Bungie Launch
Patrick Schindler, who likes fast planes, put together a nifty little portable bungie launcher to propel his Zagi wing and Multiplex Microjet skyward.
It consists of a steel plate with a latch, about 100 20 feet of surgical tubing and a stake.
In this photo sequence, you can see the surgical tubing stretching across the Great Meadow, stretched to about 50 feet. Then he launches the Microjet by kicking the latch and the tubing zips forward, collapsing in a rubbery ball midfield.
May 1st, 2008 at 8:50 pm
That guy looks like he is totally out of control to me. . . Someone should restrain him. Next thing you know he will be trying to launch a goose with that thing.
May 3rd, 2008 at 4:19 pm
Don’t need that much tubing, about 20 ft or so and about 20 ft of string will do. You are not trying to get the plane high up into the air, you are just trying to get the plane up to flying speed, just like the catapult on a carrier. My standard tension is about 4-5 G, i.e. tension = 4 to 5 times of the weight of the projectile being launch.
May 3rd, 2008 at 11:27 pm
There is only 20 feet of tube and no string. My total launch distance is 50′ at 2.5X stretch. That is absolute minimum to get this bird up.
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
May 5th, 2008 at 2:17 pm
No wing loading needed. All you are doing is to accelerate an object to certain speed (i.e. the speed that the control surfaces can function). You are not making the plane to climb with the bungee, just to accelerate the mass to a certain speed. It could be a rock or an airplane, does not matter. Actually a plane with wing would have less drag!
May 5th, 2008 at 9:45 pm
That is an invalid argument on two accounts.
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
May 8th, 2008 at 9:13 pm
I would be interested in seeing photos of your plate with kick latch release mechanism.
May 10th, 2008 at 10:48 pm
There is a mini build thread near the end of the microjet thread on RCGroups
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