Para Recovery - Failsafe Sensor Mechanism

[N2EC]

I would like help on how to acheive this. I have a parachute installed in a UAV which is used for recovery. However I would like to turn the parachute as a failsafe recovery feature. The current system allows me to eject the parachute with the help of a servo when the airplane is within visual range by switching over from CIC Mode to PIC Mode and switching the release switch. What I would like to do is that the parachute should automatically eject when the engine dies outso that the airplane would survive in the event of an engine failure...

Thank you for your inputs.

[Tom Harper]

Eagle Tree has a Hall effect pick up for their tach input. It would be easy to rig as an engine on/off detect.

[LukeZ]

N2EC, not really the point of your post, but if you care to, I'd be interested to see how you have the parachute rigged up - what it's attached to on the airframe, how it deploys, etc...


Luke

[Joe2003]

I know that some people have done them using model rocket ejection charges:-

http://www.deepskyrocketshop.co.uk/product_info.php?cPath=50_10109_10056_10059&products_id=11374

They use black powder though, so I dont know how easy it would be to sort, the electronics is easy though.

Is it an electric engine?
How would you sense engine failure, is it when the motor isnt turning or when it stops responding to input?

You could do it with a pic or stamp chip. Im not too good at the moment but I think you could do that very easly with a basic X chip. Just a constant loop sensing the signal line on the esc, if a signal doesnt exist or is below a certain pulse width, then send a high pulse width to the paracute servo. The whole unit would just plug inline with the esc.

[N2EC]

The airplane is powered by a gas engine..what could be done also is a loop around the ignition coil and if there is no current passing then the servo would be activated...i think a missing pulse detector would also be needed...i am not too good at this and would be happy with detailed info..

Luke Z...I shall post the rigging of the parachute soon..dont have pictures with me immediately

[kd7ost]

Can you do any programming with something like the Parallax Basic Stamp? You could monitor many different conditions to provide the failsafe activation.

Dan

[Joe2003]

Out of interest, how heavy is your plane. I used to do loads of rocketry and the size of chute you would need to bring down a 1kg plane at say 1ms would be huge :-s

[skogsvargen]

Joe,
That´s interesting. Is there a formula that describes the relationship between weight, drop speed and chute size?

/Magnus

[Joe2003]

There sure is, A-Level physics wasnt a complete waste of time :p The
maths only works for a flat surface area chute but gives you a good idea
of the kind of dimentions you need to be working to.

The equation is:-

F = 1/2 r C A (v 2)

F is the drag force, which is basically mass of the
object x acceleration due to gravity
r is the air desinty (1.22kg/m3)
C is the drag coefficent (0.75)
A is the area of the chute
v is the velocity through the air ( so the speed you want the object to hit the ground

so rearranged if you want the diameter of the chute, given the velocity of impact:-

D = SQUAREROOT((8*m*g) / (3.14*r*C*v2))

or for impact speed

V = SQUAREROOT((8*m*g) / (3.14*r*C*D2))

but the is a simple online calculator type thing too:-

http://www.rocket-simulator.com/parachute.php

[Unterhausen]

at approximately what speed is balsa reduced to toothpicks? Is 1 m/s the speed the rocket guys aim for?

[Joe2003]

no, they aim for 2-3ms, but that is a real impact which would easily trash a plane. I would aim for >1ms

[kd7ost]

I've experimented with bringing an RC plane down by parachute before. It worked out fairly well but I learned you do need to slow it down a lot more. I had the parachute tied at the spar. So it would be at the center of lift on descent. The chute was too small though for a gentle float. The descent was fast enough that the plane rotated nose down due to weather cock effect of the tail feathers and busted out the engine mount on landing. (Tractor design)

I learned that first, attack the chute on something like a Kite Yoke but longer. One end tied forward of the wing LE and one tied nearer to the tail might do the trick. That might prevent the rotation.

Having a impact resilient fuselage might help too.

Try searches on Spectra chutes, Rocketman and spherachutes. The Rocketman chutes are pretty heavy though.

Dan

[Trisquire]

The plane would be moving too fast if the chute opens right after the engine quits. A stopped engine could trigger a pilot chute which would allow the initial slow down. A few seconds later, the main chute would deploy.

Tom

[kd7ost]

The plane would be moving too fast if the chute opens right after the engine quits. A stopped engine could trigger a pilot chute which would allow the initial slow down. A few seconds later, the main chute would deploy.

Tom

I think thats too complicated and prone to tanglement. It's a fun topic though. We used to jump out the door of C130 and C141 aircraft going 130 knots. I weighed, with gear, about 250 pounds. The chute would open at a pretty explosive rate.

If you compare and take a 20 pound airplane even if it was travelling at 80 mph, the openning shock isn't nearly as severe. Deployment rings can be used to slow the full openning shock. As long as the chute is made pretty strong, and those chutes the rocket guys use are pretty stout, you should be OK.

Dan

[Trisquire]

Maybe just a sturdy shock cord between the chute and the plane; like the rocket guys use.

Tom

[rclinks2002]

on the UAV I am building we are using a 6 foot cargo parachute (I think it was meant to help deploy the large chutes for things like humvees and such. It is very sturdy and heavy though. It uses a spring loaded system drag chute (a flare chute off a bomber) to pull the larger chute out. If the mount points are correct there shouldnt be too much of a problem with the thing destroying itself..... As for the electronics of the operation, for my system we are using a pulse operated timer that you set prior to take off, and it will deploy so many units (minutes/hours) after a valid signal has been lost. Basically, it is a modified failsafe circuit that has a time delay on the preset position.

Ben

[N2EC]

rclinks2002..can you share some pictures of your spring loaded pilot chute....

[rclinks2002]

I will have to get back with you on that