Expert: Ray Wilkinson - 6/7/2006

Question
Ray.....
A friend referred me to you as being the Guru of Difficult questions. I hope you have the time to perhaps help out here.



A friend and I have differences on this question:-



An aircraft is loaded with many birds (as many as can be fitted in) suspended on their perches inside the fuselage. The aircraft is at max. take-off weight. The plane takes off with all of the birds sitting on their perches, and enters a state of level flight at, say, FL 250. The cabin is pressurized to 8000 ft alt.



Magically, all of the birds fly off their perches and fly 6 inches above the perches. There is no contact of birds to aircraft

No pilot induced changes occur. (Thrust, angle of attack etc)



What would happen?



The aircraft maintains straight and level flight?
The aircraft climbs due to decreased weight/mass/gravity?
Something else?


My co-worker maintains that nothing will change. His contention is that the flying birds still exert the same downward effect as they did when they were perched. He has not explained how this actually happens.



I believe that since the birds are now expending their own energy to create lift within the enclosed capsule and thus the downward force exerted on the aircraft would be lessened. The net effect should therefore be and increase in aircraft speed resulting in more lift, resulting in an increase in altitude.



Hope you can help.



Regards,



peter


Answer
Peter

This is a classic textbook question! To answer it you need to consider equilibrium and boundaries. Take what's going on inside the aircraft first - you can consider this to be a closed system where the boundary is the fuselage wall. The birds are flying, so they have to generate lift equal to their own weight to achieve equilibrium and stay in the air. From Newton's third law (every action has an equal and opposite reaction), this lift needs to react against something, so somehow the upward force providing the lift must react against the floor of the aircraft, and in aerodynamic terms this results from the pressure difference between the top and bottom of their wings. Thus the pressure against the fuselage ceiling is slightly lower than that on the floor.

Now consider the outside of the aircraft. No matter what the birds are doing inside (provided they are in equilibrium, which I'll come back to), their weight will still have to be supported by the aircraft, therefore it will still need to generate the same amount of lift.

Now back to that equilibrium thing. The exception to this comes at the moment the birds take off together. At this point they will be accelerating upwards, which will need more force than is required just to keep them in the air. For that moment, the apparent weight of the birds is increased, so the aircraft will descend momentarily. Once they have stopped accelerating, equilibrium is restored.

As an aside, consider what happens when you are riding a bike. To turn left, you must first push the handlebars to the right (try it, but prefereably when there isn't a bus coming). This causes the bike's equilibrium to be disturbed, and it begins to turn right. Because it isn't in balance, it will fall outwards (left) by centrifugal force (which does exist, whatever some people say). To restore equilibrium and keep the skin on your elbows, you now need to turn the bars to the left, controlling the fall into the turn until the centifugal force of the left turn balances the gravity effects of leaning over. At this point the bike is again in equilibrium in a left turn. To straighten up, the process is reversed - turn left harder, and the increased CF pushes the bike up out of the turn. The moral of this, from my own painful experience on a motor bike, is if you get stuck to the kerb the only way out is over the handlebars!

I hope this answers the question, and my fee is 10% of whatever your mate won from you on the bet ;-)

Regards

Ray