The wing profile on DC10s/MD11s

[Nick Figaretto]

Today my wife I spent a few hours in the ternminal at Malpensa, Milano. (Not an unknown situation for an ID-pax...) One Continental DC10 and an Alitalia MD11 pushed back while we were waiting. Apart from the beauty of these aircraft, I noticed the wing profile of these airplanes.

It seems like the angle of incidence (mounting angle) at the root of the wing (looking from behind the A/C) is very high, but it "flattens out" pretty abruptly at about 1/4th of the wing length and out. (Jeeeesus, give me a better english vocabulary!!!)

Anyway - what I am wondering is: Isn't the angle of attack at the root of the wing of the DC10/MD11 extremely high at cruise? I would imagine that the inner 1/4th of the wing would stall a lot earlier than the rest of the wing. Does this mean that the stall characteristics of the DC10/MD11 is pretty smooth?

And would the limiting mach-number of the DC10/MD11 be due to the high angle of attack at the wing root?

As I only had my wife to discuss this with (which I wisely enough didn't :)), I wonder if there's any of you who have any comments on this.

Nick.

[fly4fud]

Oh well, nothing to worry about. The MD-11 has about the same wing (but for the winglets) as the DC-10, e.g. a very old NACA designed wing profile dating from the fifties. Of last century ;)

Nevertheless, as you observed, the wing has a different angle of attack (and profile) at the root and the tip. This is called washout and should prevent the tips of the wing stalling first in the event of speed loss, retaining the full airflow over the tip of the wings and therefore aileron control. Ouf :)
Now, for the angle of incidence. This one has most probably been set TOO low. In cruise the aircraft (either one) has a pitch-up attitude of 3.5 deg. Ideally, aircraft in cruise should have the fuselage level, equals less drag, the angle of attack of the wings having been set at the necessary value. Funnilly enough, as far as I can remember the pitch attitude of both DC-10 and MD-11 is the same, despite the latter being equipped with C.G. control, fuel being xferred in the tail tank (horizontal stabilizer).

Oh well, neither aircraft will win any fuel efficiency contest, as with most American design ;) (this comment should rise some response "over the pond" friends :D )

As for the limiting Mach number, well, it has basically nothing to do with this. First, let's point out that .84 is not too slow a cruising speed.
The max speeds VMO/MMO are calculated according various factors, foremost structural integrity of the craft in turbulences or overspeed.

Well, hope all this helps, and I have no doubt you will get numerous answers on this subject :cool:

[fly4fud]

So sorry, have to add something about the stall behaviour.
Pilots of commercial jets don't know about the stall behaviour of their aircraft :eek: Yes, all these machines are equipped with stall warning, stick shaker, stick pusher and/or similar devices. Meaning the only ones ever to experience REAL stalls are the test pilots when the aircraft is certificated. Line pilots are trained to the stall in the simulator, but the stall is not really entered as the relevant safety device will prevent the aircraft from entering a full stall.

Well, again looking at the root wing profile of the DC-10/MD-11, the bunt profile reveals what must be "gentle" stall carachteristics. Of course, having the slats retract on one side (AA in Chicago following engine loss) will have this wing stall rapidly as has sadly been demonstrated :(

[Prof2MDA]

Fly4,

I disagree, and the MD-11s fuel efficiency proves my point. The aircraft burns just a bit more than a 777, whilst carrying MORE payload, with the older wing design (as you correctly pointed out) and having 3 engines. If the wing had an incorrect incidence, that would not be true.

The issue on stall characteristics is accurate. The aircraft is very docile except with flaps beyond 28 and slats retracted, for fairly obvious reasons if you look at the form in that configuration!

The limiting Mmo is not all that low, actually is comparable to and even higher than several newer designs.

[fly4fud]

Prof2MDA, well if the incidence angle would be right, the efficiency would be even better. The thing is, airplanewise, the wings do carry the weight and the fuselage the payload. So, I would dare to say that producing lift with the fuselage, as done here, is not a very effective way :)
Talking about efficiency, let's compare pears with pears. One of the first factor will be the how efficient the engines are. I don't know a thing about the 777 (ok, it's BIG :D ), but on the 747-400 the fuel burn shows a great advantage when it runs thru the RR combustion chamber, opposed to the PW engine :p

[Prof2MDA]

Fly4,

There is nothing wrong with the incidence on the wings. You obviously do not have access to the engineering diagrams and performance analysis. Perhaps you are confused by what works out to be a bit of optical illusion due to the "gull" shape of the wings? Perhaps you are confused by the airplane's body attitude on the ground (related to the larger slats, relatively, as compared to some other aircraft types)?

Perhaps you are just believing a OWT about the subject? In any event, you are not correct.

[fly4fud]

Oh well, I must be wrong for sure :o
The subject of my confusion is that the aircraft is flying with a nose up attitude and nothing else.
Your arguments beat me, I give up :cool:

[downfourgreen]

Some very, very useful and easy information to memorize about MD11 speed limits. (specially when a check airman likes to ask one by one during a night long haul flight)

When the wing tip fuel tanks are 60% full or less VMO is 320Kt (SL to 30,704ft) and MMO is .85M. There is a linear trasition between 60% and 90%. VMO/MMO speeds at 90% are approximately 350/365 kt and .87M (when
above 25,670ft).

Turbulence penetration speeds: 290 to 305Kt or .80 to .82M whichever range is lower, but when the wing tip fuel tanks are 60% full or less and the aircraft GW is less than 216,409kg (that's right, 2 1 6, 4 0 9 kg)speed must be reduced to 250 to 270kt or .80 to .82M and below 10,000ft the greater of 250kt or climb speed.

Easy?
In short, MD11 manuals and limitations are written by lawyers.
:mad:


"Three engines are cheaper than four and safer than two".

[Prof2MDA]

The tip tank fuel is a non-issue because the FMS will show that limit as the upper foot, no need to memorize it!

The turbulence numbers, well, I am suspicious that the lower set for the lighter weights are something your company came up with.

[Prof2MDA]

Fly4,

Having operated a variety of types, I don't notice the MD-11/10 operating at a higher nose up attitude in cruise. A bit higher in the approach phase with slats extended without flaps, but that's about it.

[downfourgreen]

Prof,
Ok, FMS can show automatically a large number of limit speeds, including VMO/MMO, however MD11 is the airplane (theoretically a modern one) with the largest number of limitations I've already seen, 18 pages, and limitations must, or should, be known by heart, beside any automatic feature, but, it isn't definitely an MD11 issues, and DC10 as well.
Concerning turbulence penetration speed, well, there is a "note" in the operating manual after basic speed range and it isn't absolutely my airline recomendation or restriction.
Anyway, I love to fly that big eagle...

[Prof2MDA]

Down4,

This is a note in the Boeing FCOM or your company manual? If it's in the Boeing FCOM, please give me the specific reference with page number, etc.

As to the other limitations, I've also operated a wide variety of aircraft and I don't know that there are more limitations on this one, nor more that I have to know. Each aircraft has its own set, I'm not sure what you're comparing it to, but perhaps some of this is company driven in your case? Amazing how many things those in charger of flight depts can come up with that you "need to know"! I'm comparing it to the 747, 727 and DC-8, all of which seemed to have similar numbers of limitations in our manuals.