TheFLCowboy From United States of America, joined Jan 2004, 405 posts, RR: 0 Posted (9 years 11 months 3 days 1 hour ago) and read 2990 times:
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?
Air2gxs From , joined Dec 1969, posts, RR:
Reply 1, posted (9 years 11 months 3 days ago) and read 2967 times:
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 From United States of America, joined Sep 2003, 3152 posts, RR: 10
Reply 2, posted (9 years 11 months 3 days ago) and read 2952 times:
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.
Bri2k1 From United States of America, joined Dec 2004, 988 posts, RR: 4
Reply 3, posted (9 years 11 months 2 days 23 hours ago) and read 2942 times:
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.
SlamClick From United States of America, joined Nov 2003, 10062 posts, RR: 67
Reply 4, posted (9 years 11 months 2 days 22 hours ago) and read 2921 times:
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.
RaginMav From United States of America, joined May 2004, 376 posts, RR: 1
Reply 6, posted (9 years 11 months 2 days 18 hours ago) and read 2830 times:
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.
B2707SST From United States of America, joined Apr 2003, 1373 posts, RR: 59
Reply 7, posted (9 years 11 months 2 days 12 hours ago) and read 2750 times:
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.
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:
G4doc2004 From United States of America, joined Feb 2004, 123 posts, RR: 0
Reply 10, posted (9 years 11 months 1 day 11 hours ago) and read 2563 times:
"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 From United States of America, joined Nov 2001, 1826 posts, RR: 8
Reply 11, posted (9 years 11 months 1 day 10 hours ago) and read 2555 times:
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.
Caboclo From United States of America, joined Nov 2004, 203 posts, RR: 0
Reply 12, posted (9 years 11 months 8 hours ago) and read 2491 times:
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.