http://www.rcgroups.com/forums/showpost.php?p=9638713&postcount=2253

Here is the rest of the thread:

http://www.rcgroups.com/forums/showthread.php?t=460889&page=23#post9724904

I suggest expanding on the idea and doing it better.

-methods

First I will address surface size

You just said it yourself:
quote:
“(i.e. the speed that the control surfaces can function) ”

Let object A be a 2 lb microjet with tiny elevons
Let object B be a 2 lb 3D plane with huge surfaces

What you just said is that you must accelerate a mass to the speed at which its control surfaces start to function. That part is correct. What is incorrect is that you are assuming that all 2lb objects achieve control at the same speed.

All 2lb objects do not achieve control at the same speed (which is what I said before)

2lb Object A must be accelerated to about 40mph to obtain control
2lb Object B must be accelerated to about 10 mph to obtain control

This disproves your first statement

This is because object A has TINY surfaces and object B has HUGE surfaces. I don’t know how you can argue against that kind of logic. The weight of the model is not enough to make the decision. The flight characteristics (i.e. wing loading, surface size, etc) are what determine the amount of force needed on the bungee.

Now, the second part of your argument that is incorrect is as follows:

quote:
“You are not making the plane to climb with the bungee, just to accelerate the mass to a certain speed”

It is absolutely necessary to make the model climb. This microjet needs to be (at the absolute minimum) 30 feet in the air before I hit the throttle. On power-on I easily dip 10 or 15 feet down.

You need to accelerate an object “until it starts to fly”. This “speed of flight” is not directly related to its weight. It is indirectly related to its weight and directly related to its flying characteristics.

If you are not satisfied with my arguments we can settle it with an experiment at Baylands.

I will leave you with a final visualization that I hope will end this debate:

Object X weighs 2lbs. Object X is a HUGE KITE made from unubtanium. It has a 30 foot wingspan and it just “wants to fly” real bad. You could fart and make this thing fly.

Object Y also weighs 2lbs. Unfortunately Object Y is a brick with a paper airplane taped to it.

If I “tune” my bungee to some constant times 2lbs it will be a total failure. Lets assume that constant is 4-5 like you suggested. That would be 8 – 10 lbs of “pull”.

Object X, the large kite, will be broken into pieces by the sudden force
Object Y, the brick, will tumble a few feet and stop.

This example illustrates that multiplying the weight by a constant does NOT give you a reliable tension speed. You need to take into account the model you are trying to launch. . . i.e. Its wing loading etc.

And all this after I only had 20′ of tube in the first place !?!? lol…..

-Patrick

That said this is a microjet that weighs 30 ounces with only a 26″ wingspan. The wing area is 200 square inches so I will let you do the wing loading math. “Launching Speed” as you put it is surprisingly high. This is no RTF. . . 6S lipo at 1,400W. It needs a lot of pull.

Your argument does not take into account wing loading. You need to account for wing area, not just weight.

-Patrick