Why aren't there more aircraft with engines mounted above?

Non-military aviation
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KamenRiderBlade

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Unread post09 Nov 2015, 00:14

https://en.wikipedia.org/wiki/Honda_HA-420_HondaJet

https://en.wikipedia.org/wiki/Antonov_An-74

https://en.wikipedia.org/wiki/Boeing_YC-14

It's not like this design / configuration hasn't been attempted before and I know there are technical / aerodynamic advantages to having the engines mounted on top, but I'm still confused why this configuration isn't more popular in the civilian commercial or cargo aviation category?
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popcorn

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Unread post09 Nov 2015, 00:28

The investment in ladders would be prohibitive.
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eloise

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Unread post09 Nov 2015, 00:32

i think the reason is because when aircraft fly at high AoA, the engine will stall due to lack of air
Last edited by eloise on 09 Nov 2015, 03:25, edited 1 time in total.
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KamenRiderBlade

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Unread post09 Nov 2015, 02:21

eloise wrote:i think the reason is because when aircraft fly at high AoA, the engine will due to lack of air


What airliner / cargo aircraft need to do high AoA?
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johnwill

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Unread post09 Nov 2015, 03:13

Thrust from a high mounted engine produces a nose down pitching moment. That moment must be balanced by a down load on the horizontal tail. Thus more wing lift is needed to overcome that down tail load. Heavier wing, more drag, etc.
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Unread post09 Nov 2015, 03:26

KamenRiderBlade wrote:What airliner / cargo aircraft need to do high AoA?

sorry , i thought you mean fighter
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sferrin

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Unread post09 Nov 2015, 03:28

johnwill wrote:Thrust from a high mounted engine produces a nose down pitching moment. That moment must be balanced by a down load on the horizontal tail. Thus more wing lift is needed to overcome that down tail load. Heavier wing, more drag, etc.


What are your thoughts on the YC-14?
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Unread post09 Nov 2015, 05:46

johnwill wrote:Thrust from a high mounted engine produces a nose down pitching moment. That moment must be balanced by a down load on the horizontal tail. Thus more wing lift is needed to overcome that down tail load. Heavier wing, more drag, etc.


Actually, when the thrust is inline with the center of mass this isn't true at all. You're talking about a torsion load, which is entirely dependent on the center of mass. The low mounted wing is meant to counter the high mount engine.
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johnwill

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Unread post09 Nov 2015, 06:07

Nevetheless, an engine mounted above the wing will generate more negative pitch moment from thrust than an engine mounted below the wing. So an incremental down load on the tail is required for pitch balance. That incremental down load on the tail may be in the form of less up load, but it is still there.
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KamenRiderBlade

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Unread post09 Nov 2015, 06:15

johnwill wrote:Nevetheless, an engine mounted above the wing will generate more negative pitch moment from thrust than an engine mounted below the wing. So an incremental down load on the tail is required for pitch balance. That incremental down load on the tail may be in the form of less up load, but it is still there.


https://www.youtube.com/watch?v=nxvV0kaLGgk

https://www.youtube.com/watch?v=nxvV0kaLGgk

So where in this video is the tail having to compensate for pitch?
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johnwill

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Unread post09 Nov 2015, 07:00

sferrin wrote:
johnwill wrote:Thrust from a high mounted engine produces a nose down pitching moment. That moment must be balanced by a down load on the horizontal tail. Thus more wing lift is needed to overcome that down tail load. Heavier wing, more drag, etc.


What are your thoughts on the YC-14?



I actually got to see the YC-14 fly at Edwards AFB. They were doing short rough field tests on some dirt runways west of the main base. Talk about a cloud of dust. I always wondered about the problem of engine failure right after takeoff, a double handfull of roll and yaw problems. Less concern for YC-15, as I think it had four engines blowing under the flaps.

An-72 seems to work OK.
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Unread post09 Nov 2015, 07:12

KamenRiderBlade wrote:
johnwill wrote:Nevetheless, an engine mounted above the wing will generate more negative pitch moment from thrust than an engine mounted below the wing. So an incremental down load on the tail is required for pitch balance. That incremental down load on the tail may be in the form of less up load, but it is still there.


https://www.youtube.com/watch?v=nxvV0kaLGgk

https://www.youtube.com/watch?v=nxvV0kaLGgk

So where in this video is the tail having to compensate for pitch?


Throughout the entire video. Doesn't mean you can actually see load on the tail, but it is there. If the engines were under the wing, there would be less down load on the tail.
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Unread post09 Nov 2015, 14:30

Guess that goes for the An-32 as well.
There is a the FOD issue for a tactical short take-off/landing cargo plane. Guess higher up engine means better.
One other reason for the top mounted engine was an increased prop size.

Wouldn't a modern FBW negate any side effects of haveing top mounted engines?
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Unread post09 Nov 2015, 17:03

FBW could reduce pilot work load by autotrim function, but the negative pitch moment is still there. It might also allow a more aft CG location to reduce trim load effects. The pitching moment effect I mentioned is not a serious problem, but is simply one of many considerations when laying out a new airplane design. Take a look at the VFW-614 for an unsuccessful attempt at overwing engine installation.
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Unread post10 Nov 2015, 00:10

With a small cargo plane / airliner like the AN-72 with it's STOL capabilities, what markets could open up in the civilian world by connecting major hubs to small towns with short airfields?
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