Aces High Bulletin Board
General Forums => The O' Club => Topic started by: DaveBB on June 29, 2017, 08:41:51 PM
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Lets use an MD-80 as an example. Twin engined jet. Both engines are mounted near the tail. Would the aircraft climb better if the engines articulated so that they stayed parallel to the ground as the plane increased its angle of attack?
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Without cranking the numbers, I'm going to say no, because the engines are no longer in line with the relative airstream. They'll produce reduced efficiency while increasing drag because the air is now hitting the lower portion of the intakes at an oblique angle.
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serenity sounds logical. air density is variable enough and so diffuse that gravity would have little affect on performance of the engine itself. more importantly would be the movement of the air which would be dictated by the planes movement. at takeoff and during maneuvers the engine intake would offset to the angle of attack although at 130 mph this could be negligible and quickly corrected. once angle of attack stabilizes the engine would be parallel to relative air intake, the grounds position would be irrelevant as far as the air is concerned.
i thought the engines were at a angle? some of them move? seems a engine would always want to be at zero angle of attack unless thrust was used to increase angle of attack or is vectored thrust called "pushing" the tail?
:airplane:
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Lets use an MD-80 as an example. Twin engined jet. Both engines are mounted near the tail. Would the aircraft climb better if the engines articulated so that they stayed parallel to the ground as the plane increased its angle of attack?
You want thrust in line with direction of travel. It's the wings that get tilted (angle of incidence) for efficiency.
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i thought the engines were at a angle? some of them move? seems a engine would always want to be at zero angle of attack unless thrust was used to increase angle of attack or is vectored thrust called "pushing" the tail?
:airplane:
To my knowledge, most engines ARE slightly off of parallel with the fuselage of the body, but only very slightly, and because the airstream doesn't necessarily run perfectly parallel. As for engines moving, I don't know of any. The inlets themselves on high performance fighters DO move, but that has more to do with transonic aerodynamics than anything else. (The ONE thing I learned from my API engines class lol).
[Explanation: In subsonic airflow, as the inlet gets larger, the air flowing into it slows down. The exhaust gas coming out of the aircraft at subsonic speeds moves faster as the outlet gets smaller. Once you cross into supersonic speeds, the opposite is true. Larger inlet = faster airflow, larger nozzle = faster exhaust. So, with high-performance jets expected to perform on both sides of that boundary, variable inlets and variable exhaust nozzles are necessary to maintain smooth flow and achieve the desired engine performance.]
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(http://rotatesim.com/wp-content/uploads/2015/11/MD-80-screenshot-43.jpg)
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There are other considerations when mounting the engines. Their effect on stability is one. Seems alot of a/c have them mounted so the thrust line is in a good average position. Variable position would add weight, cost etc; that could offset any benefit.
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Another factor would be the angle of the thrust vector. If it stays in line with the plane it would be driving up somewhat as the plane angles upward.
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You want thrust in line with direction of travel. It's the wings that get tilted (angle of incidence) for efficiency.
On fixed-wing aircraft, the angle of incidence (sometimes referred to as the mounting angle) is the angle between the chord line of the wing where the wing is mounted to the fuselage, and a reference axis along the fuselage (often the direction of minimum drag, or where applicable, the longitudinal axis. I think you might have meant angle of attack.
Lift on a fixed wing aircraft comes from the forward movement of the wings provided by (in the case of a jet) the thrust vector provided by the engine(s). In the case of propeller aircraft (excepting pusher props) some additional lift can be provided by the propeller blast over the wing surface at higher than idle power settings.
Varying the thrust vector of a jet would introduce huge trim implications for the engineer designing the airframe when considering that safety regulations would dictate control of the aircraft would have to be maintained should the system varying engine thrust fail.
With respect!
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Hmmm,..I wonder what the gyroscopic force of a jet engine is?
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On fixed-wing aircraft, the angle of incidence (sometimes referred to as the mounting angle) is the angle between the chord line of the wing where the wing is mounted to the fuselage, and a reference axis along the fuselage (often the direction of minimum drag, or where applicable, the longitudinal axis. I think you might have meant angle of attack.
Lift on a fixed wing aircraft comes from the forward movement of the wings provided by (in the case of a jet) the thrust vector provided by the engine(s). In the case of propeller aircraft (excepting pusher props) some additional lift can be provided by the propeller blast over the wing surface at higher than idle power settings.
Varying the thrust vector of a jet would introduce huge trim implications for the engineer designing the airframe when considering that safety regulations would dictate control of the aircraft would have to be maintained should the system varying engine thrust fail.
With respect!
No I meant incidence. I don't mean it's variable. The engine and wing are likely aligned for most efficient cruise performance. The wing incidence sets the fuselage angle for minimum drag. I imagine the engine angle is set for most efficient thrust.
Rotating the engines like BB suggested is fine for fighters but the pitch response doesn't help your climb rate.
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Hmmm,..I wonder what the gyroscopic force of a jet engine is?
Depends on the engine, but it's there. We trim the rudder differently as flight regime and engine power changes. Tail-mounted engines throw the ball around a bit.
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Hmmm,..I wonder what the gyroscopic force of a jet engine is?
It can be significant but usually only in departure / loss of control situations (in addition to throttle-up/down, which are present but not as dramatic). Standard gyroscopic effects. Nose drops in a stall, the motors will twist to the side, adding a yaw rate, and as we all know, stall plus yaw equals spin...
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I climb/descent calculations, thrust angle is calculated and thrust angle against gravity actually helps the efficiency side of the calculation when doing a minimum-fuel climb/cruise/descent profile. Not much, but its there. No I can't regurgitate the formula since it was 25 years ago, but we spent a week or two in aero engineering back at USAFA on the calculations involved in climb/cruise/descent and various other efficiency factors in aircraft design. Bottom line, a climbing aircraft with the engines angled along the wing chord (or very close to it) is more efficient in small part due to some of the thrust angled down against gravity.
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No I can't regurgitate the formula since it was 25 years ago, but we spent a week or two in aero engineering back at USAFA
Phew, for a second you had me worried they were going to expect me to do that kinda math in the near future!
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Planes generally have a lift to drag ratio greater that 1. As long as that is the case, the engine is better used to offset drag than contributing directly to lifting the plane by pointing the jet downwards. This is why planes with anvery low thrust / weight ratio can fly at all.
If you incease engine power, speed will increase till the drag is againt equal to thrust and the climb rate incease too ( lets say we fly at maximal angle of attack to keep it a constat). But as speed increase, total drag increase till L/D ratio is smaller that one. At that point, diverting the engine thrust so part of it directly contribute to lifting the plane may start to pay off. Planes normally do not fly at that regime, so it is better to align the engine with the airflow.
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I climb/descent calculations, thrust angle is calculated and thrust angle against gravity actually helps the efficiency side of the calculation when doing a minimum-fuel climb/cruise/descent profile. Not much, but its there. No I can't regurgitate the formula since it was 25 years ago, but we spent a week or two in aero engineering back at USAFA on the calculations involved in climb/cruise/descent and various other efficiency factors in aircraft design. Bottom line, a climbing aircraft with the engines angled along the wing chord (or very close to it) is more efficient in small part due to some of the thrust angled down against gravity.
so if i may. the md80 is angled upward to be in better position because of airflow over the wing (wing chord?) and it is noted a additional "upwards boost" is achieved from downward vector thrust?
Or does wing chord have a angle with respect to the "angle" the wing is mounted?
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Engine alignment is a trade-off between cruise efficiency and pitch changes encountered by changes in thrust.
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Pivoting engines AND a conveyor belt takeoff, now we're talking.
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Phew, for a second you had me worried they were going to expect me to do that kinda math in the near future!
USAF UPT students get to see/work the equations, but in a session lasting an hour or two. Just familiarization to "prove" that a min-fuel profile in a jet involves a full power climb to a pre-calculated optimum cruise altitude, a max range cruise, followed by an idle descent.
In the T-6 the min-fuel profile is a bit different due to differences between props and jets, partly because an idle descent results in a ton of prop drag.
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so if i may. the md80 is angled upward to be in better position because of airflow over the wing (wing chord?) and it is noted a additional "upwards boost" is achieved from downward vector thrust?
Or does wing chord have a angle with respect to the "angle" the wing is mounted?
If the engines are angled up, it could be to zero out the thrust angle in cruise. Or it could be to reduce the horizontal stab / elevator load during cruise which would decrease induced drag from the horiz stab and elevator.
And yes wing chord determines wing angle of incidence. Wing and tail incidence must balance out nose-down pitching moment from the wing, and it may be adjusted one way or another relative to fuselage, tail, or engines, to optimize climb, cruise, or takeoff/landing performance. Tradeoffs everywhere.
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I don't know anything, and I mean anything, about aerodynamics. But I do know something about being snappy, or drawing attention to details...
So: What kind of a climb are we discussing? VTOL planes that can climb relatively fast by turning their engines' thrust towards the ground.
The title makes me think of F1 cars where they use wing type surfaces to keep the tyres, i.e. the parts that create movement, on the track at an optimal grip. In planes there's no need for a ground connection, on the contrary, but it makes sense to try to push the plane to the direction where you want to go.
Another thing that comes to mind is the water jets where there's no flying surfaces whatsoever. All directions are created by adjusting the thrust angle.
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USAF UPT students get to see/work the equations, but in a session lasting an hour or two. Just familiarization to "prove" that a min-fuel profile in a jet involves a full power climb to a pre-calculated optimum cruise altitude, a max range cruise, followed by an idle descent.
In the T-6 the min-fuel profile is a bit different due to differences between props and jets, partly because an idle descent results in a ton of prop drag.
The differences between the Air Force and Navy still astound me lol. When they told us the bingo profile (full power climb, idle descent as you say) they literally said "just press the 'i believe' button" and that was all there was to it, no one ever hesitated or questioned it lol.
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Interesting responses. So the consensus is that an aircraft will climb better with the engines aligned with the airstream, even though the thrust is diminished in the X-axis and slightly increased in the Y-axis.
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Interesting responses. So the consensus is that an aircraft will climb better with the engines aligned with the airstream, even though the thrust is diminished in the X-axis and slightly increased in the Y-axis.
The thing is, you're thinking of the X and Y axes as being in relation to the horizon, but (if I understand aero) they should be relative to the airflow.
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What about if...
...it was on a conveyor belt?
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It depends on what color the plane has.
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Interesting responses. So the consensus is that an aircraft will climb better with the engines aligned with the airstream, even though the thrust is diminished in the X-axis and slightly increased in the Y-axis.
You want to maximize climb rate. Thrust is fixed. Your option is minimize drag. The thrust to drag ratio determines the optimal climb angle.
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What about if...
...it was on a conveyor belt?
I think you are on to something here. The two factors combined would help take off immensely.
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I wouldn't want to do V1 cuts or any single engine work for that matter withthe proposed design.