Altitude limits - RC Groups
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May 25, 2011, 10:07 PM
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Altitude limits

I am just getting started in FPV and will not be flying more than 2-3 km for a while but in due time I will push the limits. I have been reading a lot about flying long range and the limitations therein. Seems like we have limitations of video/RC link range, and there is a ton of info about that. Also we have the limits of battery/fuel and simple calculations of airspeed/run time can be done for this. What I am having a hard time understanding is why we need to climb so high as we fly away.

I suspect it is a limitation in video and RC link, perhaps signal pattern. But in the USA the FAA limits model airplanes to fly under 400 feet. I am interested in doing things legally so I would like to open discussion on flying long range under a 400 foot ceiling.

I really like how Roberto is utilizing thermals to overcome limitations of run time but it is published that a 1:10 range to altitude ratio should be used. And in the USA this is illegal.

Trappy was able to take advantage of mountain terrain to elevate his broadcast position. However I would like to fly flatland, under 400 feet.

So the questions: Why the 1:10 ratio? and can I place my video RX/ antenna tracker and Transmitter TX module on a tower to achieve longer range at lower altitude? how tall should the tower be to reach 40-50 km range or further?


- Joe
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May 25, 2011, 10:37 PM
Registered User
If you plan on reaching 40-50 km while staying under 400 feet, chances are, you're not going to get close to doing that. It might not make too terrible of a difference but small changes in elevation and the curvature of the earth are going to give you issues if you plan on getting that far legally. If you're planning on staying legal, you technically can't fly out of line of sight so your spotter is going to have to watch the plane the entire flight as well.
If you get your receiver up high, you'll definitely get better range but then you also have to deal with how you're going to get cables from your receiver to the ground, especially if you get it up on a high tower. I believe you'll run into signal degradation if you make your cables too long. I guess you could just transmit that video down using a different frequency to get around that hurdle.
If you plan on staying completely legal with everything, don't expect to be able to push the limit to 40-50 km. You're not getting that far.
May 25, 2011, 10:42 PM
Originally Posted by chemnut842
I am interested in doing things legally so I would like to open discussion on flying long range under a 400 foot ceiling.
FPV already operates in a grey area of the law, as most cool innovations do. As for 400 ft there is no law that states that you can't fly higher (yet). If you want to do things legally...well...have fun with the FAA waivers.

May 26, 2011, 01:18 AM
Registered User
Toysrme's Avatar
1) The FAA advisory on model aircraft is law
2) The OTHER FAA advisory on UAV's specifically states that hobby UAV equipment is NOT treated the same as commercial. Hobby UAV falls under the previously set forth model aircraft laws in the oft quoted advisory

Do what you will, but continuing to think the law is not such is simply showing personal ignorance.

If you wanna fly a long way without going higher than 400' you need to park yourself on something very tall like a mountain and fly through the surrounding area.
May 26, 2011, 01:54 AM
Registered User
Daemon's Avatar
Advisory Circulars are *not* legal regulations in themselves. They are by
definition, provided by the FAA for informational purposes only. They may
reference regulations, or simply provide information and guidance (best practices).
does not reference any regulations and starts with the language
"1. PURPOSE. This advisory circular outlines, and encourages voluntary
with, safety standards for model aircraft operators."
(emphasis mine)

Non-commercial model aviation is currently exempt from almost all FAA regulations
except when specifically mentioned in TFRs and similar. This is all in
the process of changing for the worse.

May 26, 2011, 01:56 AM
Originally Posted by Toysrme
Do what you will, but continuing to think the law is not such is simply showing personal ignorance.
You are wrong.

(Hope the link works.)
May 26, 2011, 02:38 AM
Registered User
Fresnel zone is one thing that will affect range while keeping a low altitude.

The interesting thing is that the radius is based on the distance away and the frequency which may mean that for low altitude flying (with clear los) a high frequency tx/rx pair may get more range than a low frequency one. The tradeoff being obstacles in LOS will more adversely affect the high frequency than the low frequency.

Fresnel Zone
May 26, 2011, 03:08 AM
Zephyr 66" Blunt
mark hitchman's Avatar
I am pretty sure 20km is possible staying below 400'
I have done 10km at 40' so doubling that range with 10 times the altitude should be possible. Keen to try this but the plane I use needs more battery capacity.

May 26, 2011, 03:15 AM
Registered User
Daemon's Avatar
10km at 40', over flat ground? Or from a high point?
Over flat ground, I don't think you can do 20km under 400ft. I think you'd
need 600-700ft minimum. From a high point, not a problem.

May 26, 2011, 03:20 AM
Zephyr 66" Blunt
mark hitchman's Avatar
From flat ground, 9dbi yagi receiver antenna at 3m above ground. I still think 20km at 400' is possible, people would think 10km at 40' was impossible.

May 26, 2011, 03:25 AM
Originally Posted by Daemon
10km at 40', over flat ground? I think you'd
need 600-700ft minimum.

I crunched the formulas according to that website, for 900 mhz at 20km you'd need a minimum of 1,000 ft for the radius. There must be some assumption with that formula about the directionality of the antenna, because 1k ft seems like way too much.
Last edited by Blueshy; May 26, 2011 at 03:30 AM.
May 26, 2011, 03:39 AM
Zephyr 66" Blunt
mark hitchman's Avatar
Here's the vid and google earth pic.

10 km out and 12 meters high (12 min 24 sec)

May 26, 2011, 07:19 AM
Registered User
That is cool, 10 km at such low altitude. So your receiver was 3m up? This is what I had in mind, or a collapsible tower that could get your station 10m up or so.

As far as spotters, I was under the impression that AMA required a spotter and that was only for AMA fields.

I just want to stay clear of any conflicts with FAA.

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