MD11/DC10 Physics quirks?


#1

Hello everyone, I’m sort of new to this forum, so do forgive any oversight(s) on my part regarding this topic.

I have searched for any related topics, but was unable to find any. I do apologise if this issue has previously been raised.

Now let me start with the “issue”.

So basically I’ve been flying the MD11 and DC10s for quite a while, and notice that they have rather interesting descent physics. When I retard the thrust levers to idle on most aircraft (787, 777, A320, CRJ etc.), they tend to start off with a initial vertical speed of around -2600fpm from FL350 if I maintain 250KIAS, and this drops to around -1800fpm when I reach 10000ft MSL. (I understand that this is the “ideal” descent flight path)

For the MD11s and DC10s, they too will start off at around -2600fpm, but once I’m down to around FL230, I start to notice that the vertical speed has to drop to absurdly low levels in order to maintain 250KIAS (800fpm or even less).

I did do some research on this matter, but could not find anything on the matter. Could someone please enlighten me? Or is it just perhaps an isolated case? I thought that it was a result of the “28%N1” instead of the usual “20%N1” when at idle, but I’m really not sure.

Thanks!


#4

Heavies are hard to slow down.
I begin my descent further out in the heavies (MD/DC, 777 etc) I even deploy spoilers if necessary.
You can also do descending in steps to stabilize speed.
It’s not an issue with the sim.


#5

Simple physics: because of earths gravity and atmosphere, heavier objects fall faster. If you descent faster you will gain more speed. It’s 100% normal. Let’s say a piece of paper, because it is so thin and light but has a big contact surface with the air, the air slows down the paper dramatically. For an airplane as heavy as the MD/DC, It isn’t thin and it isn’t light. It might have a big contact surface but the weight of the plane makes that surface hard to slow down. That’s when the spoilers come in to play. Spoilers are an extra contact surface to create more drag which will slow down the plane even more. That’s why, let’s say a 737, will be able to descend faster without gaining more speed.


#6

The atmosphere thins with height. As you are descending more and more air molecules are striking the wings, air density increases, creating more and more lift. So to maintain a set forward speed, your vertical speed will decrease as wing efficiency(lift) increases.


#7

its just the nature of wide body aircrafts. take b747 as well. it goes like. as you descent into lower thick atmosphere the same airspeed generate more lift as more air flows over the wing. it is quite normal thats why ypu nay also find on flight radar 24 tgat wide body aircraft start there descent long back compared to a320 l, 737 and other smaller aircraft.


#8

Thanks for the information, but I actually just found it rather curious that the MD11/DC10 behaves very differently from all the other turbofan aircraft out there


#9

This isn’t true, all objects reguardless of weight fall at the same speed ignoring air resistance.

Heavy jets simply have more potential energy at altitude, and if your trying to get down the energy has to go somewhere and if you don’t have enough drag to lose the energy then it’s going to start being converted into kinetic energy (speed).

As these metal tubes are fairly streamlined to improve performance and fuel consumption there isn’t a great deal of drag to disperse the energy.


#10

it should be with all heavies. the more the mass the greater the inertia. its normal. when speaking differently. you may notice on the 2 aircraft that planes pitches down initially when you throttle up rather piching up. it is due to the 3rd engine on tail mounted above tge centre of gravity of aircraft. rest all heavies and super are hard to slow down. so deacent further out from similar flight level compared medium and light category aircraft.


#11

If you read the rest it explains. I didn’t put this alone:

Blockquote
because of earths gravity and atmosphere, heavier objects fall faster.

And when saying atmosphere it means air resistance.
Please read the whole text before judging people.


#12

That’s not true.
Quick example, imagine 100kg of wool and 1kg stone…

Faster fall objects which have the smallest surface area. Therefore, if you would fit all the weight of B747F into Cessna 152, Cessna would fall much faster than Boeing.


Absolutely,

Gravitational Potential Energy = mgh, where m - mass of an object, g - gravitational field strength (9.81 N kg^-1 on the earth surface), h - height. Energy can’t just appear or disappear, therefore it should be converted into something. Mostly Kinetic Energy, but also heat etc.

Kinetic Energy = 0.5 * m * v^2 (where m is mass and v^2 is speed squared).

Since GPE is converted into KE: mgh => 1/2 * m * v^2

Rearranging, 4.905 * h => v^2. So if air resistance wouldn’t exist, for every 1m lost in height you would gain 2.2 m/s or 4.3 knots… So… Thanks air resistance!

Sorry, just love physics

If you want more fancy equations and definitions, please PM me:))


#13

Just said that. Go and read the whole thing please, it’s just annoying to see people read a small part of it and criticize. If you guys would’ve read the whole thing, you would’ve realized I talked about the atmosphere, air resistance and surface area!!!

P.S. : Sorry if I don’t use the right words cause I speak French!


#14

Alright, if we’re just gonna argue about air resistance, can we take it to a PM?

EDIT: Okay, obviously we can’t. Carry on, I guess?


#15

It don’t want to argue, I’m sure we understand air resistance, it’s just that no one takes the time to read a whole post and then they criticize!!! It’s just too funny!!!


#16

The physics of the MD11/DC10 are different from other heavys simple for the fact of the added third engine in the rear. This leads to an aft CG ”Center of gravity” therefore you need to increase the nose down attitude to gain speed when at idle like you stated. On a typical aircraft the engines are located on the wings and give a reletivly balanced CG. Therefore you need to pitch less for the same amount of speed. This also has the reverse effect at lower speeds. That’s why you get a much higher pitch attitude at landing then typical aircraft. That’s the best way I can describe it for you.


#17

If you slow down a bit more than 250kts you will be able to put some flaps out and you should be able to descend a tad quicker.

@Adam_Al-Finge I did read your whole reply before typing my response, The rest of it was fine, it was just the opening statement that was false and I was simply pointing this fact out.


#18

True, the beggining wasn’t well said!