I see what you are saying. I definitely mis-spoke when I said:
As you say, the engines will tend to push the nose down while the stab will tend to push it up. I stand corrected.
Right. You’ll notice that most jets fly with a nose-up attitude. This would put the engines in line with the airflow. Then angle of incidence is built into the airframe, so the wings will be at a set angle of attack in cruise. The engines would have an angle of incidence such that the airflow is “straight in” at cruise.
The thrust line and CG are two different issues. Weight and balance is the aerodynamic balancing of forces and moments of an airframe within a specified envelope. The placement of engines or the length of the fuselage are design decisions that result in a balanced aircraft. So an aircraft with a long nose will have to have a heavy tail, and it’s much more efficient to put the heavy engines on the aft fuselage than it is to stow hundreds of pounds of unproductive ballast there. As Q.E.D. points out, the aircraft needs to be able to fly even if both (all) engines are lost.
So the placement of the engines are a design factor that is considered in the weight and balance. The angle of the engines – where they point their thrust – is something else.
Right. You’ll notice that most jets fly with a nose-up attitude. This would put the engines in line with the airflow.
I see what you’re saying, and I have to agree. OTOH, since the rear-mounted engines are angled downward, there is an upward thrust component. Just to use round numbers, let’s say the total engine thrust on the MD-90 is 100,000 pounds, and the engine is tilted down by 5[sup]o[/sup] WRT the long axis, this means that there are about 5,555 pounds of downward thrust that must be compensated for (assuming I’ve done my math correctly). Given this, I’d say it’s likely soem combination of thwo prevailing theories here. That is, the engines are tilted to align them with the local airflow, and are mounted slightly off-axis to compensate for the downward thrust component. Make sense?
Yep, that’s exactly what I was trying to say. I think you put it better than I did.
Actually, there are three theories here;
Using some vertical component of the thrust to balance the aircraft fore/aft.
Tilting the engines so that the thrustline goes through the CG.
Tilting the engines so they are aligned with the local airflow. (This isn’t a theory, BTW )
For #1, you are relying on a force that may go away at any minute to balance the aircraft. The engines were put back there as part of the aircrafts design, not added as an afterthought.
#2 is possible, although you can make the thrustline pass through the CG by just placing the engines at the correct height on the fuselage. Design concerns may prevent this, though.
#3 is the main reason. The airflow comes over the top of the wing and starts to angle downward. The engines are placed right in this airflow, and are tilted aft slightly so the inlets are square into the wind. Like I said before, you can see this same thing on wing mounted engines also, but you don’t notice it because the engines are canted inwards, not tilted up/down. They are angled inward slighty into the spanwise airflow around the wing.
I’m not an engineer, but I do fly these things. I believe you’re on the right track with thrust line vs CG considerations.
Airplanes with engines under the wings (eg 737 / 747 / 757 / 767 / 777) have a pronounced pitch-up tendency as you add power. It’s not enough to produce a dangerous change in trim, but it does need to be corrected for every time you change power. The loss of power, whether due to throttling back back to idle or engine failure, produces a corresponding trim change nose down.
Airplanes with high mounted engines (717/ 727 / DC9 / MD80 / MD90) have the reverse tendency, all else equal.
Naturally, any particular thrust line offset will only be perfect under one combination of speed, power setting, and aircraft CG. But by setting the engine at some offset, the engineers are able to get the heart of that envelope near the common cruise design point.
As well, for drag considerations, you want the overall cowling structure aligned with the local airflow. Back in the tail area that’ll tend to have soem downward cant moving rearward.
It’s also possible that the engine centerline does not track exactly with the cowling centerline. This is particularly apparent on the 727 and DC9 (not MD80), where the tops of the engine cowlings are essentially flat, while the bottom has a pronouced bulge or belly.
That bulge’s purpose is to house the engine accessories (fuel pump, electrical generator, etc), but it also serve to camoflage the actual engine centerline for the casual observer.
Man. I went through my college textbooks and couldn’t find any reference to it, but I’m sure that’s where I learned it. You may have to settle with what you can find online and make up your own mind.
Look under the heading “Aft-engine placement” and the diagram that goes with it showing the 3 degree tilt. It’s not very plainly explained, but you can get the general idea.
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