DP Extra w/ 50cc gas SPEED TEST
06-25-2003, 08:07 PM
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DP Extra w/ 50cc gas SPEED TEST
I did some speed testing today with my Extra 330 powered by a DA 50 and Zinger Pro 22x10. This was accomplished using a GPS system with a max and average speed recorder.
I was quite surprised by the results.
My normal cruising speed, 1/3 throttle or less........68mph.
HIGH speed pass, with a shallow dive but NOT full throttle.....103 mph !!!
It just doesn't look that fast in the air....I guess the size of the plane makes it look slower.
Just wanted to pass on my results for anyone interested.
I was quite surprised by the results.
My normal cruising speed, 1/3 throttle or less........68mph.
HIGH speed pass, with a shallow dive but NOT full throttle.....103 mph !!!
It just doesn't look that fast in the air....I guess the size of the plane makes it look slower.
Just wanted to pass on my results for anyone interested.
06-26-2003, 01:08 AM
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Tach
Geistware, I calculated the same rpm. I would be really surprised that the DA50 is revving up to nearly 11,000 rpm when it unloads, and at partial throttle, to boot. With a 22" prop, it would be really ripping the air and sound very quick.
BTW, APC says the max rpm for their props is 190,000 divided by prop diameter. Thats around 8600 for a 22".
I suspect that a radar gun would be a more direct, and therefore more accurate method to measure the speed.
I did some experiments a while back with a PC/RC Digitach. It's an inexpensive (about $60) onboard tach that can be set to record peak rpm or constant rpm. The sensor is mounted to the backplate and senses the steel pin in the crank going around. It has a digital display that mounts to the plane. I found that on a Fox 46 in a sleek pattern plane, I was gaining roughly 1800-2000 rpm, increasing from 12,000 to 13,800 or 14,000 rpm in the air. This explained why that plane just SCREAMED in the air as it went by at full throttle. That's a little bit over a 16% rpm increase in a small, high revving glow engine.
I would love to see more speed data collected by another method, so see if you are truly getting what is probably a 30% increase in rpm. That would be amazing.
BTW, APC says the max rpm for their props is 190,000 divided by prop diameter. Thats around 8600 for a 22".
I suspect that a radar gun would be a more direct, and therefore more accurate method to measure the speed.
I did some experiments a while back with a PC/RC Digitach. It's an inexpensive (about $60) onboard tach that can be set to record peak rpm or constant rpm. The sensor is mounted to the backplate and senses the steel pin in the crank going around. It has a digital display that mounts to the plane. I found that on a Fox 46 in a sleek pattern plane, I was gaining roughly 1800-2000 rpm, increasing from 12,000 to 13,800 or 14,000 rpm in the air. This explained why that plane just SCREAMED in the air as it went by at full throttle. That's a little bit over a 16% rpm increase in a small, high revving glow engine.
I would love to see more speed data collected by another method, so see if you are truly getting what is probably a 30% increase in rpm. That would be amazing.
06-26-2003, 01:19 AM
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DP Extra w/ 50cc gas SPEED TEST
I'm only reporting the data obtained, I can't vouch with any certainty to its accuracy. A radar gun would undoubtedly be more accurate. We have not tested the accuracy of the GPS system using another method.....a car for instance.
On the other hand, I wonder how accurate your "calculations" are in determining airspeed from prop pitch/rpms. I'd guess my engine was turning no more than 7500 rpms unloaded during the high speed pass. I did put the plane into a dive to increase its airspeed. There was also a tailwind of about 10mph---and remember that the GPS is measuring ground speed, NOT airspeed. I would doubt that the extra could get to 100mph just under its own power.
A full power run to obtain top speed would not be "healthy" for the airframe. Giant scale planes, particularly overpowered extra 330s, are prone to flutter.
Just wanted to see how fast it would go, without risk.
On the other hand, I wonder how accurate your "calculations" are in determining airspeed from prop pitch/rpms. I'd guess my engine was turning no more than 7500 rpms unloaded during the high speed pass. I did put the plane into a dive to increase its airspeed. There was also a tailwind of about 10mph---and remember that the GPS is measuring ground speed, NOT airspeed. I would doubt that the extra could get to 100mph just under its own power.
A full power run to obtain top speed would not be "healthy" for the airframe. Giant scale planes, particularly overpowered extra 330s, are prone to flutter.
Just wanted to see how fast it would go, without risk.
06-26-2003, 01:26 AM
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speed
Understood.
You do know that engineers are cynics, right? LOL!
Anyhow, I am using ThrustHP which is a relatively useful calculator for thrust and hp based upon the affinity laws. It is not 100% accurate, but I've found it to give very good relative information. It tends to be overly optimistic on HP demand (vs my dyno) and also slightly optimistic on thrust produced, but it has to make some assumptions to work at all.
You can download it for free at my club's website, www.BlueMaxRC.com
Theoretically, a 10" pitch prop would need to be turning nearly 11,000 rpm (static mph) to move through the air at 103 mph, with 100% efficiency. In reality, the props are not that efficient at that rpm, and it would need to turn even faster to move through the air at 103 mph
Would be good to verify the accuracy with another method. Still it was a fun way to use the technology and I personally think it was cool that you used the GPS to see what it could do!!
You do know that engineers are cynics, right? LOL!
Anyhow, I am using ThrustHP which is a relatively useful calculator for thrust and hp based upon the affinity laws. It is not 100% accurate, but I've found it to give very good relative information. It tends to be overly optimistic on HP demand (vs my dyno) and also slightly optimistic on thrust produced, but it has to make some assumptions to work at all.
You can download it for free at my club's website, www.BlueMaxRC.com
Theoretically, a 10" pitch prop would need to be turning nearly 11,000 rpm (static mph) to move through the air at 103 mph, with 100% efficiency. In reality, the props are not that efficient at that rpm, and it would need to turn even faster to move through the air at 103 mph
Would be good to verify the accuracy with another method. Still it was a fun way to use the technology and I personally think it was cool that you used the GPS to see what it could do!!
06-26-2003, 03:40 AM
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DP Extra w/ 50cc gas SPEED TEST
GPS shows ground speed, not air speed just as ptebbe said. It is tough to tell what the wind is doing even 50 feet off the deck.
06-26-2003, 12:19 PM
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DP Extra w/ 50cc gas SPEED TEST
I have tested a garmin GPS unit for accuracy (using a car), and they tend to be dead on accurate. I would think they are actually more accurate than a radar gun, since a radar gun can be affected by such things as the angle of incidence.
06-26-2003, 12:21 PM
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DP Extra w/ 50cc gas SPEED TEST
so does ground speed mean it would keep up with a car on the highway going that same speed? if so it still fast ground or air speed which i think is in knots.... 
when i watch the cars pass by my field they dont look fast but i bet they are doing 45-60 mph....
when i watch the cars pass by my field they dont look fast but i bet they are doing 45-60 mph....
06-26-2003, 04:18 PM
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DP Extra w/ 50cc gas SPEED TEST
As far as the GPS [speed] accuracy goes, I think that it would be very accurate if it weren't for the resolution limit of 15 feet. However, the faster you go, the more accuracy there should be.
07-03-2003, 02:59 PM
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DP Extra w/ 50cc gas SPEED TEST
The point I noticed is that he was in a "dive" now I'm not sure about the math but if I remember right a falling object gains speed at 36ft/sec squared, so would gravity been the reason for the 100mph+ speed without looking at prop speed? A skydiver fly pretty fast without a prop! 
I have a GPS also, use it in my car all the time, seems like it's right on.
Take Care!
I have a GPS also, use it in my car all the time, seems like it's right on.Take Care!
07-03-2003, 03:30 PM
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DP Extra w/ 50cc gas SPEED TEST
knpjdad,
You're close, the acceleration due to gravity is approximately 32.2 ft per second squared. This does not take into account drag or any other opposing force.
ptebbe,
I don't think the accuracy of the GPS unit (with regards to velocity) are a function of velocity, rather a function of the distance traveled in which the velocity is measured. Remember, velocity is the distance traveled divided by the time it takes to travel that distance. The farther the subject travels, the more accurate the indicated velocity. This is because the error in the GPS unit becomes very tiny compared to the total distance traveled. It's a calculus problem
Take care!
You're close, the acceleration due to gravity is approximately 32.2 ft per second squared. This does not take into account drag or any other opposing force.
ptebbe,
I don't think the accuracy of the GPS unit (with regards to velocity) are a function of velocity, rather a function of the distance traveled in which the velocity is measured. Remember, velocity is the distance traveled divided by the time it takes to travel that distance. The farther the subject travels, the more accurate the indicated velocity. This is because the error in the GPS unit becomes very tiny compared to the total distance traveled. It's a calculus problem
Take care!
07-03-2003, 03:37 PM
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DP Extra w/ 50cc gas SPEED TEST
Ptgarcia,
That about covers it. The only other thing I would add is that everyone is assuming the 15 error is at a maximum of 15. The public signal is guaranteed to have "no greater than" a 3 meter error, this does not mean that the error is not less.
That about covers it. The only other thing I would add is that everyone is assuming the 15 error is at a maximum of 15. The public signal is guaranteed to have "no greater than" a 3 meter error, this does not mean that the error is not less.
07-03-2003, 03:43 PM
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DP Extra w/ 50cc gas SPEED TEST
Obviously. My comment about accuracy at higher speed is precisely because of the larger distance traveled. The GPS even will give you a speed at walking speed over 5 or 10 feet, so it clearly has better resolution than the "minumum" 3 meters.
07-03-2003, 04:12 PM
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DP Extra w/ 50cc gas SPEED TEST
Ptebbe,
The 3 meter resolution is the max (I'll use this loosely) error. If you set the GPS receiver in a fixed location and collect the position data over a 24hr period, you will notice that the position wonders +/- 3 meters (ocassionally you have a larger than normal spike that could have been caused by mulipath, satellite geometry, etc..). The speed will also wander slightly.
Off the shelf receivers are much better now that SA (selective availability) is turned off. I have tested Magellan receivers and found them to be very accurate even when travelling at less than 20mph.
Azhar
The 3 meter resolution is the max (I'll use this loosely) error. If you set the GPS receiver in a fixed location and collect the position data over a 24hr period, you will notice that the position wonders +/- 3 meters (ocassionally you have a larger than normal spike that could have been caused by mulipath, satellite geometry, etc..). The speed will also wander slightly.
Off the shelf receivers are much better now that SA (selective availability) is turned off. I have tested Magellan receivers and found them to be very accurate even when travelling at less than 20mph.
Azhar
07-03-2003, 09:17 PM
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GPS
If your plane goes straight up or straight down, will the GPS record a correct speed?
What I'm getting to, is if the plane is in a deep dive, it is travelling only partly in the horizontal plane, and partly in the vertical plane. Even at 30 degrees from horizontal going up or down... The GPS might not be as precise in the vertical plane, or not even record a change in distance, depending upon the triangulation of the satellites. If true, any significant flight angle would also give a false high speed reading due to lack of resolution or sensitivity vertically.
What I'm getting to, is if the plane is in a deep dive, it is travelling only partly in the horizontal plane, and partly in the vertical plane. Even at 30 degrees from horizontal going up or down... The GPS might not be as precise in the vertical plane, or not even record a change in distance, depending upon the triangulation of the satellites. If true, any significant flight angle would also give a false high speed reading due to lack of resolution or sensitivity vertically.
07-03-2003, 09:44 PM
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DP Extra w/ 50cc gas SPEED TEST
GPS receivers measure in 3 dimensions. The vertical element is the least accurate but not by much. It will measure a 3-D distance and give a velocity. Keep in mind, the better the GPS receiver, the better the results (like most everything else).
