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Using Gps for speed without mixup - launch - 12/5/2012 12:14 AM   
rcuname


 

Posts: 19
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Joined: 7/22/2012
Last Login: 1/21/2013
From: Pantego, TX, USA
Status: offline 		Since I use a good catapult for the launch then I will have to get a base reading on the launch - then and only then if the GPS reading is somewhat higher after a dive or some other aerobatics will I know my max speed. My ships are heavy so I am sure that the catapult launch speed can be beaten easily.

Am I missing something? My ship is a glider but I am sure this applies to ducted fan with bungee start as well also?

The Gps takes the max speed hit per flight and saves it for me...Havent tried it yet..


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RE: Using Gps for speed without mixup - launch - 12/11/2012 3:09 AM   
Lnewqban



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quote:

ORIGINAL: rcuname
Am I missing something?

Horizontal speed OK, not sure if GPS can measure diving speed precisely.

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RE: Using Gps for speed without mixup - launch - 12/11/2012 11:25 AM   
da Rock



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Status: offline 		In theory, wouldn't the speed GPS sees in a dive be slower than the model actually runs. Any angle not seen in the distance covered would give a longer distance than assumed.

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RE: Using Gps for speed without mixup - launch - 12/12/2012 12:15 AM   
alasdair



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Status: offline 		I believe that any of these single shot, max speed memory devices is a complete waste of time.
It will depend a bit on the frame rate.   They work by calculating the position, then calculating another position, then calculating the distance between those 2 positions and dividing by the time interval to get the average speed.

If the time interval is very short, then the two positions will be close together and the line between them will be short and almost straight. The trouble is that the positions have a random error. Is the positional error plus or minus 1 metre or 5 metres or what?  So if position A is calculated 1 metre too far forward, and position B 1 metre too far back, then the distance error is 2 metres and if the time interval is very  short you get a large speed error. Too fast, or too slow.

If the time interval is very long then the position error doesn't matter so much, but in a turn or loop the distance between the two points (straight line) will be shorter than the flight path, so the speed calculated will be wrong (too slow).

The only valid speed data you can obtain from GPS would be a continuous graph of speed (ground speed obviously), so that you can spooth out the spikes and dips due to position errors.

I have used an Eagletree GPS add-on to their data recorder and obtained some useful data. The data can be downloaded as a continuous stream that can be graphed. You can also download heading data (so you know which bits are upwind and which downwind) and if memory serves you can even download the number of satelites locked.
I found that the graph of groundspeed had many spikes and dips, and if I turned with too much bank the number of satelites used dropped, while if I rolled or looped I lost the signal altogether. Even a tight turn could lose the satelites.

You can only believe GPS speed data if it is part of a continuous data stream.
A single maximum spike is laughable nonsense.

< Message edited by alasdair -- 12/12/2012 6:25 PM >

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