Hey, anyone have experience flying over explosives? Looks like I have a shoot coming up flying over a quarry where they want aerial shots of the demolition... wondering if anyone has had that experience? Anything to consider with pressure waves? Radio interference? what else might be of interest to think about? Thanks for any advice... steve
I've never tried filming over explosives. The way I fly is already dangerous enough. But I would be concerned that the shockwave will cause you some serious problems -- might it be intense enough to cause a rotor stall on all the props as the rarefaction wave goes by after the compression wave? If that happens you might lose some altitude suddenly. The radio transmissions from the flight control, gimbal control, FPV? Hmmm. I would hope by now that the explosive wrangler's have more resilient gear that cannot be triggered by a passing trucker giving a big 10-4 on his radidio.... You better tell them all the frequencies that your copter and ground station are radiating (e.g. 2.4 Ghz, 1.2 Ghz, and 5.8 Ghz) and see if their eyes open wide. Andy.
A shock wave occurs when a sonic or subsonic pressure wave travels at or beyond the speed of sound. Consequential damage is a result of the pressure magnitude as well as the speed as in the case of the US armed forces daisy cutter or commando vault. In the case of High Power Rocketry, the flight computers are essentially just data logging air pressure sensors or baro-sensors. Special features are designed in to handle mach transition without accidental chute deployment at Mach speeds which would otherwise occur. Here is some additional background in case you are interested. http://www.featherweightaltimeters.com/Featherweight_Home_REEQ.php
Graydon: Is there a commensurate rarefaction wave that follows the compression wave? I was wondering whether the sudden drop in air density would cause all the props to stall? Andy
I don't know enough about that to answer your question. Bu I know that the aircraft that dropped the Tzar Thermonuclear bomb over an arctic island above Russia 1975 fell over 1000 feet before flight control was regained. Obviously the shock wave for a TND is vastly more powerful than a shock wave from TNT or dynamite. Keep in mind that the front of the shock wave is well above ambient air pressure which would cause the FC to think that the MR was dropping rapidly to ground level or below resulting in a prop/motor speeding up to recover altitude when in fact the altitude did not change. When the shock wave passes, the air pressure returns to normal ambient and not lower.
Andy.... After closer investigation it seems there is a possibility that the air pressure immediately following a shock wave could drop below normal ambient air pressure. This is best explained in the following which involves some calculus, specifically derivatives. http://people.umass.edu/ndh/TFT/Ch09 Shock.pdf . Much will depend on the magnitude of the shock wave and the distance as distance attenuates the shock wave effect.
Thanks for the follow up Graydon. I've taken an oath never to study calculus again, especially derivatives! But you confirmed what I thought might occasionally be the case. Andy.
Hi guys. We did quite a lot of flying explosives for a feature film back in April 2013. Mikrokopter FC in manual mode with alt hold at times. I too was concerned about pressure waves, as well as heat waves, but watching the telemetry and video feed live and afterwards, there was no noticeable change in temps or in altitude of the drone. I did keep my distance to at least 40m from the blast at all times. One thing you will also need to give a lot of attention to is the wind direction. Even it if is just a light breeze, if the wind moves the smoke and dust cloud between you and the drone you may loose LOS for a few seconds. Be prepared, and don't panic. Also, do not rely on the video link as the dust cloud is full of excited energy and static. Best is to plan around the wind, and have a few exit path options planned beforehand. In all we flew a number of explosions with no problems at all, staying 40m away or more. Of course results may vary a lot depending on the size of the blast, proximity, etc.