I donīt think that Walmart sells Garmin GPS.

They do here.

I took my GPS on a 200 mile drive this weekend after reading this thread. I had it on the entire time and compared the GPS speed with my speedometer reading. As long as I was moving, the speeds were with in .5 Mph all the time as all speeds. I also confirmed this by bringing along my radar gun and getting a reading of several over passes to get check the reading on my speedometer.

I have a Garmin GPS Map 76S but have not put it in a plane as of yet.

Just food for thought!

Enjoy,
Randy

I'm not saying GPS cannot be accurate...only that it was not made for this purpose, and may not be accurate for this job.
Radar guns, for instance, often have a given range for which they are accurate, like 0-100, for example, and may not give accurate readings over 200 mph. Depends what model. Similar things may apply to GPS, too. At a nice steady state in your car they may be fine, yanking and banking in a model airplane, maybe not.
Maybe it is getting spurious signals from the ECU or radio, entirely possible, right?
Maybe it is just the momentary acceleration off the bungee, yes?
Maybe someone should make some more runs with some more verifiable measurements, like radar and timers. I'm sure if it is doing 350mph, you will have no problem finding volunteers, and no problem verifying it, and all us doubters will have to eat crow!

Hi Veritech,

So you think that the speedometer in your car is accurate to within 0.5 mph ????!

What type of car ?

Mike C

15% error on civilians GPS ! Wow ! I wonder how sailplanes inboard Computers/Gps units can now be authorized for competitions, and give accurate informations at about 100 meters precisions for 500 kilometers circuits....?

Bernard

Hmm.... I vote C

Is the Hornet ready?

Todd

I have used Radar and Laser to check speed, they are both accurate and reliable they both have pros and cons, but in this application I feel they fall well short of ideal.as for vehicle speedos...they suck, right from the factory a 3-7mph varible is the norm, plus you have to figure in tire wear as well as proper inflation. this is why most officers wont write tickets unless you 10-15 over. (I know some write for 1 over) The biggest problem with GPS is the handheld units dont compensate well for the AOA that the jet is at, ie. the jet could cover 100m of ground distance but the net distance could be greater or less based on the plane it was travelling thus giving a indicated speed that could be higher or lower than true airspeed, plus the low cost GPS systems dont have the accuracy that 747Drvs aircraft do. B1 Driver pretty much nailed that, The most accurate timing systems for this application in my opinion would be point to point measuring systems, similar to law enforcment's VASCAR that can factor in all of theses variables. Just my two cents. also using overpasses to estimate your speed in a vehicle isnt all that accurate either.

Im sure that owners of competition ready sailplanes can afford a better GPS unit than the el cheapo ones at walmart. (not 100% sure but, since the airlines have challenged the government for the decoders of A band, I believe they have been released...hence the GPS approachs, ect..ect. Therefore, if you can afford it, you can get extremly accurate readings from GPS units. All in all..you get what you paid for...want to impress your friends..go to walmart...want to impress nonbelievers, go to NASA.

No you cannot infer that your GPS unit is accurate by comparing readings at random times with your car.....Your car, (if on a long trip) is usually moving in a semi straight line thus making it easy to calculate from where it is at point a to where it will be at point b, (cheap GPS units dont update as fast or as much as the more costly one), thus giving you good readings at THAT TIME. Add some power reductions, speed ups, curves, go up and down some hills and see if your readings are just as good. As far as the 350mph reading goes....I doubt that plane was goin in a straight line long enough to verify the speed...and as ET said, the initial acceleration from the bungee was proably the reading he got. First the plane is at point A at a standstill...next time the GPS recieves a good signal its holy cow...way out at point B...he MUST have been goin X fast. I dont think so.

Luis

Seem to me ,
you guys donīt get it.
If the GPS would include the AOA the GPS would show higher readings.
And I told you to watch carefully. You didnīt.
If the speed is from the bungee then how is it possible that three different planes get approx three different speed with three different engines? If the bungee would give the speed then how is that possible? The speed after leaving the bungee is approx. 65mph.
If there is an position error the GPS still meassures the distance between two points correct with the same error. That means the distance between two points remains the same.
And you get correct readings. And by the way using an other(larger) GPS is not possible due to the size of it.

Vmax

I think there is a lot of misinformation about GPS. Heres the spec for the garmin 35 which I use in my plane connected to my video feed.

One thing I do know is that the gps reads "absolute" speed. It doesn't care if you going vertical or horizontal. It reads the absolute distance from you previous position, and divides it by the time it took you to get there (1 sec usually, but thats programmable).

This GPS is typical for the consumer market. About 120 bucks.

Notice the "Velocity accuracy".
0.1 m/s RMS. I'm not sure how to read this. Usually accuracy is a percentage of error, IE +/- 1%. If I'm reading this right its saying there is a maximum of 1/10 meter per second error and any speed.
That would suggest the faster you go the more accurate the reading. Am I wrong?? Opinions?


BTW, Selective Availability is the deliberate induction of error by the government. But I have read that they stopped doing that.


http://www.garmin.com/products/gps35/spec.html
also
http://www.garmin.com/manuals/64.pdf

Accuracy:

Differential GPS (DGPS): 5 meters RMS
Non-differential GPS: 15 meters RMS (100 meters with Selective Availability at maximum)
Velocity accuracy: 0.1 m/s RMS steady state (subject to Selective Availability)

Dynamics: 999 knots; 6g's

Vmax...it seems to me YOU don't get it. A hand-held GPS just does not seem to be the appropriate tool for this job, and your figures come up 50mph higher than anybody else ever recorded. Guess what? You are not the first person who came up with readings that were 50mph higher than everybody else...and turned out to be false.
Go do some new speed runs with some proper measuring equipment and see what it says. Convince us all. I would be thrilled to be wrong, to find you were actually doing 355.

Sailplane, aeronautics GPS are not more precise than "Walmart" GPS, they have more features, measure from more satellites, the usual big difference being they take into account altitude....duh....

Those specifics for Sailplane are also arrivals calculators, which make them very specialised, hence very costly. Still has nothing to do with distance (and speed) measurement accuracy.

Bernard

I want to know what a "Walmart" GPS is. Walmart sells, amongst other brands, Garmin GPS units. These are among the best consumer GPS's available no matter what the name of the store is. They are very accurate and reliable.

Also, the notion that the speed reading is coming from the bungee launch doesn't hold up. If it did, then the speed would be constant. Instead, we see the speed increasing with the ongoing work on the model. Higher thrust engines, better aerodynamics, etc.

Granted, it would be nice to see speed readings from another method other than the GPS to back up the claims made by Neils, but most of the comments in this thread are to the tune of "Yeah, it's fast, but it's not THAT fast". You make it sound like it's a hopped up .40 size plane. I've seen videos of 200+ mph ducted fans, and this thing is considerably faster. 100 mph? Maybe, but until someone can prove Neils wrong, how about giving him the benefit of the doubt? In the real world of aircraft, 350mph is not that fast. I see no reason why this model cannot go as fast as Neils claims.

John

@ easytiger
So then who is fastet one? MEC? He did his readings with GPS as well. WITH THE SAME POWER AND A LARGER AIRFRAME. Or is someone else faster? How do these guys meassure their speeds? What is a proper measuring equipment in your opinion?
Why donīt you try it? Go ahead and build a plane with 40 in. wingspan and a 26lbs. jet engine on it. You will be surprised. Maybe I am not the first who claims to be faster then other. But I guess none of these guys put two years of work in three similar rc-planes and saw constantly increasing speed.
Read other comments about GPS. Why do you ignore them?
Niels

Think I have to ask: What do YOU know about hand held GPS ? Do you kow you can see yourself WALKING, on a hand held GPS ? That's pretty accurate, as a human step is usually less than 5 meters/yards....

Bernard

@easytiger
One thing I forgot to add: the three readings are 551, 571, 576km/h.
If you would be right and there would be an 50mph error then maybe 576km/h (355mph) is correct and the GPS should show 405mph. Or the other way around. The plane flies 355mph and the GPS shows only 300mph. If there is an error it could go up and down. The lowest speed is just 4% away from the highest. So in your opinion the speed should be somewhere between 300mph and 400mph. But the range (551-576km/h)is very small. If there is an error then why will it always go up? And if there is an error why is it always in the same range? If there would be an error the GPS would show something like 301mph, 347mph, 417mph. Why goes an error always in the same direction? Why would have these errors all the same diffence? If there is no error the GPS shows similar numbers. And this is the case here.
Niels

Vmax, are you using full power on your downlines? If so i have no problem believing 355 for an aircraft that small with that much power in a downline.

BTW, what kind of transmitter is that. The antenna looks 5 ft long

Yes I do.


The transmitter is a Graupner JR MC16/20.
It has the same antenna as the MC24.
I guess you guys donīt get the MC16/20 in USA.
And the MC24 only in limited numbers.
But you are right the antenna is very long.
Niels

ET keeps writing things like:

There's a whole lot of "may be" and "seem to be" statements in your posting. Yet, no facts. If you don't have the facts on GPS, how 'bout you go educate yourself before casting unsupported doubts on Vmax. THere's plenty available on the web. I don't think it's Vmax's responsibility to make up for your lack of knowledge about the system.

If we were arguing about a couple mph, there might be some validity to the whole "walmart/civilian/consumer GPS" argument, but there isn't as it's obviously accurate to those limits and Vmax has the repeatability to show it wasn't a fluke "bad signal".

Selective availability is indeed off so all of the "intentional inaccuracy" arguments are also invalid.

THen there was:

So, first Turbodog makes a joke that didn't seem too funny - but I respond with good humor and poke fun at myself and Turbodog anyway. Then the 30 post wonder gets all huffy and wants to give me advice on posting. I'd say it's you who needs to sit back and take a breath - and maybe learn some thing about posting here.

As for hiding behind the computer - here's my cell phone: (407) 701-1209. Give it a call and you can hear for yourself what I think. Or, I'll be in the DC area in a couple of weeks - happy to repeat everything "to your face".

I just saw in my last issue of RCJI (arrived yesterday) in the whats new section, an on board telemetry unit that willmeasure speed and all the other info you are after.

Why not invest in this system, and then compare to the GPS and put all this bickering to rest?

I am not sure of the cost but maybe it is not too expensive?

Just a thought....

Andrew Coholic

vmax, where can i find the trace kit?

im looking for some speed.

Hi dusteater,
I build those planes.
They are so much work and I have a fulltime job.
Sorry. Not for sale.
Vmax