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jgch
Topic Author
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Joined: Sun Nov 27, 2016 5:32 pm

Why has the C-17 four engines?

Thu Jan 20, 2022 9:34 am

As far as I know, there were more powerful engines available in the 80s and 90s than the ones chosen P&W engines for C-17.
Why has the C-17 designed with 4 engines?
 
mats01776
Posts: 171
Joined: Sun Apr 22, 2012 6:10 pm

Re: Why has the C-17 four engines?

Thu Jan 20, 2022 12:29 pm

Probably due to the issue with the take-off performance hit with one engine out.
 
Noray
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Re: Why has the C-17 four engines?

Thu Jan 20, 2022 12:47 pm

I guess it has a lot to do with the C-17's tactical capabilities.
* Blown flaps create lift for slow flight, which is important for airdrop and STOL. Two more engines with thrust to hit a larger area of flaps.
* Failure safety. With one engine failed/hit, there's still 3/4 thrust left.
 
GalaxyFlyer
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Re: Why has the C-17 four engines?

Thu Jan 20, 2022 4:22 pm

The military mission often requires the flexibility of four engines—often you cannot divert, for example, due to diplomatic concerns. PNAF, moving nuclear weapons, for instance. Operating in/out of Kabul, if necessary, doing an OEI departure, for another.

Yes, the Buddha, uses thrust across the wing for additional lift, two wouldn’t work.
 
Akita
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Re: Why has the C-17 four engines?

Fri Jan 21, 2022 1:30 am

And...C-17 is an evolved YC-15, which proved its very effective STOL capabilities using four P&W JT8Ds. As they inflated it up to a heavier weight class they simply went for a more powerful P&W turbofan.
 
JayinKitsap
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Joined: Sat Nov 26, 2005 9:55 am

Re: Why has the C-17 four engines?

Fri Jan 21, 2022 6:53 am

It was "Four Engines For Long Haul"
Besides it looks more awesome with 4,
Only the Buff has 8, all the better
 
TheSonntag
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Joined: Wed Jun 15, 2005 7:23 pm

Re: Why has the C-17 four engines?

Fri Jan 21, 2022 12:10 pm

For the A400M, Europe also went to 4 engines, while the Transall only had 2. The plane is about capabilities, not pure cost.
 
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flyingclrs727
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Re: Why has the C-17 four engines?

Fri Jan 28, 2022 12:13 am

jgch wrote:
As far as I know, there were more powerful engines available in the 80s and 90s than the ones chosen P&W engines for C-17.
Why has the C-17 designed with 4 engines?


Because it's a military aircraft that needs more redundancy to be able to continue flying to its destination even with an engine out. They have to be able to operate in warzones during wartime.
 
889091
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Joined: Sat Apr 06, 2019 7:56 pm

Re: Why has the C-17 four engines?

Fri Jan 28, 2022 12:19 am

....and the Air Force of whichever country that operates them - they don't really care how much Jet A1 costs....
 
GalaxyFlyer
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Re: Why has the C-17 four engines?

Fri Jan 28, 2022 12:31 am

I always liked the story of the airliner/private jet told to hold short of the dearm pad at a civil airport with an ANG base.

Civil plane: “how long will the wait be, we pay for our fuel?”
Lead: “you’re paying for our fuel, too.”
 
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Strebav8or
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Re: Why has the C-17 four engines?

Sun Jan 30, 2022 7:40 pm

Be cause three or five engines would look odd....????
 
tommy1808
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Joined: Thu Nov 21, 2013 3:24 pm

Re: Why has the C-17 four engines?

Mon Feb 07, 2022 11:34 am

jgch wrote:
As far as I know, there were more powerful engines available in the 80s and 90s than the ones chosen P&W engines for C-17.
Why has the C-17 designed with 4 engines?


No engine available that would do it with just two.

Best regards
Thomas
 
ELBOB
Posts: 381
Joined: Sun Jun 21, 2015 6:56 am

Re: Why has the C-17 four engines?

Wed Mar 02, 2022 2:38 pm

The PW2037 was chosen because of maintenance costs. For the first 3,000hrs of airframe operation the USAF was guaranteed a 100% discount on any spares required, and free labour. An engine replacement within 4,000hrs attracted a $40,000 penalty and there were penalties for IFSD rates. Only the PW2000 and RB211-534 met the reliability requirements, and the latter didn't have a US source ( though GE did consider a JV with RR for the contract ).

Once the engine was selected on that basis, the number required for the mission followed. Climb to 10,000ft had to be within 143 seconds of rotation.
 
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ehbowen
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Re: Why has the C-17 four engines?

Wed Mar 02, 2022 8:31 pm

Strebav8or wrote:
Be cause three or five engines would look odd....????


Izzat so?



(Tee hee!)




...and, yes, I know the 747 is ferrying a spare. But, still...
 
trex8
Posts: 5814
Joined: Sat Nov 02, 2002 9:04 am

Re: Why has the C-17 four engines?

Fri Mar 04, 2022 12:02 am

well MDC could have gone like Boeing and used 2 engines, the competing Boeing YC15 design to MDCs YC14, which eventually got developed into the C17, had 2 CF6s
https://en.wikipedia.org/wiki/Boeing_YC-14
 
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Aesma
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Re: Why has the C-17 four engines?

Fri Mar 04, 2022 2:04 am

First flight was 1991. At that time there was no twinjet that big. The 777's first flight was 1995. And militaries are conservative when they can.
 
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Strebav8or
Posts: 44
Joined: Sat Aug 29, 2020 4:03 pm

Re: Why has the C-17 four engines?

Wed Mar 09, 2022 6:15 pm

ehbowen wrote:
Strebav8or wrote:
Be cause three or five engines would look odd....????


Izzat so?



(Tee hee!)




...and, yes, I know the 747 is ferrying a spare. But, still...


LOL.....I was hoping someone would get me on these two....Don't forget the DC/MD/KC-10/-11
 
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ehbowen
Posts: 18
Joined: Mon Feb 28, 2022 10:49 am

Re: Why has the C-17 four engines?

Thu Mar 10, 2022 5:00 am

Strebav8or wrote:
ehbowen wrote:
Strebav8or wrote:
Be cause three or five engines would look odd....????


Izzat so?



(Tee hee!)




...and, yes, I know the 747 is ferrying a spare. But, still...


LOL.....I was hoping someone would get me on these two....Don't forget the DC/MD/KC-10/-11


Okay, how about this one:


Fun Fact (although probably well-known to this audience): The Douglas DC series originally started as a response to a request for a tri-motored airliner.

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