Quebecair727 From Canada, joined Apr 2001, 328 posts, RR: 0 Posted (11 years 10 months 4 weeks 2 hours ago) and read 1483 times:
Today at the company, we had a discussion on how long can a full load 747-400 can fly with four engines out. Let's pretend she was flying at 37000 feet at a speed of 850 kmh. How long would it take(theoricaly speaking)before she hits the ground? That discussion came in after we talked about the BA 747 that flew in a cloud of volcanic dust many years ago.
Uvalencia From Mexico, joined Jun 2001, 55 posts, RR: 0 Reply 1, posted (11 years 10 months 3 weeks 6 days 22 hours ago) and read 1419 times:
Whenever an airplane is in a unstalled descent, it is said to be in either a glide or a dive. There is no sharp distinction between both of them but it is usually accepted that a glide takes place at lower rates of descent.
Yours is a very interesting question and I would like to propose that we refer to a vector diagram of the forces that are acting on the aircraft during a steady glide.
Unfortunately as I have little time I can not post a drawing but try to picture it in your mind. First the aircraft MUST have an angle of glide (that in general will be different from the angle of attack in the wing) think of this as the aircraft pointing its nose down.
Now let us try to remember the basic forces in an aircraft which are the lift, the drag, and the weight of the aircraft. The weight in our vector diagram will be pointing down, the lift would be exactly opposite to the weight IF THIS WERE A STEADY FLIGHT but since we have a glide angle, let us call it GAMMA, the lift has been shifted from the weight in GAMMA degrees, of course the lift is pointing upwards. Finally the drag will be making a 90 degree angle with the lift vector, so you will have a triangle with two sides being the lift and drag and the hypotenuse being the weight.
If you draw this diagram it will become clear that the following is true:
Tan GAMMA = Drag/Lift (this is the tangent of the angle of glide equals the ratio drag/lift)
Tan GAMMA = Cd/Cl (the same as above but this time it would be the drag coefficient and the lift coefficient which basically are the lift and drag but in dimentionless quantities).
Thus, the glide angle is completely determined by the L/D ratio. It follows that for minimum gliding angle the airplane must fly at the best L/D ratio and that an aircraft with the highest possible L/D ratio will have the flattest glide or the best range in a glide. As an interesting point this does not mean that flight at best L/D ratio will result in minimum sinking speed!!.
I really dont want to extend too much on this, the bottom line is that it can be shown that the best glide angle occurs at at the best Cd/Cl and the speed for best descent velocity corresponds to flight at maximum Cl^3.5/Cd.
So basically to answer your question we need to now the aerodynamic characteristics of the 747 in the form af a set of curves where the Cd and Cl are plotted against each other. I am not sure if that information is available but I will try to find out and do the calculations for you and post the results later.
Max Power From , joined Dec 1969, posts, RR: Reply 5, posted (11 years 10 months 3 weeks 5 days 16 hours ago) and read 1324 times:
I can give you a "rule of thumb" one can only speculate on how long it will "fly" or how far. Depending on wind temp. altitude, weight etc. etc. The accepted rule is "about" 30 miles or less for every 10,000'. That is assuming you are going into the trees at your glide speed (LOD) Start to dirty up and slow and the distance gets shorter, faster. Cheers, Max
Miguel From Portugal, joined May 2001, 101 posts, RR: 0 Reply 6, posted (11 years 10 months 3 weeks 5 days 10 hours ago) and read 1299 times:
I can't tell about numbers, how many miles it will fly, how much time until hit the ground, and so on. But I think we can think of an aircraft descending normaly.
Usualy, when starting descent, pilots set thrust to idle, so the plane flies like gliding, although in a controlled way. If you know well your home airport terrain in a circle of 100 NM, next time you fly home, look at the window and try to identify the place where you are when starting descent.
I know this answer is very poor after reading the very rich answer from Uvalencia, but this is the point of view an almost ignorant about aviation!