angular acceleration of the aircraft.

2) the angular rotation (roll rate)

3) the removal of some cyclic to stop at the bank (angular roll acceleration now opposite the bank).

4) the steady turn at a bank angle (comprised of complex pitch and power and yaw re-trims due to aerodynamics of the turn).

5) the roll-out, which reverses 1, 2 and 3 above

So, the MP does change during most of the above, for many reasons!

I answered for item 4, hoping that was the original request!

Last edited by NickLappos; 18th Apr 2005 at 07:00.
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Old 18th Apr 2005, 01:03   #19 (permalink)
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Just wondering as well..........does this have any relation to the transient torque in banks? I read, a bit back, about the torque rising in a left bank and decreasing in a right, but never did find a conclusion. I tried this in a 206 and noticed a few % change. Kinda shocking. Glad someone else brought it up, cause I don't think I would have noticed it what with my eyes outside and all. I also tried it in a 120 and noticed the opposite trend. Any input appreciated.
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Old 18th Apr 2005, 15:52   #20 (permalink)
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MAP drop illustrated

To demonstrate different aspects or viewpoints explaining the MAP drop in a steady/level turn look at the following simple simulated right turn


(Quicktime, 230Kb)


(Media player, 240Kb)


with corresponding controls:

Turn is initiating by changing the horizontal flight lateral cyclus of about -0.5 left to 0.25 right (0.75 change)
To maintain altitude aft cyclic is necessary, which goes from 3.7 degrees forward down to 2.5 degrees forward (1.2 degree change)
Speed reduces because of loading while maintaing collective, from 90 to 85 knots.
Altitude is almost maintained, except of a small climb due to simulator pilot errors (still learning to fly that thing...)

The corresponding pilot views

Before turn:

During turn:

The Engineering data in the above figures show:

While level :

-> 9307 N MR trust,
-> 4.08 degrees rotor disk inflow angle
-> 2.75 degrees coning
-> 111 Main rotor SHP

While in the turn
-> 11200 N trust = higher
-> 2.39 degrees rotor disk inflow angle = lower see sketch
-> 3.32 degrees coning = higher consistent with higher loading
-> 106 Main rotor SHP = lower, so lower MAP

So indeed power needs are lower by approximately 5% in this case.

One way to explain the MAP decrease is to look is at the rotor disk inflow:

A lower inflow angle tends so to speak towards autorotation so reduction of SHP.

An other way to explain is look at the detailed rotor power needs. This consists of profile drag (the drag of the blade profile when it is rotating around the mast)
and the induced drag (the horizontal part of the lift due to the fact that net inflow is not horizontal, so the lift component is not strictly vertical as it were in an airplane fixed wing)

Drag before turn:

During turn:

The drag plots show 3 quantities :

- in blue the profile drag, this is pretty much the same in both cases (slightly higher for more loaded disk in turn).
- in green the induced drag, the is lower when turning because the inflow comes more from below, so lift is more forward (or better less backward)
- in red the total drag, again lower in turn, so lower SHP.

Remark that profile drag is greater for the forward going blade the for the retreating blade, but that the induced drag is greater for the retreating blade.

Finally a very detail view consists at exploring all the angles of Attack

Before turn:

During turn:

The meshed figure shows the mechanical angles of the rotor blades: this is the same in both cases (front view) corresponding to same collective and almost same lateral cyclic.
The solid blue figure show the effective angles of attack taking all airflow components into account : this also shows an increase corresponding with lower inflow and higher disk load.

Finally an examples of a detailed airflow plot, snap shot begin of turn right advancing blade fully to the right (270 degrees in my references, 0 starts centre front), at the 50% cord position. I indicated manually the major changes that occur in these flows when in level turn.
Air flow is influenced of course by helicopter speed and the blade rotation. But also by blade flap, induced air speed, and total rotor disk attitude changes:

So all these ways of looking at turns explain the drop in MAP.

Perhaps a final remark.
During rapid turns the effective rotor RPM equals the relative rotor RPM, which is kept steady at approx 404 rpm = 42 rad/sec = 2400 degrees/sec, plus the absolute yaw rate of the heli.
See the pilot view while in the turn (lower right corner): Yaw = -8, so in this turn there is a extra yaw angle of -8 degrees per second. This is a change of -8/2400 : -0.3 %. This has a cubic general influence on power (so -0.9%) The effects of this small change are just on the edge of the precision of this simulator (0.5-2%), so if precision were
to be increased, which is possible at the cost of more CPU time, it should be detectable, but in view of all the other much larger influences this may not be easy. It would also be outside the precision of onboard measuring instruments.

Last edited by delta3; 18th Apr 2005 at 16:19.
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