Duration

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Re: Duration

Post by jander14indoor » February 25th, 2013, 3:25 pm

This is a debate that has gone on since Helicopter was introduced. At first, it was attributed to pointy ceiling bumpers somehow 'sticking' into the ceiling and holding the helicopter up when the rotors did not provide enough lift to hold up the copter. Alternatively some claim that somehow stopping one rotor, without it hanging up vertically, and transferring power to the other rotor in some way that's more efficient than two rotors and increasing efficiency. Some claim that ES are not properly recognizing when a helicopter is hung on something and not self supporting.

The pointy ceiling bumper discussion resulted in the round disk at the top of your copter rule. Sticky substances on the disk have been banned when recognized.

One rotor stopping (without providing vertical support) doesn't match reasonable aerodynamic theory. L is proportional to velocity. Drag is proportional to velocity squared. All other things (form, area, temperature) being constant (as they are in a single copter). If two rotors have to spin at x rpm (translates to velocity x1 at the tips), one rotor will HAVE to spin at 2x to give the same lift, creating 4x drag. The rubber CANNOT spin twice as long. Never believed that theory.

There is a 'theroretical' possibility if BOTH rotors keep spinning. A wing flying closely above a horizontal surface gains lift above the normal due to interaction with that surface, something called ground effect and very demonstratable. I'm sure a wing would also interact with a surface above it, though I haven't seen the math/data which says whether it helps or hurts lift. If it helps, that might explain longer flights when touching the ceiling. But as long as all teams have access to that same smooth ceiling, I don't see a problem with that. Though frankly I haven't seen it demonstrated in practice

Now, before we added the disk, I saw no reliably reported data that did not either fit the stuck in ceiling probe, or ES not recognizing a copter was not supporting itself.

Oh, in a site with raftered AND smooth ceilinged areas we did see that competitors could wind harder if they flew in the smooth ceiling areas (even with a lower ceiling) and get longer flights than if they flew in the raftered areas because there was no way to avoid hanging up and stopping the flight if they wound that hard in rafters. In fact, last years nationals winning teams exploited that knowledge.

If you are seeing reports of this, the rules writers would appreciate reliably and carefully described first hand examples so we can see if a new rule is needed, or if students have somehow discovered a valid aerodynamic behavior and exploited it.

Oh, I'm not sure 1:20 is all that wicked a time. 2 minutes has been posted on this list, and seems reasonable given this years design constraints.

Thanks,

Jeff Anderson
Livonia, MI

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Re: Duration

Post by wlsguy » February 25th, 2013, 4:11 pm

Jeff, I propose the following reason why stopping the top rotor gives longer times;

The typical helicopter with 2 rotors has more than enough lift to reach the ceiling and would probably continue to climb for a very high distance without the obstruction. To maintain the altitude requires far less lift. If the top rotor is stopped, the bottom rotor has enough lift to keep the helicopter at the ceiling for a much longer amount of time since the top rotor is not using the winds. Once the bottom rotor slows and cannot hold the helicopter it usually falls away from the ceiling. At this point, the top rotor will generally start turning again and the combination of the 2 rotors quickly use the remaining amount of winds.

Just my theory....

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Re: Duration

Post by foreverphysics » February 25th, 2013, 6:30 pm

sparkchaser wrote:What was the ceiling height on some of these posted times? Our team could try the 24 foot ceiling and loose their helicopter or 9 foot dropped ceiling. We played it safe after seeing two other teams loose their helicopters.
I assume (for various reasons) that you went to the Auburn University Regionals. If so, that was my copter that you saw stuck in the rafters. -_- (to be fair, we did both runs on the tall ceiling, lost only one copter, and still ended up getting third).

9 foot ceiling time: 32 sec with good copter
Higher ceiling time: 54 sec with good copter (I really cannot build)

That was the only reason we chose to take a risk.

The 1st and 2nd place times at Regs were 1:34 and 1:21. Both did them on the low ceiling. (I watched them, and also, the kids were on my team, only not on my competitive team.)
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Re: Duration

Post by rmp509 » February 26th, 2013, 5:45 pm

i consistently get 2 minutes on a ten foot cieling. would that be competitive for a regional competition?
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Re: Duration

Post by A Person » February 26th, 2013, 5:52 pm

rmp509 wrote:i consistently get 2 minutes on a ten foot ceiling. would that be competitive for a regional competition?
I would say so. Depends on your regional's competition.
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Re: Duration

Post by talicoa » March 3rd, 2013, 2:13 pm

If you can stop the top rotor, you can get better flight times. The torque in the rubber band is only able to drive the rotor at a certain speed. It won't spin twice as fast. The rubber motor unwinds at a slower rate due to higher drag. With both blades spinning they more efficiently use up the winds driving the helicopter into the ceiling and creating a wasted upward force. Now, how you stop the top rotor legally without causing damage to the helicopter is a good question. It is relatively easy to slow the blades down by using a short stick beneath the disk and letting your rotors bang into the ceiling. but you risk breaking the rotors, ruining the flight covering, and weakening your wings over time. It also makes it less stable as the bottom rotor is over driving the top rotor. I think time is better spent getting an efficient design matched well to the motor, and not trying to stop the top rotor. It will provide for much more consistent times which are easier to improve through testing.

Tom

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Re: Duration

Post by fifty_missions » March 6th, 2013, 5:48 am

At one of the Indiana regionals recently, the best time was 2 min, 3 secs in a 27' ceiling with plenty of beams/bracing clutter. The rest of the other competitors' times were clustered around a minute or less. Rumors are already afoot that the team was able to "slowdown" the rotors for the longer duration. Not true; the helicopter weighed 3.2 grams and the motor was matched to the site. Yes, they have some curious features on their helicopter. These add nothing to performance but do aid survivability at the "top". The same team will be going to the state finals at Indiana University and plan one more build that features a helicopter with bright colors on the rotor tips. Why? the ceiling is over 60' high and jammed with steel beams and braces. The bright tips will simply allow the officials to keep track of the otherwise 'stealth-like' vehicle.

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Re: Duration

Post by Hosj » March 9th, 2013, 7:25 am

We got second at the Central Maryland Regionals, probably one of the least competitive regionals in the nation, with a time of 55 seconds. We used the freedom flight kit, somewhat poorly built. The moajority of the helicopter falied to get 10 feet of the ground.
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Re: Duration

Post by NerdGirl314 » March 10th, 2013, 6:56 pm

That's not to bad. We used the 2013 freedom flight model and got 1 minute 41 seconds despite it being broken in several places :D We ended up getting first place in the region but there's always room for improvement. Can't wait for states!
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Re: Duration

Post by hogger » March 12th, 2013, 8:45 am

wlsguy wrote:Jeff, I propose the following reason why stopping the top rotor gives longer times;

The typical helicopter with 2 rotors has more than enough lift to reach the ceiling and would probably continue to climb for a very high distance without the obstruction. To maintain the altitude requires far less lift. If the top rotor is stopped, the bottom rotor has enough lift to keep the helicopter at the ceiling for a much longer amount of time since the top rotor is not using the winds. Once the bottom rotor slows and cannot hold the helicopter it usually falls away from the ceiling. At this point, the top rotor will generally start turning again and the combination of the 2 rotors quickly use the remaining amount of winds.

Just my theory....
I was just reading this again. It makes some sense but if you can't build it to weight at 3 grams that extra rotors that stop are just a lot of weight taking away a lot of efficiency. And the bouncing against the ceiling should also result in a lot of energy wasted. I think the key to 2 upper rotors (4 blades) is that the helicopter flies so much steadier than the 1 rotor (2 blades), especially during the descent, that is where the efficiency gain comes from.

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