TurboTristar From , joined Dec 1969, posts, RR: Posted (11 years 4 months 1 week 6 days 6 hours ago) and read 2915 times:
Why do the Avro RJ/ BAE 146 have the engines mounted on top of the fuselage? Is it because increased lift or is that why the plane is so quiet? I am asking this for my dad because he would like to know also.
Aerokid From Belgium, joined Jun 2000, 348 posts, RR: 1 Reply 1, posted (11 years 4 months 1 week 5 days 23 hours ago) and read 2808 times:
Hi there,
I have yet to see the first BAe 146 with the ENGINES mounted on top of the FUSELAGE!
Anyway, I get your question. Aircraft with a so called "high wing" have better roll stability because the mass of the fuselage acts like a damper. When the aircraft goes into a roll maneuvre because of a gust, the mass of the fuselage will force the aircraft to level again.
Also, the lift of any wing is caused by decreased pressure (vacuum) on top of it and increased pressure below it. The vacuum on top of the wing accounts for more than 60% of the total lift produced. By placing the wing on top of the fuselage, the airflow on top of the wing (which is the most effective) is not distorted by the boundary layer of the fuselage.
There may be other reasons but that's all I can think right now.
Prebennorholm From Denmark, joined Mar 2000, 5810 posts, RR: 56 Reply 2, posted (11 years 4 months 1 week 5 days 16 hours ago) and read 2791 times:
Dear TurboTristar,
There are three basic configurations for jet transport planes. They all have their advantages or disadvantages:
1. Low wing, wing mounted engines, wing mounted landing gear.
2. Low wing, tail mounted engines, wing mounted landing gear.
3. High wing, wing mounted engines, fuselage mounted landing gear.
1. Low wing and wing monted engines gives long and heavy landing gear structures. On large planes they are needed anyway to give tail ground clearance, so that's the preferred configuration on large planes.
The 737 is kind of compromise. Landing gear is rather short, but then the engines are mounted so high on the wing that they disturb the airflow on the wing slightly.
2. Tail mounted engines are popular on smaller planes. But it calls for a generally stronger and heavier structure because the heavy weight of the engines is not placed where the lift is generated.
3. The high wing and fuselage mounted landing gear has a too narrow landing gear track. And then the passengers always bang their forehead against the wing main spar which passes through the cabin ceiling - yes, I did that myself on the 146!!!
So there you see, three different configurations, three different sets of disadvantages.
The 146 is supposed to be a hot runway performer. That's probably the reason why they chose the only configuration which doesn't pay with weight penalties or earodynamic penalties.
That's only my guess. It's a very interesting subject. But I shouldn't wonder if I am right. You see the same on other planes which were also designed for ultimate runway performance, most noteably the milirary C-17 and IL-76.
I remember many years back when the 737 was designed. Everybody was wondering why Boeing didn't put the engines on the tail as on the 727. Boeing said that by putting the engines on the wing they saved 1500 lbs of structure, and that more than compensated for the disadvantage having the engines disturb the airflow slightly on the wing. There is probably no exact result on that calculation. We see that most competitors (Caravelle, BAC 1-11, DC-9) went the other way.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs, Preben Norholm
EssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 2 Reply 3, posted (11 years 4 months 1 week 5 days 1 hour ago) and read 2740 times:
Hmmm...
Delta wings don't count? What about midwing, above mounted engines? Check Jane's...
RE: item 2: thrust is a force, or a line of action, and doesn't care where lift is generated. Item 2's explanation is weak...Where does the Dash 8 fit in to this classification?
Aerokid From Belgium, joined Jun 2000, 348 posts, RR: 1 Reply 4, posted (11 years 4 months 1 week 4 days 22 hours ago) and read 2726 times:
For military (transport) aircraft, the criterium for choosing a high wing is whether or not the aircraft will operate in remote areas without asphalt runways. If so, a high wing is prefered because you want the engines mounted as far away from the ground as possible to make sure that as little as possible dust and dirt is sucked in by the engines.
Prebennorholm From Denmark, joined Mar 2000, 5810 posts, RR: 56 Reply 6, posted (11 years 4 months 1 week 4 days 12 hours ago) and read 2699 times:
EssentialPowr wrote:
------------------------------
RE: item 2: thrust is a force, or a line of action, and doesn't care where lift is generated.
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Item 2 talks about engine weight and airframe weight and does not mention engine thrust.
EssentialPowr wrote:
------------------------------
Item 2's explanation is weak...Where does the Dash 8 fit in to this classification?
------------------------------
Dash 8 is not a jet plane. Therefore I excluded it on line #1.
Most turboprops hide the landing gear behind the engines, or at least behind the fat gear box. That way it adds very little drag. That's not possible on a jet plane.
Best regards, Preben Norholm
Always keep your number of landings equal to your number of take-offs, Preben Norholm
EssentialPowr From United States of America, joined Sep 2000, 1820 posts, RR: 2 Reply 7, posted (11 years 4 months 1 week 4 days 10 hours ago) and read 2700 times:
"That's probably why they chose the only config that doesn't pay with weight or aerodynamic penalties..."
Any a/c design is a series of compromises. If the above statement were correct, every a/c design would be a high wing. Landing gear become more complex, and therefore heavier, when all of the componentry is stowed in the body of the aircraft. That takes cargo room, or forces the a/c packs and hydraulic pumps to be relocated to less than optimal locations. If a separate fairing is used for the gear, a la C17, what about the additional weight and drag that adds?
The reason the C17/141/5/130 and IL 76 are high wing aircraft is for STOL performance and flap clearance, but more importantly, to enable paratroopers and palletized loads to be dropped. Low wing a/c aren't very suitable for this primary purpose of a mil transport.
What about flight characteristics and stability? In the 737/57/67/47/77, the tail has to have more authority, and therefore be larger with more drag, when compared to an equivalent tail mounted application b/c the line of thrust is so far from the center of gravity.
From item #1, tall gear are needed for engine, not tail clearance. The 767-300 and 727-200 both have tailskids to protect from a tailstrike. Boeing designed the 737 to sit as low as possible to maximize maintainablity, as a result the engine cowlings are squared off to provide clearance.
Your explanation was way too simplistic...but a good attempt.
Mit From United States of America, joined Dec 2000, 166 posts, RR: 0 Reply 8, posted (11 years 4 months 1 week 4 days 10 hours ago) and read 2691 times:
I can't say much about the wing position contributing to STOL performance, but the 4 engines are a major contributor. An engine loss on takeoff extracts a vastly smaller perfomance penalty in the 146 than in a twin engine plane. That's why you see them in places like Aspen and Bhutan.
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