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

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


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.

No you don't. If you did, it would be called a RAP. The clue's in the name. :O

You're reading something into my post that is not there. I was looking for confirmation and could not find it.

RAT vs RAP vs ADG!!!

@ DaveReidUK

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/Em...esentation.pdf

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.)

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.

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

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.

I'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

@DaveReidUK

So "Yes" we both agree on that one. Which was the intent of my original post.

Also: We 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.

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.

DaveReidUK

What part of post #22 did you not understand?

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.

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.

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.

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.

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.

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.

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.

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.

Correct, see more in my answer below.

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.

^^

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

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

I credited your post, too.

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

nor this question

@27/09


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.

Two 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/ --

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