Any aeronautical engineers provide the straight dope on this?
Here is a photo of an MD11 and here is a shot of an MD90. Notice how the rear mounted engines have a noticeable slope to them, they appear to be pointing downward. You can see the difference in the first picture between the rear engine and the wing engines - the wing engines look straight while the rear engine is pointed downward.
Any ideas why this is?
The down angle provides a vertical force component that helps overcome drag when flying at slow speeds. Useful for takeoffs and landings…
Sure - most aircraft have the thrust line angled downward - but why so severe on rear mounted engines as opposed to wing mounted engines?
I don’t know, but I work with some of the best aerodynamicists in the world. I’ll ask…
Since the nose of an aircraft is heavier than the tail, the horizontal stabilizer is there to provide downward pressure on the the aft end. (I’m sure someone will be in to discuss weight and balance and “forward-CG” and “aft CG” situations shortly; or I will if I have time later.) Pointing the engines down provides the downward thrust that’s needed to balance the aircraft in flight, so that it takes some the load off of the horizontal stabilizer (or stabilator, on most – if not all – commercial jets).
The downward angle of the engines in the first photo isn’t nearly as severe as it looks. Notice the plane is taking off, and the front wheels are well off the ground. If you compare the angle of the fuselage to the angle of the rear engine, you’ll see they’re nearly identical. In the second photo the apparent severe angle seems to be an artifact of the orientation of the photograph. Look at this side view of an MD-90, and you can see the angle of the engines appear to be more nearly zero with respect to the long axis of the fuselage. The engines may be tilted downward slightly, but it isn’t by much, if any.
Here and here, I’ve attempted to show the engine tilt with respect to the long axis. I take back my previous comments. There definitely is a noticeable tilt to the rear engines. That’s what I get for holding up a ruler to my monitor. 
I got this reply to my query. Basically, the same this as what Johnny L.A. said.
Are the engines on the MD-80 capable of swiveling? Like some sort of crude thrust vectoring.
If you look at this picture, you’ll see that the engines are clearly parallel with respect to the ground, but the rest of the aircraft isn’t. Here’s another view that shows the distinctly downward-sloping aspect of the fuselage. A couple more for you:
So the question isn’t why are the engines pointed downwards, but why is the rest of the airplane!
The plane points down because the forward landing gear is made as small as possible. This is because the front gear doesn’t need to be as strong as each the mains and because there is limited space to store the gear when it’s retracted. Remember that when landing, planes always touch down on the rear wheels first and then the pilot gently sinks down to the nose gear.
I’ve always noticed the same thing and wondered why most jet engines are slanted. Some jets have it more than others, but MDs seem to have more slant than other jetliners. I remember seeing a smaller business jet that had two huge engines mounted on the rear of the fuselage, and the slant was very noticable.
Sorry Johnny this does not fly inside my 15 watt pea brain 
Look on something like a Cessna 150 the nose of the plane is way heavy, that is where the engine is. Makes sense that the horiz. stab holds the tail down/nose up.
But on the MD-11 linked there is no engine in the nose; two of the engines are on the wings (at or near the CG) and the third engine and all of its assorted plumbing etc is in the tail. I have a hard time believing that that aircraft is nose heavy. If that bird is nose heavy man those pilots have to start doing Atkins.
In the case of a plane with rear-mounted engines, they are offset from the centerline of the aircraft to counteract the rearward weight bias of having all that weight hung on the tail. This would prevent the pilot from having to run with the trim all the way tail up, and allow the trim to be run nearer to the center of it range. Needless to say, if the trim runs near the center (no deflection on the trim tabs*) the plane will get better fuel mileage since it will be cleaner aerodynamically.
If the rear engine were to quit, the pilot would then have to adjust for the lack of thrust from that engine and retrim the A/C.
If that MD-11 were nose heavy having the rear of the rear engine point downward would only make the plane feel more nose heavy as the engine would be pushing the tail up. In a nose heavy A/C I think that this would be considered a bad thing.
As far as the plane being nose down on the runway, when the plane accelerates, as the wings start to fly the nose will lift off the ground. As the plane continues to accelerate, the plane is rotated and takes off. Landing the nose wheel is held off until the wings stop flying and then settles back onto the runway.
*I think I got the terms correct, IANAP and all that.
Yes. They are aligned with the airflow coming off the wings.
Wing mounted engines are usually canted inwards slightly, for the same reasons.
Ah, but take a look at where the wings are! You have a lot of fuselage forward of them. With weight and balance, you have moment. For the sake of simplicity, consider the datum to be at the centre of lift (CL). (The datum can be any arbitrary point, even if it’s not anywhere on the aircraft. It may be defined as “100 inches forward of the nose”, for example.) But let’s say it’s at CL. Any position forward of the datum would be positive, and any position aft would be negative. It doesn’t really matter; you can switch them around. It’s just math.
So let’s say that you put a 200 pound weight at 50 feet from the datum (which in this example is the CL). This would give you a moment of 120,000 pound-inches. You put another weight 20 feet aft of the datum, but this one is 500 pounds. Now you have a moment of -120,000 pound-feet. The result is that the CG is at the datum, which we’ve said is at the CL.
Aircraft have a “CG envelope” within which the aircraft is designed to fly. Here’s why it’s important: If your CG is too far forward, you may not have enough elevator authority to keep the nose up. This may be particularly bad on landing if you don’t want to plant the nose first! If your CG is too far aft of the CL, then you may not have enough elevator authority to push the nose down. Let’s say you get slow and stall. If you’re “tail heavy” and you don’t have the elevator authority to push the nose down, you’re in trouble. Aircraft have to be a little “nose heavy” so that they can recover in case of a stall. (An aft-CG situation is more efficient in normal flight; but safety is paramount, so a little bit of efficiency is sacrificed for the sake of it.) Variations in CG within the envelope can me compensated for with aerodynamic trim.
Now in the case of the MD-whatever, (and remember that my ratings only cover light planes and helicopters) the wings are very far back. You have two heavy engines on the aft fuselage. In order to balance the moment caused by the engines, you need to have a good deal of fuselage forward of the CL. Hence, the wings are far back.
So here are the pieces: The CL is on the wing. (It varies on different aircraft, but trust me that it’s there somewhere.) The moments forward and aft of the CL are more or less balanced. In any case, the CG is within the envelope. The aircraft, being a safe design, has more weight forward of the CG than aft of it. Therefore, the tail has to be pushed down. In a Cessna or a Beechcraft, this is done aerodynamically with the aft flying surface; which, conveniently, can be moved to cause the aircraft to pitch up or down at will. In a larger aircraft, the flying surface can provide an aerodynamic downward pressure and you can put engines back there that are “pointed down” so that they also tend to keep the nose down. Is it more efficient to position the engines this way instead of just doing it with the stabilator? Yes. Otherwise they would not design the aircraft so.
So what about the landing gear? (I’ve always liked “alighting gear”, though I never use the term.) The mains are aft of the CG. This keeps the aircraft from standing on its tail. (Of course on a conventionally-geared aircraft, the mains are forward of the CG so as to keep the tailwheel on the ground.) I would suspect that having the nosewheel lower helps keep the nose on the ground and aids in braking. Dad’s Cessnas (bottom of the page) have a jaunty nose-up attitude on the ground. This is how I would land them: I almost always used full flaps. After flairing and touching down, I’d hold the nose up until the wings stalled. Still holding the nose up, I’d retract the flaps to get the aircraft’s weight on the mains. The nose would come gently down, and I’d rarely have to use the (expensive) brakes to make the first turn-off.
Deceptive ain’t it? I first looked at the MD11 taking off and thought there was very little tilt. However I copied the image and put it in my drawing software, drew a line parallel to the blue stripe and copied a line parallel to that through the engine. Wow! A very noticeable tilt.
The MD-11, MD-80, and rear engine mounted aircraft are interesting cases. The aircraft’s CG as a result of the engine becomes Tail heavy. If you look at the wing mounted engines there is not tilt since it is generally in line with the CG. Since the engines in this case are placed in the rear, its going to give the aircraft a tendency to pitch up. To correct for this, the rear engine has a downward deflection which in essence lifts the tail up, trimming the nose down. The ‘tilt’ usually isnt more than a few degrees.
Yes, the tail has to be pushed down. So why would they tilt the engine so they are pushing the tail up?
I think you are contradicting yourself, Johnny. Going by your example, the engines are fighting the tail. The tail is pushing down, the engines are pushing up. You might have a point if the engines were angled the other way.
For those saying the engines are angled to compensate for the rear CG that comes with having aft mounted engines, consider this: They knew the engines were going to be back there when they built the airplane. Why apply a band-aid fix to the CG when they could have just designed it out to begin with? If you depend on the engine for a correct CG, what happens when you lose one or both of the engines?
The reason the engines are tilted is to align them with the airflow where they are mounted. Tail engines are affected by the airflow coming off the wing. Wing mounted engines, flap track fairings, etc. are angled inwards for the same reason.
That’s different from what Johnny was saying. If the engine is mounted higher or lower than the CG, changes in power could cause the aircraft to pitch up or down with power changes. You can compensate for this some by tilting the engines so the thrustline is more alighed with the CG. You ever seen those seaplanes with an engine mounted high on a pylon above the fuselage? They will try to pitch up or down with power changes because the thrustline is out of line with the CG.
At this point the problem of off-axis thrust becomes moot, anyway.
That’s not what I’m saying. If you rely on an upward component of the engine’s thrust to balance the plane fore/aft and then lose that thrust, the airplane’s balance will be off. Even with a severe loss of power you want the aircraft to be controllable. Why rely on something to balance the aircraft if that something might go away at any second?
