theflcowboy
Posts: 357
Joined: Sun Jan 04, 2004 3:44 am

Engine Transmissions - Why Not?

Sat Mar 05, 2005 10:53 pm

This may seem like a stupid question, but I have been wondering about it for a while.

In a car, to get to better fuel effeciency, the transmission changes gears based on the RPM's of the engine. More throttle means more upshifting, less throttle means down shifting. Why not employ this in an airplane engine?
A318, A320, A332, A333, B1900, B722, B732, B733, B734, B735, B737, B738, B772, CR1, CR2, CR7, CR9, MD80, MD81, MD82, MD8
 
air2gxs
Posts: 1443
Joined: Mon Jun 18, 2001 1:29 pm

RE: Engine Transmissions - Why Not?

Sat Mar 05, 2005 11:11 pm

I'm not sure what you're getting at, but I'll give it a shot.

In order to get a car moving the engine must be able to overcome the weight of the vehicle standing still (and friction). In order to do this a transmission is used (automatic or manual doesn't make a difference). The transmission's low gear(s) allows the engine to transmit the power to the wheels and get it moving. As the car accelerates, the engine needs less power to keep the car moving and the transmission shifts to a more efficient gear.

On aircraft, there is no direct linkage from the engines to the wheels. That would be inefficient. The aircraft is moved simply by the airflow through the engine (or prop).

Now, an interesting situation exists on some high-end prop aircraft. The constant speed propeller and the adjustable propeller (for the life of me, I can't remember the "official" name). These do act somewhat like a transmission, but not in the traditional sense. They (the props) adjust their pitch to allow the engine to operate at its most efficient RPM for the given flight condition.
 
pilotpip
Posts: 2820
Joined: Fri Sep 19, 2003 3:26 pm

RE: Engine Transmissions - Why Not?

Sat Mar 05, 2005 11:34 pm

Would you want to disengage your prop while in flight to shift gears? I wouldn't. That just seems like one more place to break. Making a prop spin faster doesn't make it more efficient, it does the opposite. As it goes faster it approaches the speed of sound and gets very inefficient. Turboprop engines spin in the tens of thousands of RPM but the props have a reduction gear attached that will bring it down to around 2000rpm.

Helicopters have a transmission but for a different reason. Where's SlamClick? I saw a little Hughes 269 yesterday for the first time and the pilot did a clutch check during run up. Maybe a rotorhead like Mr Click can enlighten us but I don't think it's used to make the aircraft more efficient.
DMI
 
bri2k1
Posts: 952
Joined: Sun Dec 19, 2004 4:13 am

RE: Engine Transmissions - Why Not?

Sun Mar 06, 2005 12:10 am

The constant-speed propeller, commonly called the variable-pitch or controllable-pitch propeller, is analogous to a car's transmission. The engine of airplanes with constant-speed propellers is controlled directly with the throttle and indirectly with the propeller control. This control allows you to select a low blade angle and high RPM for takeoffs, since a higher RPM means more power. After you reach cruising altitude, you can use a higher pitch (taking a bigger "bite" of air) and a lower RPM setting to produce adequate thrust at a lower fuel consumption rate. Most of these systems use a high-pressure hydraulic oil system that opposes aerodynamic forces on the propeller to regulate blade pitch and, resultantly, RPM.

Because a higher pitch takes a bigger "bite" of air, there is a maximum engine power setting (measured with manifold pressure gauges on piston engines and turbine speed gauges on jet engines) for a given propeller RPM. Operating above those limits causes undue stress on the engine. In general, you should avoid high engine power settings with low RPMs -- just as you don't try to get your car moving from a standstill in top gear.
Position and hold
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Engine Transmissions - Why Not?

Sun Mar 06, 2005 1:08 am

Most large recip engines did have reducing gears (usually a planetary set) to get prop RPM lower than crankshaft RPM. Then they had the constant-speed props to optimize their performance. Turboprops are, AFAIK all reduced.

With most modern fanjet engines a different tack is used for optimization. That is variable-angle inlet guide vanes. These change angles in various regimes of operation. You can see the linkage for changing the angle around the outside of the compressor section of these engines. There is a lever for each blade and a ring that connects them all to an actuator.
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
ThePinnacleKid
Posts: 497
Joined: Wed Feb 23, 2005 9:47 am

RE: Engine Transmissions - Why Not?

Sun Mar 06, 2005 1:52 am

Quoting Air2gxs (reply 1):
The constant speed propeller and the adjustable propeller (for the life of me, I can't remember the "official" name).


-Constant-Speed (this one adjusts the prop blade angle, uses a prop lever to adjust RPM and a throttle to adjust manifold pressure)

-Fixed-Pitch (as it sounds, the blade angle is fixed and RPM is directly adjusted with throttle)

-Chris
"Sonny, did we land? or were we shot down?"
 
RaginMav
Posts: 335
Joined: Tue May 25, 2004 5:22 am

RE: Engine Transmissions - Why Not?

Sun Mar 06, 2005 5:30 am

Wasn't there an experiment with a transmission between the prop and powerplant on one of those really weird planes from the 50's? I think it was in the flying flapjack one, whatever that one is called. As far as I know it was not successful.

Another thing, why is the MU-2 (you know, the one that sounds like a damn industrial vacuum cleaner) so loud, while other turbo props aren't (to the same extent). I'm told it's the gear reduction that is making a good portion of the racket as they taxi around.


View Large View Medium
Click here for bigger photo!

Photo © Jarrod Wilkening

 
B2707SST
Posts: 1258
Joined: Wed Apr 23, 2003 5:25 am

RE: Engine Transmissions - Why Not?

Sun Mar 06, 2005 11:11 am

Pratt & Whitney has been researching geared turbofans for almost ten years now. Turbines generally function best at much higher RPMs than the bypass fan, so Pratt's designs use a planetary gear set to reduce fan RPM by a factor of about 3. This allows them to eliminte turbine stages, reducing weight and complexity. So far, though, it seems that the added weight and complexity of the gear system are still too high for geared turbofans to be broadly accepted.

http://www.flug-revue.rotor.com/FRheft/FRH9810/FR9810h.htm



Pratt is developing the geared PW800 in the 10,000-20,000 lb. thrust category for regional and busines jets, but no one has ordered so far. P&W pushed the larger 25,000-35,000 lb. PW8000 as an option for the A318, but Airbus never bit, and given Pratt's disastrous experience with the PW6000, the PW8000 may never see service.

Pratt & Whitney may have offered a geared design for the 7E7 (I can't recall if their proposal was conventional or geared), but were of course passed over in favor of RR and GE.

Another Flug Revue piece on advanced engines, including the geared turbofan:

http://www.flug-revue.rotor.com/FRheft/FRHeft04/FRH0401/FR0401d.htm

--B2707SST
Keynes is dead and we are living in his long run.
 
MD11Engineer
Posts: 13916
Joined: Sun Oct 26, 2003 5:25 am

RE: Engine Transmissions - Why Not?

Sun Mar 06, 2005 12:39 pm

Helicopters need a clutch to allow free rotor movement in case of an engine failure to get back to the ground using autorotation. It is usually a type of centrifugal clutch.

Jan
Je Suis Charlie et je suis Ahmet aussi
 
SlamClick
Posts: 9576
Joined: Sun Nov 23, 2003 7:09 am

RE: Engine Transmissions - Why Not?

Mon Mar 07, 2005 2:15 am

Whoever created that graphic must not live in this solar system. Where is it that stars orbit suns?

I think the "star" gears should be planet gears, else why is it called a planetary gear system?
Happiness is not seeing another trite Ste. Maarten photo all week long.
 
G4Doc2004
Posts: 120
Joined: Thu Feb 19, 2004 7:49 am

RE: Engine Transmissions - Why Not?

Mon Mar 07, 2005 12:55 pm

"why is the MU-2 (you know, the one that sounds like a damn industrial vacuum cleaner) so loud, while other turbo props aren't (to the same extent). I'm told it's the gear reduction that is making a good portion of the racket as they taxi around."

The MU-2 utilizes 2 Garrett (nee Honeywell) TPE-331 engines, that at 100% RPM have a shaft speed of 41,000 RPM. On the ground, the engine is "idling" at around 80% RPM. The noise you hear is the prop spinning at a high RPM (~3000 RPM) and the high frequency noise generated by the first stage centrifugal compressor.
"Failure to prepare is preparing to fail"--Benjamin Franklin
 
411A
Posts: 1788
Joined: Mon Nov 12, 2001 10:34 am

RE: Engine Transmissions - Why Not?

Mon Mar 07, 2005 1:45 pm

It indeed may surprise some here to learn that 'shifting' with large radial engines used on piston-engined airliners was quite common.
The 'shift' was done to the engine supercharger (blower) and was normally completed (ie: shifting from low to high blower) at 'round about 12,000msl, in the climb.
The general procedure was to reduce the throttle slightly, then shift two blowers at a time (on 4-engine types), then reset climb power/BMEP.

A conical clutch/drive gear arrangement internally in the engine was used to spin the blower impeller.

Worked very nicely...and was quite reliable.
 
caboclo
Posts: 175
Joined: Thu Nov 11, 2004 12:34 pm

RE: Engine Transmissions - Why Not?

Tue Mar 08, 2005 3:11 pm

One more point which hasn't come up yet: cars need transmissions because they are always starting and stopping and changing speed. Aircraft engines are generally designed to work best at cruise speed, with sacrifices made for the other regimes of flight for simplicity. Therefore, on aircraft with reduction gears, there is only one gear ratio which is set to optimize all the variables at cruise speed. Other factors, such as the TPE331's horrible noise, are more or less overlooked. Another example fo this engineering philosphy is vacuum advance, or the lack there of. I can't remember about the big radials, but small aircraft piston engines don't have variable ignition timing; you just set it for cruise and let it idle rough.
Freight dogs have more fun

Who is online

Users browsing this forum: VTBDflyer and 8 guests