Why did mixing engine types never take off ?
- Thread starter VA5124
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Not entirely true. Where fitted they are generally available for use in the air in an emergency if required. Also for some longer ETOPS certifications it is a requirement that they are capable of being run in the air in case of engine shutdowns.
Actually, I should correct my word usage, not many APUs can be used when the engines are running rather than in the air (I also said not many, not none). There aren't that many aircraft types are ETOPS qualified. In earlier 737s the APU can't be run with main engine power on. I have never worked on 777s, A350s or Dreamliners, so I can't say about them. I've worked on A320s, but not engine systems, only airframe.
Using the APU inflight would only take place during a dual engine shut down. All big jet engines have AC and DC gennies attached, so even if your ETOPS type has a single engine failure, power to essential systems can still be run from the working engine gennies. The other thing for hydraulics is that big airliners have the equivalent to power transfer units (they might be called something else depending on the type) which work independently of the engine driven pumps and provide boost to hydraulic systems (on Airbus aircraft this sounds like a muffled 'barking' coming from beneath the floor of the pax cabin). These can be switched on in flight to aid in flap actuation and depending on the aircraft, undercarriage retraction. The APU would be used to provide bleed air for engine start in a dual engine failure, but in a single engine failure cross starts are possible without APU use.
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special ed
2nd Lieutenant
- 5,563
- May 13, 2018
With wingtip mounted engines, how does one manage an engine out flight or landing?
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Assuming that aircraft is capable of flight with an engine out situation, they'd most likely react the same as if the engines were mounted further inboard.
But looking at those stubby wings, I'm thinking that their best option would be to punch out in that situation.
The jets on the B-36 burned avgas to keep from having two fuel storage systems. As I understand it, 115/145 was hard on the jets and they didn't last long. I guess it wasn't a big problem, since they were only used on takeoff under normal circumstances. There were always a lot of deposits on the tail cone from the leaded fuel, but not being a jet engine expert, I don't know exactly what the internal effects were. I would suspect excessive heat would be a problem.
I would add that, for high performance aircraft, one would likely do better going with a clean sheet of paper design using jet engines. With jet engines, one could normally use a shorter undercarriage and it was worth making design choices for operation at high subsonic speeds.View attachment 669782
Now for WW II we can 'claim' that the piston engines were close to the turbo prop curve.
The orange and green curves didn't exist (n service engines).
We will go with the red turbojet curve, adjust a bit if needed.
Basically a mixed propulsion aircraft could not use both propulsion systems to very food efficiency at the same time.
Altitude will have some effect on this which is why the B-36 worked, kind of.
What complicates things is that the turbo jet had a much better power to weight ratio than the piston engine so it could provide a useful "booster" engine function.
However the turbo jet engines of the time got truly horrendous fuel consumption so you wanted to use them for very short periods of time.
For bombers or transport planes perhaps the "booster" engines did make sense. Even a B-29 could use between 400-800 gallons of fuel just climbing to cruise attitude depending on weight and perhaps a couple of "booster" engines and the fuel for them might have weighed less than running the piston engines for required time.
Most piston planes climbed at around 200mph (or less) where the turbo jet was horribly inefficient.
In single seat fighters the weight penalty of the "booster" engine gets harder to put up with since the proportion of aircraft weight for the power plant/s is a much higher percentage than the with percentage in a bomber or transport.
Any once the fighter is doing much over 500mph the prop is just slowing the airplane down.
The same way you would in any aircraft, very carefully, of course
Dave, the Trident didn't have an ejection seat, the whole forward fuselage encompassing the cockpit broke away during time of emergency. Yeah, not the best idea, but the 50s and 60s were full of real crazy ideas and crew survival appeared to be low on the priority list back then!
GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Yeah, I recall reading that the French borrowed some ideas from the Germans, the rocket engine initially was based on the Me163's Walther engine and the detachable cockpit was based on the DFS228 and He176 concepts.
Looking at the size of the rudder/vertical stabiliser, I think the best course of action is the handle between your knees, especially if its on take-off.The same way you would in any aircraft, very carefully, of course
Yup, as you know, the French aviation industry survived WW2 by being a shadow German production facility during the war.
Nuuumannn,
I currently fly the A320/319 and we use the APU for engine start, electrical, and environmental. And that's for both ground and air. Ive flown the 757 / 767 and we used the APU for all the above plus as a third electrical source when doing Cat 3 (autoland) approaches (it's how the plane was certified IIRC). I plumbed the DC-10, DC-8, and the 727 (the latter two being very "vintage jets"). The DC-10 used the APU as a backup electrics, starter, and environmental the same as the modern twins.
As for when you use the APU airborne it's really for the same reasons as on the ground: engine start, electrical (generator), and environmental (pressurization). It can't be used for de-ice / anti-ice, but it can raise your service ceiling should a motor lose or not have bleed air. As for electrical, on the airliners I've flown, they can't power the entire electrical load from one engine driven generator. In the event one should fail we start the APU (predominately non-essential electrics are load shed with the biggest draw being the galley). Airborne use can be summed up as another level of redundancy.
As for hydraulics it's as you mentioned. Douglas called them Reversable Motor Pumps, and Airbus calls them Power Transfer Units (the barking dog).
On ETOPS, from what I remember the APU was held to a higher standard of reliability. We would on occasion get a message to start the APU at altitude to verify its operations/ reliability. As for the non-ETOPs twins I can't answer, however my airliner is limited to 162NMs offshore for planning and that's due to life raft configuration from what I've been told.
Cheers,
Biff
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GrauGeist
Generalfeldmarschall zur Luftschiff Abteilung
Yes indeed.
And if I remember correctly, France was one of the largest post-war operators of Luftwaffe aircraft.
SaparotRob
Unter Gemeine Geschwader Murmeltier XIII
...and PzKpfw Vs.
When I was a kid, my brother's "Observers book of Aircraft" seemed to have a lot of experimental V/Stol aircraft with vertically mounted lifting engines. Apart from the difficulty on controlling them on take off, in level flight there is a lot of dead weight and space. Dassault Mirage IIIV - Wikipedia
Oooo good stuff, thanks Biff.
drgondog
Major
Simplest answer is operational logistics - mixing spare parts, coolant and different fuels would have been foolish. Would have been just as dumb to put two Allisons to combine with Wright's on a B-17.
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