confusion

[Phil Winks]

Where ever I look at at aerodynamic theories I find one statement that always confuses me

"in level flight all forces concerned must be equal"or put another way lift must equal weight and thrust must equal drag. now I see many heads nodding in agreement but. its the thrust must equal drag that confuses me surely if the forces pulling the aircraft forward are only equal to those holding it back then what we have is a stationary aircraft. surely thrust must be greater than drag to overcome it and allow the aircraft to move forward.? I do understand that enough difference between thrust and drag causes acceleration or conversely deceleration. perhaps I'm missing a key point can some one explain please.

Perhaps some one could also explain why its not enough for me to know an aircraft works without questioning why

[Former Member]

[This posting has been removed]

[Peter Miller]

Knowing how an aeroplane works will help you understand why it just crashed or, possibly, might prevent you crashing it in the first place.

[Phil Winks]

Now why didn't I think of Inertia   Jetsome? Cheers mate made my headache go away just knew there was a simple answer out there some where

[Mike Hawkins]

All this is related to Newton's Three Laws of Motion.

1) Something in uniform motion stays that way unless acted upon by a external force (Inertia) - because thrust = drag the net force is zero so velocity stays the same.

2) F = ma - the net force (Newtons) applied to the plane cause it to accelerate (m/s/s) depending on the weight (kg). ie. You apply 10 N to a 2kg plane it will accelerate at 5 m/s/s.

3) Every action will have an equal and opposite reaction -  the force pulling the plane forward (Law 2) minus the force at which the air is being pulled back = 0. ie from the example above the force on the air would be 10 N, if the air weighs .5kg it will accelerate at 20 m/s/s to keep the forces equal.

[Lee Burke]

Newton's Fourth law of Motion:  Every tree, bush and fence has the inverse square power of attraction to that of clear air.

[Mike Hawkins]

I thought trees were magnetic, like metal magnets attract certain metals, trees attract wooden planes. There is nothing dangerous about flying, it's the edges and stopping you have to worry about.

[Lee Burke]

Well, my science is pretty sloppy, Mike - just keep hitting trees is all 

[Mike Hawkins]

Actually I like trees, well I haven't got a reason to hate them yet. A big row of trees are just of the southern end of our strip, one of the bolts came out of my undercarriage (not enough threadlock) and it was at right angles to the way it was meant to be. I didn't realise I was so far out but was going really slow, the wing hit the trunk, no mark left, the only damage was a broken wheel from when it 'landed' facing the oppposite way to it was two seconds ago. Much less damage than if I had tried putting down.

[Phil Winks]

Ah newtons 4th law and magnetic trees how we've all suffered those oh and another thing power lines attract electric airplanes evidence my late lamented west wings beguine that lost half  its port wing while in a knife edge turn   the ensuing spiral earth wards was very educational  the choppped of wing section landed several moments after the rest of it had comprihesively re-kitted itself how we live and learn

[Peter Miller]

I remember many, many years ago, flying a Free Flight Junior 60 in the middle of an airfield.

There was one solitary post stuck up right out in the middle. The model hit it on the glide.

Going back to the orginal point being made at the start of this thread.

Drag equals thrust as has been said. If you increase the thrust the model's speed will increase until the drag again equals the thrust. Now the important bit,

Double the speed and the drag increases four times so if you want to double the top speed of a model you need four times the power.

[Former Member]

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[Simon Chaddock]

Never mind Newtons Laws, flying has much more to do with Sod's laws:-

 "If it can - it will"

 "If it hasn't - it will soon"

 "If it has - tough!"

[Mike Hawkins]

Eric, there is going to be a little bit of wood in most planes, the formers, the firewall, motor bearers in a foamie.

Anyway the early plastics were made from tree products, celluloid from cellulose (trees) and bakelite from coal (trees), although most modern plastics are made from crude oil products (of which a minority is formed from trees, though trees more readily form coal) with the'green' movement we are going back in the direction of plastics from trees. (My Chemistry teacher last year was really interested in this stuff).

[Phil Winks]

Not only do you need 4 x the power to double the speed of your model but 4 x the money  and 10 x the rebuild time when it gets away to quick and finds the trees