Boeing 747

I always like to use formulas! 😀

CD = CD0 + f*CA²
(f is a factor, which is in our region of interest constant)
So basically:
CD ~ CA²
or in words
drag is proportional to the square of lift.
CA = W/(rho*v²*S)
S: wing area (constant), rho: air density (independent of weight), v: airspeed, W: weight

W1=500klbs (which is something like the lowest possible flying weight of a B747).
W2 = 850klbs (which is something like maximum weight for any B747-200 at altitude)
rho=.5 kg/m³ (~28000ft)
S = 5500ft²

v: 250KIAS = M0.66 = 200m/s TAS

CA(W1=500klbs) = 0.45
CA(W2=850klbs) = 0.77
When you look at the graph above, you’ll see that Ca of W2 is actually outside the y-axis limit. I actually think it must be close to buffet onset boundary.

If we increase airspeed to 290KIAS, we have for W2:
CA = .57 with M=0.77.

Schorsch has explained it but Paul if you visualise a right angled triangle as pictured below, then look at the formula of

Vimd or minimum drag speed (which is the speed we have to fly at for best glide ratio) will be higher by a factor proportional to the square root of the weight increase.

This formula is basically saying that if we increase our weight then to obtain the best glide ratio or Vimd (minimum drag speed), we have to increase our speed by a factor proportional to the weight increase. Remember regardless of what our Vimd speed is we will still travel the same distance specified on that triangle provided we glide at that new Vimd speed, so the glide angle as shown on the triangle stays the same, as does the distance, we will just travel along the path of the glide path line on the triangle faster, meaning we will have a higher rate of descent.

Does this make it any clearer? (cos I’m confusing myself now LOL)

Dean

The only transport that can glide 1200 miles is the Space Shuttle.:D

FLYBYDONNI

It wouldn’t be able to glide 1200 miles, that’s here to North Africa

If an a/c wanted to glide for best range then it will fly at the best lift:drag ratio which is Vimd or velocity for minimum drag, if we remain here the glide angle will remain the same, however if the a/c is heavy then Vimd will be higher by a factor proportional to the square root of the weight increase.
So if we increase speed to a new Vimd the only thing that changes is the time in the air, you should maintain the same glide angle and the same distance covered on the ground, you’ll just meet the ground a bit quicker.
This is for still wind conditions, if we had a headwind then the best result would come by increasing your speed, the opposite for a tailwind.
There are graphs to work out the best speed to fly in certain conditions.

The 744 as with any plane will have a glide ratio worked out from the above, it could be something like 1:10, this means for every 1,000ft lost in height it will travel 10,000ft horizontally, again if you do the maths 33,000ft will give a Horizontal distance of 330,000ft which works out about 63 miles in still wind conditions.

Hope this helps too

Dean

I’m totally confused as to how you can have a higher rate of decent but have the same glide angle and travel the same distance. Sorry, Deano, my poor little brain isn’t understanding!

Is there any way reply 9 can be explained in a different way so that stupid and simple people like me can vaguely comprehend it? :confused:

Paul

The weight determines the lift coefficient. Depending on that the optimum speed changes. Actually, a very light aircraft would achieve a less desirable glide ratio than a heavy one, although the lighter one has overally less drag.

The problem with glide ratios is that it puts lift (=weight in unaccelerated flight) in relation to drag.

That is basically what Deano said: To have more lift at best lift coefficent the speed has to be increased. The glide ratio will stay the same, but with the higher speed the vertical speed is higher.

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This is the drag over lift for the B747-200.

I must have a vivid imagination.
Can only find incidents involving a DC-8 and a B727 connected with Mt.St.Helens.I was so certain about it but obviously wrong.
wawkrk

Although I am well known over in historic for my er Canberra fetish (as I was so informed today LoL) I also happen to like in a big way 2 of Boeings Big Birds, 747 – xxx & the 707 – xxx and I thought Id studied all the 747 incidents and I was beginning to wonder if Id missed one. Glad its cleared up at last. Bex

I must have a vivid imagination.
Can only find incidents involving a DC-8 and a B727 connected with Mt.St.Helens.I was so certain about it but obviously wrong.
wawkrk

I seem to remember the incident over Africa was caused by Mt.St.Helens in Washington state. The massive explosion sent out a volcanic dust cloud which circled the earth. Even cars in the UK were covered in the red dust.
This happened in 1980.
wawkrk

I’ve never heard about the second BA flight which lost all engines over Africa, but I have heard about a similar incident regarding a KLM Boeing 743 losing all engines after flying over an erupting volcano in Alaska sometime in the late 1980s.
I believe it went into something of a steep dive upon losing power but recovered power after leaving the dust cloud.

You are describing the Jakarta incident unless you are confused with the Air Canada 767 incident on 23/7/83 where the plane glided 45 miles from 35,000 feet to land at Gimli.

No, I remember very clearly reading the story as told by the F/O or could it have been a Flight Engineer then.
He spoke about the grinding noises coming from the Rollers as they tried to re-start one by one.
I think BA had to fly in support equipment because of the primitive facilities at the African airfield. Even no steps to leave the aircraft.
The passengers formed a club afterwards to keep in touch.

I cannot find any other incident with British Airways in Africa, hope someone else can provide further information if it happened.

You are describing the Jakarta incident unless you are confused with the Air Canada 767 incident on 23/7/83 where the plane glided 45 miles from 35,000 feet to land at Gimli.

No, I remember very clearly reading the story as told by the F/O or could it have been a Flight Engineer then.
He spoke about the grinding noises coming from the Rollers as they tried to re-start one by one.
I think BA had to fly in support equipment because of the primitive facilities at the African airfield. Even no steps to leave the aircraft.
The passengers formed a club afterwards to keep in touch.
The whole incident was miraculous and a huge credit to the crew that they survived.I am sure I did not mix it with Jakarta where the facilities are not so bad I remember.
I am familiar with the Gimli incident where they landed at the race track.

Maybe there was 2 BA747 volcanic dust incidents.
I am sure there was one over the African continent.
All four engines also failed and then restarted I think it was in darkness.
The crew found a small airfield with no navaids and runway lighting partly missing. The cockpit windscreen had been shotblasted and the captain landed the aircraft manually looking out of the side window.
Does anyone else remember this?
wawkrk

You are describing the Jakarta incident unless you are confused with the Air Canada 767 incident on 23/7/83 where the plane glided 45 miles from 35,000 feet to land at Gimli.

http://aviation-safety.net/database/record.php?id=19830723-0

Newforest

Mate the Air Transat glided 21,500ft in 77nm (or 88.5 statute miles) to end up at 13,000ft 8nm from the runway threshold. This is a glide ratio of about 1:21 which is excellent.

Dean

Isnt that the world record for flying with no engines the A330 Air Transat one?

James

Makes no difference, see post #9, adding weight only makes you reach the ground quicker, i.e. a higher rate of descent, the glide angle & distance travelled along the ground (glide ratio) is the same

I’m totally confused as to how you can have a higher rate of decent but have the same glide angle and travel the same distance. Sorry, Deano, my poor little brain isn’t understanding!

Is there any way reply 9 can be explained in a different way so that stupid and simple people like me can vaguely comprehend it? :confused:

Paul

Maybe there was 2 BA747 volcanic dust incidents.
I am sure there was one over the African continent.
All four engines also failed and then restarted I think it was in darkness.
The crew found a small airfield with no navaids and runway lighting partly missing. The cockpit windscreen had been shotblasted and the captain landed the aircraft manually looking out of the side window.
Does anyone else remember this?
wawkrk

Well, it was flying with empty fuel tanks at that stage, wasn’t it!

Makes no difference, see post #9, adding weight only makes you reach the ground quicker, i.e. a higher rate of descent, the glide angle & distance travelled along the ground (glide ratio) is the same

Newforest

Mate the Air Transat glided 21,500ft in 77nm (or 88.5 statute miles) to end up at 13,000ft 8nm from the runway threshold. This is a glide ratio of about 1:21 which is excellent.

Dean

The Transat also converted some speed. Basically the glide ratio of an A330 even exceeds 1:21, but under those conditions with wind-milling engines it wouldn’t achieve more than I think 14 – 18. With all conservatisms the glide ratio can be assumed 1:10 for an airliner, which then includes some margin. The aerodynamic glide ratio is normally much better (B747-200 is about 1:18, up to 1:20), but the drag of the engines and the external turbine, non-optimal trim and the normal difference between wind tunnel data and real life performance reduces glide efficiency.

And I agree Deano, it can not fly on one engine except maybe for some very remote conditions (basically empty).

Newforest

Mate the Air Transat glided 21,500ft in 77nm (or 88.5 statute miles) to end up at 13,000ft 8nm from the runway threshold. This is a glide ratio of about 1:21 which is excellent.

Dean

Well, it was flying with empty fuel tanks at that stage, wasn’t it!

Someone mentioned this on The travel city direct forum recently!!

I was reading it on the tcd forum thats why i asked

Newforest

Mate the Air Transat glided 21,500ft in 77nm (or 88.5 statute miles) to end up at 13,000ft 8nm from the runway threshold. This is a glide ratio of about 1:21 which is excellent.

Dean

Cheers mate, thanks for the correction!:)

will a 747 fly on 1 engine?

well, yes and no,
it is totally weight dependant. using the 747-2 if it is empty or close to, yes, it can make it around the pattern, in a case where you lose the other three prior to an established final,
it will not maintain level flight with the gear down on one, so you have to be on or above the glide, loc established and on your way down, it will not go around, so gear down is a commitment.
Two engine work in the SIM is done at weights of approx 195 T, usually with the loss of the first one at or just beyond V1, and the second in the pattern prior to intercept of the Final app crs. To be honest not much is gained by trying single engine work in the sim.
The loss of multiple engines at ALT if heavy will require fuel dumping asap.

driftdown as mentioned is generally computed on an enroute card, usually at 10 ton intervals, or on the hour, based on the loss of one, it is a three engine drift down ALT, the speed to best accomplish this, and the power setting (EPR/N1) required, max con for the weight and altitude.
essentially the procedure on the loss of one is the balance to max cont, allow the speed to decay to the computed drift down speed, and start down on that speed to the computed DD ALT, which is “should” maintain.

Keep in mind that as weight decreases, it is quite possible that you can not only maintain the chosen cruise ALT on 3, but actually still climb, if the “driftdown” ALT computed for the weight shows you can maintain a higher alt that your existing cruise.

regards all engine out glide, at idle thrust the engines still produce about 55% thrust, so the 3 to one rule is pretty accurate, 3 times your alt is the distance you will go, 33000 ft – 100 miles, but remove all that residual thrust, this no longer applies, and you will decrease this, again weight dependant, heavy the airplane will glide further, light, it will glide less, but a rough figure, at normal landing weights, (under 285.7) I would not expect to get any better than 85 DME.