I want to get a propeller to get a helicopter to hover about 4-5 feet above the ground. I am trying to figure about how to do so, should i use a pressure sensor? Ping? Infared Distance Sensor? Any links to projects with good desriptions and code examples? Thanks so much everyone!
should i use a pressure sensor?
No there is not enough variation in atmospheric pressure to detect 4 feet.
Infared Distance Sensor?
No you get light interference and the uncertain nature of the reflecting surface will lead to unreliable operation.
Ping?
Best of the three but not to sure about the reflectivity of the surface and any turbulence caused by the copter.
I've read about people achieving surprisingly good resolution with a pressure sensor so that seems like a viable option as long as you get a sensor with adequate resolution. There are ping sensors that operate up to that sort of range, but I don't know how well they would work just reflecting off ground, especially if they're in a noisy environment.
I assume you have recognised the separate problems of holding the vehicle level and stationary.
I'm thinking tie a string to a big rock and hold full up hover power 
No serious suggestion tho ...
PeterH:
I've read about people achieving surprisingly good resolution with a pressure sensor
To within a few inches of altitude? Underneath a helicopter rotor?
Grumpy_Mike:
Ping?
Best of the three but not to sure about the reflectivity of the surface and any turbulence caused by the copter.
I don't think low frequency turbulence will affect ultrasonic sounds much... but yeah, lawns, carpets, etc. aren't very reflective.
I got no ideas though. You might be able to cheat if it's in a fixed, known environment.
If you just want to go out onto a field and expect it to work...that's a difficult problem. The only thing that occurs to me is have an external sensor that can see the height of the helicopter (a "human" is commonly used for this but you might be able to automate it...)
Google Ardupilot Mega.
They use barometric pressure for higher (more than a few feet) altitudes, and sonar near the ground.
-j
fungus:
PeterH:
I've read about people achieving surprisingly good resolution with a pressure sensorTo within a few inches of altitude? Underneath a helicopter rotor?
I'm not sure about the 'few inches' but I was surprised at the resolution that was achieved - I don't remember the exact figure but it was adequate to maintain a hover ten feet or so off the ground. This was on a quadrotor, so the sensor may not have been directly above or below the rotor, but with a sensor designed to pick up static pressure I think the effects of dynamic pressure could be eliminated unless you are moving into the supersonic realm.
Could you use a linear CCD array as a detector and an offset and angled laser pointing at the ground. The CCD (maybe through IR filter) sees the laser dot on the ground and knows to hover higher/lower to get dot in required position on CCD array. See Using a CCD linear array (TCD1304AP) with C2000 microcontroller - YouTube
Hover means to be so close to the ground that you can produce a pressure which will allow you to "sit" on it like a cushion.
This pressure is a static pressure and can be measured, and depends highly on the distance from ground.
Only little active control is required to keep a hovering state at a constant level.
It's of course independent from atmospheric pressure and its variance in height.
However, I'm not sure if a quadcopter at a couple of feet above ground is really hovering in this strict "hovercraft" sense.
Riva:
Could you use a linear CCD array as a detector and an offset and angled laser pointing at the ground. The CCD (maybe through IR filter) sees the laser dot on the ground and knows to hover higher/lower to get dot in required position on CCD array. See
I thought of that, too, but it needs image processing to work. You'll need something more powerful than an Arduino.
I thought of that, too, but it needs image processing to work.
Yes, but it is only 1D image processing.
michael_x:
Hover means to be so close to the ground that you can produce a pressure which will allow you to "sit" on it like a cushion.
That's only one of many possible definitions. A helicopter stationary in mid air is usually referred to as 'hovering' even though it may be well above the height where ground effect occurs. Submarines can hover, too.
Harriers (AV8) hover too, and not in ground effect.
You need one of these: http://www.roke.co.uk/resources/datasheets/mra-type-2.pdf
Bit expensive though. Also may be too large for your application.