Expert: Paul Soderman - 6/6/2007

Question
QUESTION: Hi, Paul.  I work for a children's museum.  I am developing an exhibit on flying machines (both aircraft and spacecraft).  After two days of study, I'm trying to boil-down the what I have learned.  I've come up with this:
"A machine can fly if it has at least one of the following qualities:
1. it is lighter than air,
2. it has a shape that creates enough lift to overcome gravity,
3. it has sufficient vertical thrust to overcome gravity and drag,
4. or it is outside of earth’s atmosphere."
Yes, it's very simple- and I wish it could be simpler.  What I need from you- is it true?  Have I added or missed anything?  Thanks so much!


ANSWER: Hi Tom
I like your list, but I would offer some modifications. A machine can fly if:
1.  it is lighter than air to create buoyancy greater than its weight,
2.  it has a shape and speed that creates enough lift to overcome gravity and drag,
3.  it has sufficient vertical thrust to overcome gravity

Note that number 2 applies to wings and also applies to rotors because a helicopter can hover as long as the blades are moving.   Speed is essential to lift.  Number three as I stated allows the machine to hover.  If it does more it has to overcome drag as well.  And number 4 of your list is not really considered flying (in my mind) because the machine is out of the earth's atmosphere.  I guess you could say that a machine can travel in space if it has enough thrust to escape the earth's gravity or enough speed to orbit the earth.

Hope this helps.


---------- FOLLOW-UP ----------

QUESTION: Thank you for helping me fumble through this!

I like your #1.
I understand the addition of "and speed" to #2 (an important addition, I agree), however, I do not understand the addition of "and drag."  It seems to me that if a helicopter is hovering, the rotors are providing lift to oppose gravity; and if the helicopter moves, the rotors are also providing thrust that is overcoming drag (not lift overcoming drag).  Am I wrong?

Answer
Hi Tom - You are right, I said that incorrectly.  To fly you need shape and speed to provide lift, and you need thrust to overcome drag.  Shape is also important to minimize drag.   I'm sure you can say that in fewer words.

For the helicopter, the rotors also need shape, speed, and thrust to provide lift and overcome drag of each rotor - even in hover.  For forward flight the helicopter tips forward and the combined lift of the rotors provide the overall thrust to overcome drag of the vehicle.  But that is probably too complicated for your purpose.  Sticking with a simple airplane concept is probably sufficient.  In any case, contact me again if anything is unclear.

Paul