Would a plane climb better if its engines stayed level?

[DaveBB]

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?

[Serenity]

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.

[NatCigg]

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?

 

[FLS]

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.

[Serenity]

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?

 

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.]

[Zimme83]

[zxrex]

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.

[Snork]

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.

[Busher]

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!

[Skuzzy]

Hmmm,..I wonder what the gyroscopic force of a jet engine is?

[FLS]

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.

[Vraciu]

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. 

[eagl]

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...

[eagl]

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.

[Serenity]

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!