Why do they do that....

[Cuban_8]

All,

Something on the technical front that has been bothering me for a while now. I'm currently studying for a degree in Aero Eng, but have never got round to asking one of my departments "academics"!

Why do commercial jetliners cruise with a positive fusulage incidence?? It's easy to appreciate that the wings of the aircraft have to be positioned at 3 degrees or so in the cruise, but why does the fusulage also sit at that angle to the flow. In doing so, the effective frontal area of the aircraft is increased and, thus, the drag!!! Surely it would be of benefit in todays efficincy driven industry to mount the wings to the fusulage at cruise incidence, thus having the fusulage pointing in the direction of travel through the cruise. Am I missing something big here??

Cheers for reading,

Cuban

[BOING]

If you are missing anything big it is probably that the manufacturers have spent many, many hours of computer simulation and wind tunnel testing to ensure that the bits fit together for maximum efficiency. You can bet they have got things pretty much correct. A couple of points to consider. First, bearing the above in mind, an aeroplane is one big flying compromise. A long range aircraft will be optimised for cruise a short range aircraft might be optimised for field performance. Therefore, you may see some odd design aspects but there is probably a good reason for them. Secondly, make sure you are differentiating between wing angle of attack, wing rigging incidence and the attitude shown on the aircraft attitude indicator. The latter is purely arbitrary depending on gyro installation and calibration. An aircraft could be flying with an indicated two degrees nose up while the fuselage is actually perfectly streamlined with the relative airflow. Also consider that we normally only see an aeroplane operating in the low speed regime near the ground. The fuselage angle will be much more nose down in high altitude, high speed cruise flight.

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[This message has been edited by BOING (edited 31 December 2000).]

[VH_KAM]

As an example, in a typical cruise the B767 nose attitude is about 2.5 degrees nose up.

[Vandelay]

I'm not positive about this, but in Aerodynamics/Flight Theory class, I remember the professor discussing on how the fuselage contribute to lift/stability. In effect, if the airplane is a couple of degrees nose high during cruise, it would contribute to positive lift ...as opposed to few degress nose low or zero degrees being a deadweight for the wings.

[DCDriver]

Vandelay is right, a significant proportion of lift comes from the fuselage.
This is most noticeable to pilots when existing designs are stretched , and the fleet mixes both very short and long-bodied variants of the same a/c. The short ones tend to be close-coupled and twitchy, whereas the longer ones are very stable to fly...(eg) DC9-15 vs DC9-41 or -51, for example.
I was once told what proportion of lift my current mount (DC10) generates from the fuselage, I cannot remember the exact figure but it was a lot!