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Time delay switch on 3

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ffinybryn

ffinybryn

Structural
Mar 17, 2010
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I wonder if anyone can help me with an idea...

Problem:
I am building a large (2m wingspan - 3Kg) RC plane which is propelled by a 11" propeller situated at the rear ("pusher prop").
The plane is designed to be hand launched. However, this has resulted in other pilots having their hands seriously cut by the rotating blades as it is released at full throttle.

Proper launch:

Possible solutions:
Gloves. Chain-mail or leather is a possibility.
Wheels. These add weight and are not suitable on grass strips.
Bungee. Some pilots stretch 30m lengths of bungee and use this to launch the plane to flying speed. This requires a lot of setting up and adding hooks etc. Cumbersome.
Launch then throttle. The plane is capable of being thrown into the wind and being able to glide before the throttle is applied. However, this means fumbling for the throttle and controls of the transmitter.

My solution:
Time delayed throttle switch actuated by removal of pin:
Full power is applied to the prop, but the "switch" remains off whilst a pin is inserted into the rear mounted switch.
The plane can now be thrown with the "throttle hand".
As the plane leaves the hand, a string attached to the wrist pulls out the pin.
After a time delay to allow the pin to be clear of the prop (say 0.2s?), the switch closes and the propeller starts.
The plane is controlled by the other hand already on the aileron/elevator stick.

delay_on_relay.jpg


The switch needs to be small to keep down weight. The power would be 3S to 4S Lipo. Max 12.6v(3S) to 16.8v(4S). The time delay would preferably be electronic rather than mechanical.

Any ideas? (BTW I'm Structural Eng, not Electronic!!)
 
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Buyond a time delay you could PWM the motor with a simple PWM rampup. That would give the operator some warning. I would consider a transmitter on the wrist. When the distance from the hand increases, that turns on the motor. safeguards are tricky. A look at youtube provides endless examples of what happens when humans get involved.
 
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Thanks. I had to Google the first sentence!
I had thought about proximity switches etc, but think the margins are too tight.
I may just move the pin/contact breaker away from the propeller, but this may cause eccentric force...
 
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I think it's the wrong approach, because too many things can still go wrong. The person will keep their fingers for an extra few months and then they'll be chopped off anyway, just a bit later than without the delay. It's not a foolproof solution.

Anyway, here one technology that might be applicable to this and other safety interlocks. Arduino IDE for 8-pin ATtiny chips.
 
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I see some copper tape where you hold it. The resistance of your fingers shorts out a delay cap with a 1 meg ohm pullup to power. This supplies gate voltage to a FET. So. toss the plane and a second later the motor starts.
 
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Sorry, but this is scaring me. We're talking about chopping off human flesh in chunks. This really needs to be fail-safe if the motor is that powerful. Any fixed time-delay will be defeated by the spouse or compadre of the plane-tosser hollering at them for some other purpose. Or the typical teenager hears the "bing" of an incoming text message. In either case, they'll forget that they're holding the equivalent of a lawnmower that's about to start.

OperaHouse has the safest idea so far. I can't comment on the failure modes of the the cap and FET, though. I'm not an electronix guy.

I would "hand launch" that puppy from the end of a long pole.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies
 
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>>I would "hand launch" that puppy from the end of a long pole"<<
This was exactly my suggestion, when I first saw this beast!!!

This is why I'm seeking alternatives. My plane is virtually ready to maiden. I have the chain-mail glove and 10m of bungee (which stretches to 30m). Launching is very much the issue in the RCGroups forum (also for those that are interested in the electronics involved):

Unfortunately, I don't think that there is a ready-made electronic solution to this. Thanks to all.
 
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The pull-the-pin (shudder) method will ultimately result in the pin-with-cord unpredictably flying upward into the prop where it will either become a legendary snarl that sucks the plane back into the lanyard hand or instead clips the pin so it leaves the area of the prop at about 300ft/s hitting.. what or who's eye?

The best method would be a microprocessor looking at an accelerometer. The throwing method described has a very unique repeatable acceleration profile that could easily be identified by the micro and as soon as the acceleration stops the prop can take off because the plane has left the hand.

Barring that, the chain-mail glove is the simplest method. Its only drawback is that it will reduce the impulse, probably significantly, to the plane, due to its additional weight.

DRWeig; Don't fear loss of fingers. That size prop/engine/motor won't chop fingers off. It will, as mentioned, cut them though.


Keith Cress
kcress -
 
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How about the way they toss fighter planes off an aircraft carrier? You can probably make one of those and power it with a portable bottle of compressed air. A little pneumatic catapult to shoot the plane up at an angle, then jam the motor throttle to full with the RC controller?

Thanks Keith, but I'll put this in the same category as bungee jumping and rollercoasters. Not for me. I'd probably wind up with bruises from the thing hitting the chain-mail glove. My paranoia has grown with age. See if you can pick out which one is me in this little video:

Goober Dave's A Capella Band



Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies
 
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I like OperaHouse's most recent suggestion. No cables to tangle, no moving parts to get stuck. At worst it doesn't work and you're either back where you started with the motor starting early or the motor fails to start at all and the plane hits the dirt. I don't think either of these scenarios are made significantly more likely by adding the feature, and it's not like most safety controls that breed complacency - even with the control in place you're going to want to avoid contact with the prop.

See the attached sketch for the circuit. 5 discrete parts (or 4 if R2 can be eliminated because of batt voltage and FET choice, or 3 if there's a kill signal you can use instead of the FET) are required with minimal weight. The switch is normally open and the resistors bias the FET on. When you hold the plane the switch closes, discharging C1, turning the FET and the motor off. When you release the plane the switch opens again, charging C1 via R1, turning the FET and motor on after a delay.

As OperaHouse suggests, the switch could be as simple as two copper pads that your finger bridges. This requires minimal contact pressure but could be susceptible to subtle movements of the hand. It could be as positive as a push button switch that must be held down while launching the plane. Making the switch natural and reliable would probably be the hardest part.

 
 http://files.engineering.com/getfile.aspx?folder=c64db7f5-f51b-404e-9d22-5efba97c35e3&file=delay_start.PNG
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DRWeig, the catapult method has also been used, with some amazing contraptions:
However, it's more luggage that needs to be packed into the car and more setting-up time.

My only problem with the chain-mail is that it reduces finger-tip sensitivity and ability to grip, which could cause the plane to slip at the crucial moment.

LiteYear. Although I wouldn't know where to start with your components, I appreciate your input and can see your train of thought. This is certainly an alternative to the pin method.
 
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That is the basic circuit although on further thought I would replace R2 with an 18V zener, Typically the FET gate voltage must be kept under 20V to prevent damage, R2 would act like a voltage divider for battery voltages over 20V. Do you use something like a Turnigy 130A to tune your prop? I have one I use for a solar project and I'm quite impressed. In a search through message boards they complained that 60V and 130A was not enough range. Some of you RC guys use serious power.
 
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Most use 3S or 4S for this model (12.6v or 16.8v). Some use 4S in parallel for longer flying times. I had planned on 3S, but bench-testing gave the impression that it wouldn't be sufficient - even though an online prop calculator had said it would. I have an 80A speed controller which is more than adequate.
It's the props that are frightening. Up to 13" (folding, so as not to be damaged on landing).
Anyone finding a cheap, simple yet reliable method of launching "pusher prop" RC planes would be onto a winner!
 
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It's historic, before my time, but I guess less damage to property/people during unscheduled landings (crashes) and to the props themselves.
Smaller planes have the props sitting on top of the fuselage well out of the way of the launcher, but due to the size and weight of the X8, it requires a thrust through the CL of the plane. (Folding props allow them to fold backwards on landing - assuming the throttle is off).
Any loss in efficiency is made up with bigger motors and increased power.
It's also beneficial to have the props at the back for this FPV (FirstPersonView) plane as the forward facing camera (by which the plane can be flown remotely) is not obscured and allows for some great aerial footage.
 
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