Kaddyuk From Wallis And Futuna Islands, joined Nov 2001, 4125 posts, RR: 38 Reply 1, posted (3 years 1 month 3 weeks 3 days 22 hours ago) and read 1601 times:
Quoting Captaink (Thread starter): Anyhow knows the principle behind this? How does it work?
They work in the same way that props infront of the aircraft work... just they're somewhat more complicated due to the rear prop...
Whoever said "laughter is the best medicine" never had Gonorrhea
Captaink From Mexico, joined May 2001, 4339 posts, RR: 15 Reply 2, posted (3 years 1 month 3 weeks 3 days 21 hours ago) and read 1580 times:
Quoting Kaddyuk (Reply 1): They work in the same way that props infront of the aircraft work... just they're somewhat more complicated due to the rear prop...
Well that is what I am questioning.. the complications involved. Where is the air intake? Where does the thrust go?
Starlionblue From Greenland, joined Feb 2004, 13564 posts, RR: 68 Reply 3, posted (3 years 1 month 3 weeks 3 days 21 hours ago) and read 1578 times:
Some aircraft with pushers:
- B-36.
- Piaggio Avanti.
- Beech Starship.
- Do-335 Pfeil (both puller and pusher prop).
- Adam A-500 (both puller and pusher prop).
In the case of the Do-335 and the A-500 the configuration with centerline pusher and puller eliminates asymmetric thrust concerns in engine out situations.
Quoting Captaink (Thread starter): There are some airplanes (can't remember the names right now) with their props facing backwards.
As Kaddyuk says, the principle is the same. One of the reasons for using a pusher on a wing mounted engine is to avoid prop turbulence interfering with lift. However, there may be disadvantages. Quoting from Wikipedias B-36 entry:
The design of the R-4360 engine tacitly assumed that it would be mounted in the following conventional tractor configuration:
carburetor -- 28 cylinders -- air intake -- propeller
with air flowing from right to left. The carburetor is bathed in air warmed by engine cooling, and so is unlikely to ice up. The B-36 employed the following pusher configuration:
propeller -- 28 cylinders -- carburetor -- air intake
Because the carburetor is now in front of the engine, it cannot benefit from engine heat. Hence when the intake air was cold and humid, ice gradually obstructed the carburetor air intake, increasing the richness of the air/fuel mixture, until the unburned fuel in the exhaust caught fire. Engine fires of this nature led to the first loss of an American nuclear weapon, described above. Adding electric heating to the carburetor intakes eliminated the problem.
[Edited 2006-09-17 22:33:55]
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
Jetlagged From United Kingdom, joined Jan 2005, 2027 posts, RR: 13 Reply 5, posted (3 years 1 month 3 weeks 3 days 21 hours ago) and read 1557 times:
One disadvantage of pusher (as opposed to tractor) propellers is that they generally operate in the wing wake, so there is some loss of efficiency.
There have been enough pusher prop aircraft to show that it is a viable option. Quite a few WW1 fighters had pusher propellers until interrupter gear could be developed to allow a forward firing machine gun (e.g. DH-2).
FredT From United Kingdom, joined Feb 2002, 2101 posts, RR: 29 Reply 6, posted (3 years 1 month 3 weeks 21 hours ago) and read 1291 times:
Quoting Jetlagged (Reply 5): One disadvantage of pusher (as opposed to tractor) propellers is that they generally operate in the wing wake, so there is some loss of efficiency.
In a pusher config, the propeller gets the boundary layer air off the fuselage to chew up, which is a disadvantage.
One of the main advantages is, however, that you avoid having the propelller slipstream go all the way down the fuselage, which steals a lot of the efficiency of the propeller through drag.
The main problem is usually cooling. It is hard to put air intakes that far back, where the boundary layer is thick and the air flow changes significantly with e g varying AoA. That kind of air intake tends to come with lots of drag and/or poor air flow. Boundary layer ingestion etc.
You do lose the benefit of propeller slipstream over control surfaces.
Makes gun installation a lot easier. On the other hand, it makes ejecting harder. There's a reason for the Saab J21 having the first operational ejection seat!
Rgds,
/Fred
I thought I was doing good trying to avoid those airport hotels... and look at me now.
Usnseallt82 From United States, joined Jan 2004, 4885 posts, RR: 62 Reply 10, posted (3 years 1 month 3 weeks 19 hours ago) and read 1243 times:
Quoting 2H4 (Reply 8): During ground ops, debris can be thrown up from the gear into the rear prop.
I remember this being a problem with the CE-337 because just about anything that went into the front prop would get flung back to the rear. Also, if you ever made a hard landing or kicked up something with the nose gear, it would pop up in that rear one as well.
Needless to say, I rarely ever saw a 337 with a rear prop having NO nicks or dents on it.
Jetstar From United States, joined May 2003, 1245 posts, RR: 4 Reply 12, posted (3 years 1 month 3 weeks 16 hours ago) and read 1200 times:
Quoting Starlionblue (Reply 3): The design of the R-4360 engine tacitly assumed that it would be mounted in the following conventional tractor configuration:
carburetor -- 28 cylinders -- air intake -- propeller
with air flowing from right to left. The carburetor is bathed in air warmed by engine cooling, and so is unlikely to ice up. The B-36 employed the following pusher configuration:
propeller -- 28 cylinders -- carburetor -- air intake
Not true on the last sentence, I worked on the R-4360 used on the C-97 in my Air Force days, I was assigned to the engine shop and have extensive experience on this engine.
Contrary to popular belief, unlike all other engines that operate in the pusher mode like the Cessna 337 or the Beech Starship where the engine was reversed, the R-4360 was not turned around in the B-36. It always operated with the first bank of cylinders, the “A” bank in the front. All radial engines except the R-4360 have all the accessories like the starter, generator and hydraulic pump mounted on the rear case of the engine, on the R-4360 the accessories were mounted on the side of the rear case where the dive gears were. On the C-97 there are 2 DC generators on each inboard engine and a DC generator and an alternator on the outboard engines. On the back end of the engine there was a large 2 foot cover plate with nothing attached to it. On the B-36 version of this engine, the front case is blocked off with a cover plate and the prop shaft extended out of the rear case.
The R-4360 was designed to operate in either mode, depending on the version, but always facing in the same direction. There are 7 cylinders in each bank with the topmost cylinder number 1. Each row was lettered with the first row lettered “A, then B, C and D”, So the last row top cylinder is “D1” and the lowest cylinder is “D4”, so “A1” was in the same location on either the B-36 or C-97, the topmost front cylinder and if I remember correctly, it was over 40 years ago that I worked on these engines “A1” was designated the master cylinder for magneto timing. Incidentally on the B-36 and other earlier versions there was 7 magnetos, but on the later model engines which I worked on, the –59B there was only 4 magnetos. The magnetos can be seen in this photo, they are inside the oval shaped housing mounted on the upper side of the nose case.
If the engine were turned around, there would not be proper cooling of the cylinders. Normally in a twin row radial engine like the R-2800 or the R-3350 the rear row was offset to place the cylinders between the front row of cylinders to the rear row would get adequate cooling air. On the R-4360 each bank was offset about 20 degrees from the bank in front and each bank had its own tight baffling to keep control the cooling air. I never have seen an engine with as much baffling as the R-4360, but it worked. Anytime we had to work on the engine during scheduled engine maintenance had to remove all the top baffle covers and to change a cylinder we had to remove the side covers of that cylinder as well
In the photos, you cannot see the cylinders, but you see black cover plates that spiral rearward, hence the nickname the corncob engine. The brown tubes running the length of the cylinder row are part of the exhaust system. Each row of cylinders, there were seven had a single exhaust pipe and they fed into a collector ring.
Because of vibration, the exhaust pipes is not permanently attached to the exhaust port of the individual cylinder with nuts, but a spring loaded coupler we called a pineapple was used. When we had to change a cylinder, to install the pineapple we had to compress them and hold them compressed with safety wire and then put them in place and cut the safety wire and then the spring on the pineapple would expand and keep the joint sealed.
This was an amazing experience working on this engine, although in those days I used to curse the engines every time I worked on it, especially when working on the lower cylinders when you would have oil leaking all over you. One of the scheduled maintenance items was to check the valve timing on each valve every 200 hours, all 56 of them. If a valve was out of time, and at least 25 percent were we had to remove the valve cover and manually adjust the valve, they did not have hydraulic lifters all while oil was dripping from the open rocker boxes above you. We wore old oil stained clothes and hats but still oil would get on you and the drip pans on the floor making the area as slippery as ice, this was probably the worst job on the engine.
KELPkid From United States, joined Nov 2005, 4127 posts, RR: 7 Reply 14, posted (3 years 1 month 3 weeks ago) and read 1104 times:
Quoting FredT (Reply 6): The main problem is usually cooling. It is hard to put air intakes that far back, where the boundary layer is thick and the air flow changes significantly with e g varying AoA. That kind of air intake tends to come with lots of drag and/or poor air flow. Boundary layer ingestion etc.
Anyone who's owned a Cessna 337 for a while (like my former flight instructor who was also an air charter operator) will tell you that the front engine usually makes TBO, however the rear engine usually has to be pulled about 200-300 hours early. My former flight instructor would shut down the rear on the landing roll (once landing was assured), because, as he said, the cooling air intake was insuffecient for ground ops. He also didn't fire up the rear until being close to the run-up area, unless it was a really cold day (and the oil temp always came up faster on the rear than the front ). My instructor's 337 was a turbocharged, pressurized variant with the TSIO-360's front and rear.
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
KELPkid From United States, joined Nov 2005, 4127 posts, RR: 7 Reply 15, posted (3 years 1 month 3 weeks ago) and read 1102 times:
Quoting Starlionblue (Reply 3): Some aircraft with pushers:
- B-36.
- Piaggio Avanti.
- Beech Starship.
- Do-335 Pfeil (both puller and pusher prop).
- Adam A-500 (both puller and pusher prop).
Other notable certified aircraft with a pusher engine:
Cessna 337 (also had a tractor prop. Cessna's model name was Skymaster, but I've heard it called the "Mixmaster" and the "Push-me pull-me").
Lake amphibians
Republic SeeBee (the ugly plane from the early James Bond movies ).
Celebrating the birth of KELPkidJR on August 5, 2009 :-)
MD-90 From United States, joined Jan 2000, 7391 posts, RR: 17 Reply 18, posted (3 years 1 month 2 weeks 3 days 12 hours ago) and read 885 times:
Quoting EMBQA (Reply 4): They are no more or less complicated then forward props
There are two other considerations. Debris from the engine and the exhaust. The exhaust creates extra noise since it passes through the propstream (this is one reason why the Avanti has such a distinctive sound). In smaller aircraft, with exposed engines, anything that comes off the engine will go through the prop:
ThirtyEcho From United States, joined Dec 2001, 1264 posts, RR: 1 Reply 19, posted (3 years 1 month 2 weeks 3 days 12 hours ago) and read 880 times:
Another peculiarity of pusher props is from a Flying magazine article, many years ago, called "The Big, Bad B-36."
It seems that somebody was machine-gunning B-36s while flying at very high altitudes; the aircraft would come back with holes stitched through the empenage.
You guessed it. Small oil leaks were congealing on the engine nacelles and running back so that they were hard frozen. The props would then bat these "bullets" throught the vertical stabilizer.
On descent, the oil droplets would thaw and leave not a trace in evidence.
Starlionblue From Greenland, joined Feb 2004, 13564 posts, RR: 68 Reply 20, posted (3 years 1 month 2 weeks 3 days 6 hours ago) and read 845 times:
Quoting Sovietjet (Reply 17): Quoting Jetlagged (Reply 5):
propellers until interrupter gear could be developed to allow a forward firing machine gun
Sorry for the off-topic..but how did the interrupter gear work?
Actually on WW1 aircraft it wasn't really interrupter gear, it was technically a synchronizer. A cam on the propeller shaft pulled the trigger. When the pilot pulled his trigger, the cam was positioned in place to do this. This allowed firing to be synchronized with the prop and thus bullets would not hit the propeller blades.
Interrupter gear actually stops firing when the blades are in the way. There was no reliable way of constructing such a system in WW1.
Tact Is For People Who Aren't Witty Enough To Be Sarcastic
Bond007 From United States, joined Mar 2005, 4492 posts, RR: 11 Reply 21, posted (3 years 1 month 2 weeks 22 hours ago) and read 704 times:
Actually, remember when discussing airflow through the engine, that the most common turboprop, the PT-6 is 'backwards' to start with. The air, although it enters through the front (in a front facing prop), it is directed to the back of the engine and then forwards through the compressors etc. and is expelled out the front.....I think!
Jimbo
I'd rather be on the ground wishing I was in the air, than in the air wishing I was on the ground!
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). My instructor's 337 was a turbocharged, pressurized variant with the TSIO-360's front and rear.
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