If you look closely, the engines are angled slightly upwards. What’s the rationale for this? It seems like it would be inefficient to direct part of the jet thrust downwards, pushing the nose down.
Um, wouldn’t the nose be pushed up, given that the thrust is applied behind the pivot point of the plane? :dubious:
Wrong up: the intakes are pointed upwards, the exhausts downward. This is probably to give the plane better short-runway performance by directing the airflow from the engines down towards the pavement. The angle looks pretty minor, and once you are in the air, it’d be pretty minor to trim that out.
Pushing up behind the center of gravity would push the nose down. To segue off Raguleader, planes on the ground or about to touch down/take off behave a little differently because the presence of the ground changes the airflow under the plane. Shifting design to provide better performance in the most dangerous part of any flight might seem prudent.
I had a teacher who used to fly B-52s (I think I mention him pretty often in these threads) who said that they were trained to put the B-52 down into ground-effect if they had to evade enemy fighters. The B-52’s redunkulously wide wingspan let it safely surf along in the ground effect above most trees and telephone poles, at the same time losing themselves in the ground clutter against search radar, while a fighter jet persuing them on visual targeting would run the risk of plowing themselves into the ground due to a jet fighter’s much less forgiving flight characteristics.
I seem to recall from an earlier thread (paging ElvisL1ves) that the angling of the engines is to align them with the expected incoming airflow during normal straight-and-level flight, for maximal efficiency. It has nothing much to do with thrust direction.
I don’t know this one and I’m not going to pretend to, sorry, but thanks. There are too many things going on aerodynamically to make it obvious from a glance, too many issues revealed only in the wind tunnel (or in CFD models, now) that tweaks like this fix, to say without having been involved
FWIW, that feature was on the original DC-9, too, and didn’t visibly change with the stretches to make the MD-8x series. A quick look at some photos show it on the Caravelle and Fokker 28 too, but not any other rear-engine plane I know of.
QED, a few degrees doesn’t matter much in that sense. The cosine of any angle up to 10 degrees or so is close enough to 1.0 not to matter to the total thrust. That also means short-field performance isn’t affected, either.
Just ftr, it’s common for tail-mounted engines to be canted outboard a few degrees so that there is less rudder authority required if one engine is out. That isn’t a cosine effect, it’s an effect of the distance between the CG and the thrust centerline.
What advantage might there be in directing engine thrust toward pavement?
If we can believe that the guy in this thread is accurately citing the manual for the DC9 (which is the direct forerunner of the MD80):
The last sentence is the key to understanding this. Though it’s not visually obvious from the photograph, the nozzle portion is angled upward (and rearward, obviously) to avoid the asymmetric thrust that would result if the nozzle were axis-aligned with the upward pitched nacelle.
Ah, okay. So they are tilted to be aligned with the airflow in that region at high speed, but the nozzles are also angled so that the the thrust vector is not affected.
Thanks, all.
The Navy’s now (sadly) defunct A-6E Intruder all-weather attack plane used the same terrain-following radar and display as the B-52. An amazing system that allowed the plane to fly ‘under’ radar, following the contours of the terrain.
Ok, apologies to the OP, since this is turning into a complete hijack, but what the heck…
In a previous life, I did some work at a power plant near Holbrook, Arizona, where they had a radar training facility. Occasionally we’d see a B-52 come cruising along about 200 ft above the ground running north to south near the plant, and it seemed to be cranking along pretty fast. I-40 runs along there, and it probably scared the *(^# out of the drivers. The B-52s supposedly used the plant as either a target or a navigation point, and had previously flown right over the top of the plant before they installed a couple of new units and taller stacks. Interesting to see, and loud as anything, even from about a half mile away.
I bet. 
Of course, modern airborne search radars can pick out an aircraft traveling nap-of-the-earth, from what I understand, so you no longer have the same advantage that you would have gained from the practice before. If you want to avoid enemy interceptors, you had best have your fast movers knock them out of the air for you.
Anyhow, Cosmic Relief’s post sounds like it pretty effectively answered the question for us. As for why you’d need to tilt the engines up, I think it would probably be because of how the wings redirect the airflow (they sorta bend it downwards behind the plane, which is part of what provides the lift, as I think one of Cecil’s columns mentioned)
Now, what if we put this MD-80 on a treadmill… d&r 
It may just be my imagination, but whenever I’ve flown somewhere on an MD-80 it seems that the pitch of the cabin never becomes completely horizontal; the back seems to be just a few inches below the front.
Next time I’ll bring along a marble and see what happens when I let it go…
Ah. :smack: :o
I flew from NY to Dallas on an MD-80 last week, and noticed the same thing. It seemed like when I went to the john, that I was walking downhill, and uphill back to my seat, even though we were in smooth, level flight.
That’s normal for all aircraft. It depends on speed, the faster you go the more level the deck will be as the wing is at a lower angle to achieve the required lift. The deck can be designed to be level in normal cruise flight by angling the wing slightly relative to the fuselage, but I don’'t think it gets designed all the way out. Also, the airline’s preferred cruise speed may be slower than what the designers were intending.
The thrust goes like the cosine, but won’t the drag of the outer shell of the engine go more like the sine?
Yes, but the degree you’re taking the sine of depends on the nacelle’s orientation relative to the *local * airflow, not the ambient airflow.