Paradox of constant speed propeller

[Dreamlinerwannabe]

Hello fellow aviators!

I am quite confused by the principle of constant speed/variable pitch propeller for awhile. After a few days of research I have a good idea about its' purpose and how to achieve it.

But, I don't understand why do we have to have a "rev-up, throttle back" principle.

Why do we have to do this? Why sometimes we have to select RPM first, then MP?

And how do we know if that RPM setting is optimal?

Also, it's saying that propeller setting controls RPM, throttle control MP.

How come when I increase throttle, MP increases, but RPM constant because of the governor? What's next after MP increases? Crankshaft connected to cylinder rotates faster to gearbox, and the gearbox eventually lower the rotation so RPM doesn't increase?

Thank you very much for your professional helping!

Cheers!

[Basil]

I'm sure someone will be along with a better explanation but, for the moment:

On poppet valve engines it is normal to increase RPM then boost, throttle or MP.
I believe that his is to prevent overboosting and overheating of the exhaust valves.
When reducing RPM then reduce MP first.

An exception, in my experience, is the Bristol Hercules sleeve valve radial engines where we did the opposite and increased boost then RPM.

As you increase MP, the prop pitch will coarsen in order to absorb the greater power and produce more thrust.

Re optimal settings: there should be guidance in the flight manual.

The plus point, when you make it onto the Dreamliner, is that you don't have to worry about all that stuff;)

[LLZDME]

The point of a constant speed propeller is to get maximum efficiency throughout the full range of speeds and power of the aircraft.

How come when I increase throttle, MP increases, but RPM constant because of the governor? What's next after MP increases? Crankshaft connected to cylinder rotates faster to gearbox, and the gearbox eventually lower the rotation so RPM doesn't increase?

If you increase the MP, you increase the power output of the engine, thus the propeller turns faster. The propeller governor (which has nothing to do with a gearbox) detects this, and increases the pitch of the propeller blades. Your blades now have a higher angle of attack, which means more drag, thus reducing the speed of the propeller back to initial value (constant speed). It also means more lift, which gives you thrust on a propeller. You therefore have an increased thrust, which was the initial point of increasing power.

But, I don't understand why do we have to have a "rev-up, throttle back" principle.

Why do we have to do this? Why sometimes we have to select RPM first, then MP?

If you have an important manifold pressure, with low RPM (Low RPM means blades have a very high angle of attack, and thus very high drag.), you will exceed the maximum torque supported by the crankshaft and could damage the engine.

If you want an increase in thrust, you must increase the RPM before the MP so that you do not exceed max torque, and the opposite when reducing power, for the same reason.

[barit1]

While larger engines may have a gearbox to reduce the prop rpm (and thus tip speed), the gears are a fixed ratio (generally between 0.40 and 0.75) and can be ignored when studying the governor operation.

The biggest effect of the governor can be felt whenever the IAS is changing. If you set e.g. 2400 rpm at start of TO roll, and keep the nose down and let the aircraft accelerate to whatever its max IAS may be, the engine will still be turning 2400 rpm. Similarly during aerobatics.

Another note - the tachometer on the panel shows ENGINE rpm; if a geared engine, the prop rpm will be slower than the tach reading.

[thing]

I think you're getting your gearboxes mixed up. A CS prop doesn't have a gearbox, it has bob weights that move with relation to prop speed. There's only a limited amount of movement which is why at low throttle openings the prop spins slowly rather than at 2400rpm or whatever you have selected.

I fly a 182 and on take off it's full throttle and fine pitch; get off the ground to a safe height and back to 23 inches on the manifold and 2450 rpm, however the rpm will be at around 2450 anyway so basically you just leave the prop where it is for climb. Cruise is whatever you want although I fly at 22 squared which is 22 inches manifold and 2200 rpm, reducing the manifold first. This gives around 120 kts depending on weight.

The descent is where you have to plan ahead a bit. Reduce manifold pressure an inch and start a 500' minute descent. Every minute reduce the manifold pressure by one inch. Eventually you will get to the point where the prop has reached it's stops and the rpm will start to fall. If you plan this correctly you should just be turning downwind and you know it's safe to go full fine on the prop which is what you want at this point.

The prop control isn't something that's fiddled with throughout the flight, on a normal A to B flight you would probably touch it twice.

The simple way to remember the sequence is PTTP. Power ie an increase in power is pitch then throttle. Decrease in power is throttle then pitch.

You can really feel the effect of a CS prop on a touch and go; normally on a fixed pitch prop say a 172 you feel the the power come, stick in a little right boot and lift it off. On a 182 especially if it's light once you hit around 45 kts there's a real kick in the back as the prop starts to work at it's optimum, you can certainly feel those 235 hp.

[GearDownThreeGreen]

I think you're getting your gearboxes mixed up. A CS prop doesn't have a gearbox, it has bob weights that move with relation to prop speed.

There most definitely is a gearbox in connection with CS props. It's not what's controlling the RPM for different engine outputs, but I would love to see the results if the prop on any TP engine was allowed to run at non-geared engine speed :eek::eek:

[DaveReidUK]

I would love to see the results if the prop on any TP engine was allowed to run at non-geared engine speedI distinctly recall the OP talking about cylinders and crankshafts ...

[GearDownThreeGreen]

I distinctly recall the OP talking about cylinders and crankshafts ...

Yes he was, however, the principle remains the same. And to say that a CS prop does not have a gearbox is very often wrong. As I said it has nothing to do with the regulating mechanism, but it's often an integral part of the total engine/propeller assembly. Be it either piston or turbine.

[thing]

Just to clear up any confusion, I was talking about the prop mechanism, it most definitely isn't a 'gearbox'. I assumed from the OP's original post he was talking about SEPs, apologies if I was mistaken.

[A Squared]

You can really feel the effect of a CS prop on a touch and go; normally on a fixed pitch prop say a 172 you feel the the power come, stick in a little right boot and lift it off. On a 182 especially if it's light once you hit around 45 kts there's a real kick in the back as the prop starts to work at it's optimum, you can certainly feel those 235 hp.

You don't suppose that might have more to do wit the fact that the 182 has in excess of 50% more horsepower in a plane that weighs about the same?

[A Squared]

And to say that a CS prop does not have a gearbox is very often wrong.

Umm, actually, it's never wrong. I haven't ever seen a prop (CS or fixed pitch) with a gearbox. A few are mounted on engines with gearboxes, but the prop is just a prop, no gearbox included.

[thing]

A Squared: It is a bit heavier than a 172 but I take your point. I'm referring to the sweet point on the prop pitch/RPM/IAS where the prop really starts to bite. There's a noticeable surge in 'oomph' around 45-50 kts. I fly a 172 as well and there's certainly a difference in prop response at certain speeds, nothing to do with the extra 55 hp, (where do you get 50% extra power from?) which is as it should be otherwise there would be no point in having a CS prop, other than it's a damn sight quieter in the cruise than a fixed pitch.

Edit: Error on my part, the 182 has a 230 hp donk so an extra 50 hp.

[A Squared]

where do you get 50% extra power from?

172 = 150 hp

182 = 230

230-150= 80

80/150 = 0.533 = 53%

I'm sure that you can point to 172 which have other than 150 hp and 182's with other then 230, but there's a whole bunch with those numbers. Anyway, the point is, generally the 182 has a higher power to weight ratio than the 172.

[thing]

Not disputing that, just that the acceleration is not constant on take off. I know that it isn't in a fixed prop either, it's just more noticeable (to me anyway) in a CS a/c. Maybe I have a sensitive seat of the pants. Have you never noticed the surge you get at take off at around 45 knots in a 182 as compared to a 172 ?

The 172 I fly has the A3A donk at 180 hp.

[A Squared]

I would suggest that the surge you feel is not the prop "really biting" as the efficiency of a constant speed prop plots as a fairly constant curve. ie: no airspeed/rpm/etc regimes where it is dramatically more efficient at producing thrust. Additionally, the prop installed on a stock 182 is most leikely going to be one which is most efficient in the cruise regime (vs the aftermarket seaplane prop on my C-180 which is optimized for low speed thrust)

Rather the difference in push you feel is due to the fact that the constant speed prop allows the engine to turn up to max RPM (and thus produce max power) at low airspeeds, vs the fixed pitch prop where at low airspeeds the engine RPM is necessarily low, and thus power output is also well below max. A fixed pitch prop airplane will not produce max rated power until such time as the airspeed is high enough to allow the RPM to increase to redline. Unless you have a very flat pitch prop, that probably isn't going to happen on the takeoff roll.

[thing]

Good call, never thought of that.

[Uplinker]

For what it's worth; just a non-technical thought from when I used to fly turbo-props:

I think of the prop pitch being like the gears on a car. To start off, or go 'uphill' you need a low gear (fine pitch). To cruise along the 'motorway' you use a high gear (coarse pitch).

On approach to land, you need fine pitch again just in case you need to go around and climb.

As others have said; a constant speed system keeps the prop RPM constant and varies the blade pitch according to how much power the engine is producing. Turbo props generally have two propeller RPM settings - fast for take-off and fine pitch, slower for the cruise.

[aw ditor]

Basil.

Hercules 264, Lollipops before Crinkly Chips' to increase power and vice versa to decrease power . Sleeve valve engine, though I never got a good explanation as to "why". Was it to prevent chatter' on the sleeve valve mechanism?

[MX Trainer]

Dreamlinerwannabe


But, I don't understand why do we have to have a "rev-up, throttle back" principle.

Why do we have to do this? Why sometimes we have to select RPM first, then MP?

And how do we know if that RPM setting is optimal?

Also, it's saying that propeller setting controls RPM, throttle control MP.

The reason for the change in speed before increase in manifold pressure is to ensure the engine doesn't detonate with the increase in BMEP. This is often refered to as "OverBoosting" the engine - but it can also occur in normally aspirated engines as well.

You can easily achieve detonation in a motor car with a manual transmission - with a slow speed in a high gear push the throttle all the way open and you will hear - and sometimes feel - the engine "Ping". This is detonation and it is very destructive to aircraft engines.

Below is an excerpt from the Pratt & Whitney Engine Operation Manual 01 - 100 that was used in training during WWII.


Page 27 - 29


DETONATION
Normal combustion is rapid, but it is by no
means an instantaneous explosion. The charge
burns evenly and smoothly, the flame front advancing
at a measurable rate-about 35 feet per
second when combustion begins, increasing to
roughly 150 feet per second, and finally slowing
down as the process nears completion.
If sufficiently heated and compressed, any
combustible mixture of gasoline vapor and air
will catch fire. Accordingly, if the temperature
and pressure of the unburned portion of the fuel-air
charge reach critical values, combustion
will begin spontaneously and simultaneously
throughout the unburned charge. The result is
a sudden and violent explosion known as detonation.
Detonation occurs so quickly that even high
speed cameras, which slow down normal combustion
to a snail's pace, fail to retard its progress
sufficiently for exact analysis. It is accompanied
by an abrupt pressure rise and violent
pressure fluctuations of extreme rapidity. The
engine is unable to turn into useful work energy
so explosively released. The recurring shock
pressures are carried to piston, cylinder, and
hold-down studs, and the fatigue stresses set up
in the materials quickly lead to the failure of
these parts.


Detonation also causes a rapid rise in cylinder
temperatures, and thereby aggravates the very
conditions which produced it. These high temperatures
can rapidly destroy the piston, cylinder
head, exhaust valve and guide, and damage
other parts by burning and erosion.
Similar in its results to detonation, and frequently
accompanied by it, is pre-ignition. The
latter is caused by uncontrolled ignition of the
charge ahead of the normal flame front, because
of contact with some "hot spot" in the combustion
chamber, such as an incandescent spark
plug. As a result the timing is too far advanced;
the engine loses power and overheats ; local temperatures
at the hot spot rise rapidly; and the
engine may be damaged, if it is not quickly
stopped.


Detonation-free operation is altogether normal
and entirely possible over the full range
of rated engine performance, even under the
most adverse conditions. Nevertheless, detonation
is the most likely as well as the most destructive of

the possible consequences of improper engine operation.


CONDITIONS LEADING TO DETONATION


Among the conditions which may lead to detonation
the most important are :


1. Excessive manifold pressure. As manifold
pressure is increased, so is the pressure of
the charge entering the cylinders. The latter
is multiplied many times during compression
and combustion, and, if the initial pres-
sure of the charge is too great, a critical
value may be reached which will result in
detonation. Excessive manifold pressure may
be caused by too wide a throttle opening or,
on some engines, by the use of too great a
degree of supercharging.


2. Excessive carburetor air temperature (c.a.t).
As the temperature of the charge air at the
carburetor is increased, so is the temperature
of the fuel-air mixture entering the cylinders.
The latter is further raised during compression
and combustion, and, ' if the initial
temperature of the charge is too high, a critical
value may be reached which will also
result in detonation. Hot "free air" entering
the induction system, inadequate inter-cooling
in the case of multiple stage superchargers
or too much carburetor pre-heat, may
cause excessive c.a.t. High impeller speeds,
the consequence of high engine rpm or of
improper operation in the "high" impeller
gear ratio, will cause a sharp heat rise -
through the supercharger, and, as a result,
the charge will not be sufficiently cool when
delivered to the cylinders.


3. Excessive cylinder head temperatures. The
temperature and, indirectly, the pressure of
the unburned portion of the charge may be
raised to critical values as a result of excessive
cylinder head temperatures alone.


4. Improper grade of fuel. If the fuel used has
an anti-knock rating (i.e., resistance to detonation)
lower than that called for by the rating
of the engine, detonation will follow any
attempt to operate in the high power range.


5. Malfunctioning of the ignition system:
Whenever the engine is operated in the high
power range, detonation is likely to occur if
the timing of the spark is too far advanced.
It may also occur during high power operation
in a cylinder where only one of the two
plugs is functioning.


6. Lean Mixtures. The tendency to detonate
varies with the fuel-air ratio, and mixtures
at or near best power are the ones most
likely to detonate. Combustion chamber
temperatures may be lowered most effectively,
and detonation thereby most readily
inhibited by enriching the mixture beyond
the best power setting.


Detonation imposes one of the most important
limitations on engine performance, and the operator
must at all times so control conditions as to
avoid any which might lead to detonation and
the consequent damage to his powerplant.




Page 101


TRANSITION FROM TAKE-OFF
TO CLIMB


As soon as the field and surrounding obstacles
are cleared, reduce power at least to
Normal Rated. With constant speed (variable
pitch) propellers the reduction should be accomplished
in steps as follows:


1. Retard throttle to reduce manifold pressure
to about 2 in. Hg below that for Normal
Rated power (with fixed part throttle,

manifold pressure will rise as rpm is reduced).


2. Retard the rpm control to Normal Rated
rpm.


If a further reduction in power is desired,
proceed as follows:


1. Lower manifold pressure by 2 in. Hg.
2. Lower engine speed by 200 rpm.


Continue in successive alternate steps until
the desired engine speed is reached, finally adjusting
the throttle to the desired manifold pressure.






This is not to be construed to mean that the throttle
should never be advanced with a low rpm. The engine -
is not affected by the position of the throttle.
It is affected by the manifold pressure resulting from the
throttle position.



Bold and Italics for emphasis are mine.


Hope this helps

[MX Trainer]

Dreamlinerwannabe

Quote from Basil

"The plus point, when you make it onto the Dreamliner, is that you don't have to worry about all that stuffhttp://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/wink2.gif"


Actually you still have a constant speed propeller in the form of a RAT on the 787.
You won't have to worry about manifold pressure or propeller pitch control unless they further develop a variable pitch fan.

Variable pitch turbofan : Patent US3946554 - Variable pitch turbofan engine and a method for operating same - Google Patents (http://www.google.com/patents/US3946554)

A good read here for operation of the 787 RAT:
Boeing 787: A Pilots Perspective (http://www.flight.org/blog/2012/06/05/boeing-787-a-pilots-perspective/)

The Rat Part!!


If 3 of the 4 engine drive generators fail in flight, the APU will start automatically. Two APU generators can be operated to the certified ceiling of 43,000 feet. If all four generator fail in flight, the Ram Air Turbine will deploy (RAT) will deploy and power only essential buses and, if necessary, hydraulic power to the flight controls (should the RAT itself fail, standby power will ensure continued use of the autopilot, captain’s flight director and instruments, FMC, 2 IRSs and VHF radios in addition to some other essential instruments).

Ram Air Turbine: Ram air turbine - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Ram_air_turbine)

So even the basic theory of the constant speed propeller is still applied to a modern aircraft.

[HazelNuts39]

Where do your references say that the RAT is constant speeding?

[MX Trainer]

Rat Constant Speed System


Every one that I have had the pleasure of working on in the last 20 years all had a constant speed system to maintain a fairly constant speed over the variations in load and airspeed. The system was set to deliver approximately 400 Hrz from the AC generator which of course requires a specific speed. RAT electrical AC power is considered to be "Wild Frequency" due to the larger variations in input speeds to the generator as opposed to the Constant Speed Drives (CSDs) that were a part of the engine gearbox to generator drive system of many aircraft.

I don't have a schematic of the RAT installed in the 787 but I would assume it to be the same - but I reserve the right to be proven wrong.

Here is a link to a patent that explains it pretty well:

Ram Air Turbine With Secondary Governor (http://www.docstoc.com/docs/47846788/Ram-Air-Turbine-With-Secondary-Governor---Patent-5487645)


HazelNuts39 - if you have something that proves this wrong please post it so we can all learn new things. In other words: please post your reference to it not having one.

[DaveReidUK]

Actually you still have a constant speed propeller in the form of a RAT on the 787.No you don't. If you did, it would be called a RAP. The clue's in the name. :O

[HazelNuts39]

if you have something that proves this wrong please post itYou're reading something into my post that is not there. I was looking for confirmation and could not find it.

[MX Trainer]

RAT vs RAP vs ADG!!!

@ DaveReidUK

No you don't. If you did, it would be called a RAP. The clue's in the name. http://images.ibsrv.net/ibsrv/res/src:www.pprune.org/get/images/smilies/embarass.gif

Call it whatever you want but it is still a constant speed multi-bladed propeller that may be shrouded or unshrouded.

Ram Air Turbine sounds so much sexier than a Ram Air Propeller or as the military likes to call them an Air Driven Generator (ADG). I have never heard of the term RAP but I don't know everything.

Read the link below and you can see that someone with a whole lot more experience than I; has on page 17 of the presentation described exactly what I have been talking about.

http://ieee.rackoneup.net/rrvs/06/Emergency%20RATs%20Presentation.pdf


Description of RAT Operation
•
Deploy signal commanded (automatic or manual)
•
Uplock releases
•
Deployment actuator provides force to open RAT
compartment doors and deploy RAT into airstream
•
Turbine locked in position until blades clear aircraft
•
Turbine is released and accelerates to rated speed
•
Turbine governor maintains speed control
•
RAT provides emergency power to aircraft
•
RAT remains deployed for rest of flight

Page 18 and 19 show in detail the speed governing method for the RAT.

If anyone actually working on the 787 has the time, please post a description of the RAT operation for us all. (See - a nice polite way of asking for information rather than just a demand.)

[DaveReidUK]

Call it whatever you want but it is still a constant speed multi-bladed propellerIf it's a propeller, what does it propel ?

[MX Trainer]

If it's a propeller, what does it propel ?


The constant speed operation of a propeller is exactly the same for either being driven from an engine or being driven by the airflow. In some turboprop aircraft they differentiate air driven mode from engine driven mode - and sometimes have a NTS (Negative Torque System) to sense engine failure. Negative torque being air driven mode.

Again like I said "CALL IT WHAT YOU WANT" The constant speed operation of the "turbine" is exactly the same as for a propeller. I live in an area that uses huge wind turbines for generation of electricity - they are speed controlled variable pitch and full feathering in operation. Huge 3 bladed propeller systems correctly called turbines.

My original comment that there is still "Constant Speed "Propeller" operation on the 787 still stands until proven otherwise - and I have no problem with being incorrect and learning something.

Semantics my friend semantics - it is what it is - and you can call it what you want.

[DaveReidUK]

The constant speed operation of the "turbine" is exactly the same as for a propeller.I'm not disputing for a moment that both can be governed to rotate at a constant speed.

However the function of one is completely opposite to the other's.

Propeller: a device to convert work into airflow

Turbine: a device to convert airflow into work

Spot the difference. :O

[A Squared]

Turbine: a device to convert airflow into work


Yeah, exatly, just like the compressor turbine on a turbine engine which converts ..... uhh wait, that turbine doesn't turn airflow into work, does it , it turns work into airflow .. Hmmm.

[DaveReidUK]

just like the compressor turbine on a turbine engineI'd be fascinated to know of an example of an engine that features one of these mysterious "compressor turbine" devices.

Are we talking about some kind of perpetual motion machine? :O

[MX Trainer]

@DaveReidUK

I'm not disputing for a moment that both can be governed to rotate at a constant speed.So "Yes" we both agree on that one. Which was the intent of my original post.

Also: However the function of one is completely opposite to the other's.

Propeller: a device to convert work into airflow

Turbine: a device to convert airflow into workWe also agree on this one as well. See my comment about wind turbines!!!

Dictionary result for propel - propelling - propelled - of which the device used to do so is called a propeller:

pro·pelled, pro·pel·ling, pro·pels To cause to move forward or onward.
[Middle English propellen, from Latin prhttp://img.tfd.com/hm/GIF/omacr.gifpellere : prhttp://img.tfd.com/hm/GIF/omacr.gif-, forward; see pro-1 + pellere, to drive.

pro•pel•ler


1. a device having a revolving hub with radiating blades, for propelling an airplane, ship, etc.
2. a person or thing that propels.
3. the bladed rotor of a pump that drives the fluid axially.
4. a wind-driven, usu. three-bladed device that provides mechanical energy, as for driving an electric alternator in wind plants.

So a propeller can be used to cause the airflow to move forward and a propeller can be used to drive a shaft from the airflow. Both cases are correct usage.


If you can comprehend what I have posted then you would be able to see the similarities of the two and not the differences is what I am illustrating.:ugh:

At any event I am waiting for someone with actual information on whether or not the specific RAT is in fact a constant speed controlled unit on the 787.

[DaveReidUK]

At any event I am waiting for someone with actual information on whether or not the specific RAT is in fact a constant speed controlled unit on the 787.I don't know either - but given that the engine starter/generators on the 787 produce variable-frequency AC, I can't see any reason why the RAT would need to have the cost and complication of CSD or IDG functionality to produce fixed-frequency AC.

[MX Trainer]

DaveReidUK

What part of post #22 did you not understand?

RAT electrical AC power is considered to be "Wild Frequency" due to the larger variations in input speeds to the generator as opposed to the Constant Speed Drives (CSDs) that were a part of the engine gearbox to generator drive system of many aircraft.As far as I have been able to get info on the 787 RAT my internet sources have only mentioned that it is used for both hydraulic and electrical production and I have been unable to determine the make and model with an internet search so don't know for sure that this one does or does not have speed control. Still waiting for an expert before changing my mind.

AFAIK - an unloaded RAT has the capability to over-speed and possibly spit blades - so some form of speed control is usually fitted to protect the unit from catastrophic failure if nothing else.

[Tarq57]

I am quite confused by the principle of constant speed/variable pitch propeller for awhile. After a few days of research I have a good idea about its' purpose and how to achieve it.

But, I don't understand why do we have to have a "rev-up, throttle back" principle. Explained rather well in post 19. Although the item relates to an engine fitted with a supercharger, the general principles are applicable to many piston engines.

Why do we have to do this? Why sometimes we have to select RPM first, then MP? It's a generalised procedure "across the board" to minimise the risk of detonation. On some types, it's not the procedure (Bristol Hercules radial engine mentioned in other posts) but in most light aircraft, that principle is the safe option.

And how do we know if that RPM setting is optimal? Performance data in the flight manual. For any required performance, up to the maximum available, there is a recommended combination of MP/RPM to achieve this. In fact, there may be more than one. (ie: higher MP/lower RPM, or lower RPM/higher MP to achieve the same power output. The use of the second option usually requires some careful monitoring to ensure detonation will not occur.)

Most training organisations will advise the use of "pre-set" RPM/MP combinations for each stage of flight. (eg: Max RPM/MP for takeoff, 25"/2500 for climb, 23"/2400 for cruise.)These will be pretty close to being efficient for most training sessions without having to constantly fine-tune RPM/MP and mixture.

Also, it's saying that propeller setting controls RPM, throttle control MP.

How come when I increase throttle, MP increases, but RPM constant because of the governor? What's next after MP increases? Crankshaft connected to cylinder rotates faster to gearbox, and the gearbox eventually lower the rotation so RPM doesn't increase? As mentioned above, the governor is associated with the propeller. Counterweights will act on the blades to coarsen the pitch as a result of increasing RPM, due to the increased torque caused by opening the throttle. The weights are arranged in such a way that centrifugal force moves them. So an increase in RPM will move the weights outboard, which will coarsen the propeller pitch, and prevent the RPM increase.

Without a working governor, an increase in torque will result in an increase in RPM. Oil is supplied to the governor from the engine-driven system to adjust the required RPM. (An aside- you may notice if the throttle is opened rapidly on takeoff or overshoot, the engine briefly over-speeds before the governor "catches up" and reduces the RPM to that set. Hint: Don't open the throttle too fast.)

Any gearbox fitted to an aircraft engine is a fixed-ratio, and can be disregarded in terms of the process of setting propeller RPM.

All engines are a compromise - efficient within a relatively narrow range of power output and density altitude. They turn a lot of the energy within a fuel source to heat and noise. The remaining energy is converted into rotational force, or torque. The greater the RPM that torque is produced at, the greater the power output. So if you want the full power available, it is only achieved at the maximum RPM available. When reduced power is required, such as for cruise or descent, it is usually more fuel efficient for this to be produced at a lower RPM. For any required power output there is a "best" (=most fuel efficient) RPM and MP to achieve it. The details should be found in the performance section of the flight manual/POH.

[27/09]

I'm surprised that after 31 replies (Edit make that 32, Tarq 57 posted while I was writing mine) to the original post there's only one (two) post(s) that has actually come close to answering the original posters question, and that is the one (two) posted by LLZDME (and Tarq57).

To expand a bit further on what LLZDME posted.

But, I don't understand why do we have to have a "rev-up, throttle back" principle.

Why do we have to do this? Why sometimes we have to select RPM first, then MP?

The reason we do it is so that we don't "over boost" the MP. You may have noticed that as you reduce RPM, MP will increase. If you have maximum allowable MP set and you reduce the RPM the MP would increase above the maximum allowable.

On most GA non turbocharged piston engines it doesn't matter which way you do it. However on many turbo charged engines it does matter so as a rule of thumb when reducing power we set MP first and RPM second and when increasing power we set RPM first and MP second.

And how do we know if that RPM setting is optimal?

By using the power setting tables in the Pilots Operating Manual. There will be a table giving combinations of MP and RPM settings for various situations. There will be several combinations of MP and RPM for the same power output. Remember power is directly related to Torque X RPM. In the case of a piston engine MP and Torque are directly related so power is directly related to MP X RPM.

Also, it's saying that propeller setting controls RPM, throttle control MP. Correct, see more in my answer below.

How come when I increase throttle, MP increases, but RPM constant because of the governor? What's next after MP increases? Crankshaft connected to cylinder rotates faster to gearbox, and the gearbox eventually lower the rotation so RPM doesn't increase?

Yes, the RPM remains constant because of the governor but as "Thing" pointed out there is no gearbox involved in the situation you are talking about.

The crankshaft RPM and propeller RPM are directly related. (Some piston engines do have gearboxes to reduce prop RPM but you cannot change gears so to speak) The speed ratio between the engine and propeller does not change.

The RPM control is connected to the governor and not the propeller.

The governor controls the prop RPM, generally through changing oil pressure, this oil pressure acts on a piston in the propeller. The oil pressure is balanced on the other side of the piston by a spring and or compressed air or nitrogen. The movement of the piston causes the propeller blades to change their angle to the airflow.

As the governor senses an increase in RPM it changes the oil pressure which in turn changes the propeller blade angle to maintain the set RPM.

Within the the propeller there are coarse and fine pitch stops which limit how far the blades can move in either direction. As you reduce power for landing you will reach a point where the MP does control RPM, the propeller is now on it's fine pitch stop and can no longer maintain the set RPM.

[MX Trainer]

^^

And post #19 said it before you guys did!!!:ugh:

[Tarq57]

MX trainer, yep.
My answer was intended to try and clarify the overview at a sort-of basic level.

I credited your post, too.

[27/09]

MX Trainer, true you dealt with the MP and RPM setting procedures very thoroughly but you didn't bother with this question.

How come when I increase throttle, MP increases, but RPM constant because of the governor? What's next after MP increases? Crankshaft connected to cylinder rotates faster to gearbox, and the gearbox eventually lower the rotation so RPM doesn't increase?

nor this question

And how do we know if that RPM setting is optimal?

[MX Trainer]

@27/09


MX Trainer, true you dealt with the MP and RPM setting procedures very thoroughly but you didn't bother with this question.

Quote:
How come when I increase throttle, MP increases, but RPM constant because of the governor? What's next after MP increases? Crankshaft connected to cylinder rotates faster to gearbox, and the gearbox eventually lower the rotation so RPM doesn't increase?
nor this question

Quote:
And how do we know if that RPM setting is optimal?

That would be because they were previously covered in:
post #2 by Basil
post #3 by LLZDME
and post #4 by Barit1

I thought they did a pretty good job of it so no need for me to expound further.

I did feel the need to expand the discussion on why the change of speed before the change in manifold question because although over-boosting had been mentioned the concept of detonation had not. In addition it is only 1 of 2 places that I have actually seen it written - the other place was in a WWII USAF training manual.

Whenever possible I like to have the link to or paste in the information about what I am talking about.

Hope this clears up any confusion.

[DaveReidUK]

I have been unable to determine the make and model with an internet searchTwo minutes with Google comes up with the answer to the first part of your question:

Hamilton Sundstrand's Boeing 787 Team Supporting First Flight and Entry Into Service -- PARIS, June 14 /PRNewswire-FirstCall/ -- (http://www.prnewswire.com/news-releases/hamilton-sundstrands-boeing-787-team-supporting-first-flight-and-entry-into-service-62110622.html)

As of 2012, HS is now part of UTC Aerospace Systems. In its previous guise as Hamilton Standard it has a long history of making propellers as well as Ram Air Turbines. :O

[Volume]

An exception, in my experience, is the Bristol Hercules sleeve valve radial engines where we did the opposite and increased boost then RPM.
Can anybody quote a Bristol manual, just like the P&W some posts ago?
I think it is not the thermal stress to the exhaust valves (mechanical issue), it is the hot exhaust valves causing detonation (combustion issue) which limits MP at low RPM. No hot exhaust valve - no problem.

[Tarq57]

No quote or link, I'm afraid, but I remember seeing about 61" MP at takeoff power.

They were supercharged directly so the post take-off power reduction dropped the MP as the RPM was reduced. Throttles were left where they were. Reducing to cruise power (the way they were operated here) involved a large RPM drop - back to about 1800-ish IIRC - and then the MP was fine-tuned using the throttles.

[MX Trainer]

Re - Constant Speed and Over-Speed Control of Sundstrand Ram Air Turbines

Thanks to DaveReidUK for the link to manufacturer.


Here is the link to the Sundstand patent for about 1988

Patent US4743163 - Ram air turbine control system - Google Patents (http://www.google.com/patents/US4743163)

- this has the speed governor that is referenced in the second patent info regarding the Over-Speed protection.

RAM air turbine over-speed protector using redundant yoke plate linear bearings - Hamilton Sundstrand Corporation (http://www.freepatentsonline.com/7074010.html)

Since I still do not have info regarding the 787 RAT I can not say for sure this will apply - but I would expect it to be the same or similar as a RAT is a RAT is a RAT.

This excerpt from the over-speed patent is probably the biggest reason for at least overspeed control.

BACKGROUND OF THE INVENTION

A ram air turbine (RAT) is a device for generating emergency supplemental power in a wide variety of aircraft. A RAT may generate hydraulic power, electric power or both. A RAT incorporates a turbine that extracts power from an air stream proximate the aircraft in flight. A typical RAT in current use is described in U.S. Pat. No. 4,743,163 to Markunas et al., owned by the assignee of this application, and incorporated herein by reference. The turbine is coupled to suitable power generating equipment, such as a hydraulic pump for hydraulic power and an electric generator for electric power.
As described in Markunas et al., the turbine includes a speed governor that changes the turbine blade position to maintain a relatively constant shaft speed to the power generating equipment. Failure of the turbine speed governor can cause an over-speed condition that may ultimately cause the release of a turbine blade at very high speed. Due to the high speed, the wayward blade has very high energy as well. The most common cause of governor failure is due to seizure of the bearing between the governor shaft and the governor yoke plate that controls the pitch of the turbine blades.
The potential release of a high energy blade proximate the aircraft is a concern for both commercial and military RAT applications. Should the wayward blade strike the aircraft fuselage, it may penetrate the skin and cause damage to electric or hydraulic power equipment or control systems. It may also injure passengers or crew. If one of the propulsion engines ingests the wayward blade, the engine may suffer severe damage that results in loss of thrust.
Current methods to minimise hazards caused by turbine over-speed-induced release of a turbine blade have involved strategic placement of key elements or shields to prevent penetration. These methods no longer satisfy increasingly stringent certification and safety requirements promoted by airworthiness authorities.
I will cross post this in the thread opened on info for the 787 RAT as well.

[Uplinker]

Pedants Corner, (and off topic):

A Squared; I think you will find that a Turbine only converts airflow into 'work', but Compressors, or Fans only convert 'work' into airflow.

As far as I know there is no such thing as a "compressor turbine"

[DaveReidUK]

As far as I know there is no such thing as a "compressor turbine" I still think it would make a great basis for a perpetual motion machine. :O

"When I use a word," Humpty Dumpty said, in rather a scornful tone, "it means just what I choose it to mean - neither more nor less."

[Chu Chu]

Those constant-speed propellers on sticks that are popping up wherever the wind blows do seem to get called wind turbines. On the other hand, an airplane propeller doesn't become a turbine just because it's windmilling.:)

[Uplinker]

Fair points, but a windmilling propeller isn't doing anything other than going round, although I suppose you could argue it might supply some hydraulic pressure via the hyd pump - but that's not what it's designed to do, and the performance is not guaranteed.

DR UK: Can I buy some shares in your perpetual motion machine? I like the sound of it ! Perhaps if you get it going then we can forget all this ridiculous cost cutting to save fuel......:)

[barit1]

Chu Chu:... an airplane propeller doesn't become a turbine just because it's windmilling

No, the words in the parts catalog don't change, but functionally it DOES become a turbine(*). Much the same as a starter motor can become a generator once the engine is started.

* I can point to several related accidents.

[MX Trainer]

@ volumn

Can anybody quote a Bristol manual, just like the P&W some posts ago?

I finally got access to my old Bristol Freighter Pilot Notes - so the following info might help. It's been too many years for me to just go and quote something that I might get wrong. So for what it is worth ??

Re: Bristol Hercules power setting


Quoted-


As per the B170 Mk 31 – (734 engines) Pilots Manual the procedure after takeoff and first stage climb is to retard the throttle to the ECB setting first and then adjust the propeller speed as needed.


The throttles have 3 positions – Take Off , Cruise, and Shut.
The throttle was connected to the mixture system and it was important to have the position set correctly as a small difference forward would affect the fuel burn upward.


Take off – 5 min limit (15 Min emergency) - 2800 RPM @ +13 Psi Boost ( 56.25 inches Hg)




I can remember the crew calling for a “Yard and a Half of Boost” for the start of the take off run!!


Max Continuous – 2500 RPM @ + 8 ½ Psi Boost ( 47 In Hg)


Max Continuous – Lean – 2400 @ + 3 ½ ( 37 In Hg)




Cruise @ 44,000 Lbs ---- Maximum Weak Mixture Power – 2400 RPM @ + 3 ½ Psi Boost (37 In Hg)


At S/L = 149 Knts Ind - 137 Gal Per Hour


At 5000' = 131 Knts - 110 GPH


At 10,000' = 145 Knts – 145 GPH


Cruise Recommended – Throttle in ECB position – engine speed as required


SL = 137 Knts - 111 GPH


5000' = 131 Knts - 110 GPH


10,000' = 125 Knts – 114 GPH




Note: Never permit the RPM to exceed 2400 when the throttle lever is at or below the Maximum Weak Mixture ( ECB ) position, or engine damage may result.




End of Quoted information.




As per the note above – I would take that to mean that an increase above 2400 RPM would require the throttle to be moved first – which moves the mixture out of the maximum weak mixture position before the propellers are selected to give a higher speed.


This makes sense to me as the mixture needs to be addressed first so as to prevent detonation at the higher power settings. It is a funny way of doing things but it works very well and it got rid of the possible addition of power by the flight crew and the omission of setting the mixture rich first.


In a nutshell:


The reduction of power from Maximum is by throttles first and then set desired engine speed.


The addition of engine power below 2400 RPM is not addressed – but the notes do say that you adjust speed and boost to get the desired target airspeed – usually using the charts supplied.


To go above 2400 RPM – throttle is moved first – which affects the mixture as well – and then propeller speed.


There are no mixture controls on this aircraft – there is a Carburetor Fuel Cutoff which has 2 positions – OFF and RUN. Mixture control is done automatically with the position of the throttle.


Hope that helps.

[MX Trainer]

Information on the B787 Ram Air Turbine:


I have 3 different sources of information relating to the speed control of the RAT on the 787 - all 3 confirm that the blades change pitch as a function of the speed limiter - which at the higher aircraft airspeeds or the offloading of the RAT will maintain a maximum constant speed of the turbine.

Quick link to the manufacturers site with just a quick overview of their product line;

Aircraft Ram Air Turbine (RAT) | UTC Aerospace Systems (http://utcaerospacesystems.com/cap/products/Pages/ram-air-turbine-systems.aspx)