Perfectflite 15K Altimeter Question

[Grover]

I had a rocket weather cock, turn horizontal, fly for 1000 feet sideways, never getting much above tree level and reported back an altitude of 416 ft.

explanation ?

 

[Pantherjon]

The software was probably plotting the rockets 'projected' altitude based upon its previous readings up to the moment of the weather cocking..Just a guess.

 

[n5wd]

I had a rocket weather cock, turn horizontal, fly for 1000 feet sideways, never getting much above tree level and reported back an altitude of 416 ft.

explanation ?

Altimeter sampling holes blocked / too small / too close to air turbulence? Coulda been a lot of things. Have you ever flown the altimeter and it reported a reasonable altitude? Have you downloaded the data to look at the curve?

 

[brianc]

I had a rocket weather cock, turn horizontal, fly for 1000 feet sideways, never getting much above tree level and reported back an altitude of 416 ft.

explanation ?

Tall Trees??





:roll:





Seriously, what Wayne said. We need more info to offer an opinion...

 

[Handeman]

The other thing is "not much above tree level" can be deceiving when it almost a 1000 ft. away. I assume you didn't have trees between the launch pad and the point at which it deployed. It might have actually gotten 400 ft up and maybe came back lower by the time it deployed. Without trees along the whole flight path, it can be very difficult to judge altitude.

I would assume the altimeter is correct until you find some definitive evidence to the contrary. Eye witness estimates without instruments would not be definitive in my opinion, no matter how sure the eye witness may be.

 

[Grover]

Well it flew across an open field landing between two trees, but seriously, it couldn't have reached more than 100 ft. and I dont have the computer interface yet to download the flight data. I'm thinking turbulance was the best answer here. thanks

 

[Larry Curcio]

I had a rocket weather cock, turn horizontal, fly for 1000 feet sideways, never getting much above tree level and reported back an altitude of 416 ft.

explanation ?

Two guesses:

1) It could be that the approximate formula used to produce the beep is inaccurate at low altitudes. If that were true, then the altitude from the downloaded data would be more accurate; or

2) The reproted maximum altitude could include a spurious spike from the shock of ejection. Such spikes are typically on the order of 300 feet (when they are positive). In such a case, the downloaded data should display the spike and they should also reveal a better rendering of the true altitude.

 

[UhClem]

Well it flew across an open field landing between two trees, but seriously, it couldn't have reached more than 100 ft. and I dont have the computer interface yet to download the flight data. I'm thinking turbulance was the best answer here. thanks

Previous flight because it never saw enough pressure change for the launch detect algorithm to trigger.

 

[n5wd]

Well it flew across an open field landing between two trees, but seriously, it couldn't have reached more than 100 ft. and I dont have the computer interface yet to download the flight data. I'm thinking turbulance was the best answer here. thanks

OK, but you didn't answer the question: "Have you ever flown the altimeter and it reported a reasonable altitude?"

I buy the Perfectflite altimeters for my school, by the half-dozen and have gotten one that was waaaay off on it's altitude reporting. That's why we want to know if you've flown your altimeter before.

 

[Zeus-cat]

I have both a Perfectflite 15K and a Perfectflite 15K Rev 2. Both are prone to ejection spikes. I NEVER believe the max altitude reported by the beeping after the flight.

Perfectflite 15K
3 flights
2 clean
1 spike

Perfectflite 15K Rev 2
12 flights
3 clean
9 spikes

Some spikes are up and others are down. Most are small (+ or - 50 feet), but I have had a few as high as 500 feet on flights around 1,000 feet. The spikes are very short in duration, usually only 1 or 2 data points, so they are easily corrected. The software even warns you that an ejection spike (if the spike is up) may have occured before it shows you the graph.

 

[bobkrech]

0. I assume you read and understood the altimeter manual before you put it in the rockect.

1. Your altimeter must exceed 80' altitude or it will not detect lift-off. https://www.perfectflite.com/Downloads/A15KManual.pdf pages 2 and 10.

2. If your rocket flight exceeded 80' and lift-off was detected, then the altitude reading may be correct if the altimeter compartment was properly vented (page 5 of the manual), the ejection charge did not effect the pressure measurement and the altimeter is working properly.

3. If your rocket flight exceeded 80' and lift-off was detected, then the altitude reading low be low if the altimeter compartment was not properly vented with the holes being too small, and the altimeter is working properly.

4. If your rocket flight exceeded 80' and lift-off was detected, then the altitude reading could be high if the altimeter compartment was properly vented (page 5 of the manual), and the ejection charge effected the pressure measurement and the altimeter is working properly.

5. If the rocket did not reach 80', then lift-off would not have been detected, and you are likely hearing the previous flight.

6. The altimeter is not functioning properly.

7. You absolutely need to download the flight data to conclusively figure out which possibility occurred.

Bob

 

[Grover]

Sorry ,I was away on a busy mother's day.

Yes ,I have One Flight that appeared to be the altitude it reported.

I better get the USB software. I was unaware of ejection spikes

Thank You.

 

[Larry Curcio]

Previous flight because it never saw enough pressure change for the launch detect algorithm to trigger.

Seems he's hearing beeps. That indicates launch detect did trigger. No?

Regards,
-LarryC

 

[Zeus-cat]

Here is a jpeg of the graph from the Perfectflite software showing a "beautiful" spike.

 

[n5wd]

Sorry ,I was away on a busy mother's day.

Yes ,I have One Flight that appeared to be the altitude it reported.

I better get the USB software. I was unaware of ejection spikes

No problem on being away - Moms get priority! Here's a repost from the folks that make Perfectflite altimeters, in response to a question posed by a TARC Team sponsor last year. He noted that the altitude spikes were only on one sample (the Alt15K Vers 2 samples 10 times/second) and did not usually affect the beeping altitude readout post-flight, but was clearly visible when they downloaded the data into the viewer that Perfectflite has available for the altimeter. Perhaps it will help explain what everyone is talking about, though I don't think that is what happened on your aberrant flight.

- - - - - - - - - - - - - -

All barometric altimeters sense the pressure around them to determine
altitude. *Any spurious increases and decreases in the pressure
sensed will be reported as decreases and increases in altitude. *For
this reason it is important that the pressure in the altimeter bay be
kept as close as possible to the true outside pressure, with minimal
perturbations.

When the ejection charge separates the airframe, and also when the
shock cord abruptly stops the separation, the payload section is
subjected to sudden changes in velocity and attitude. *This can cause
pressure fluctuations to be introduced through the static sampling
ports in the altimeter bay.

The Alt15K/WD will reject momentary spikes from being reported in the
beeped-out altitude reading, but all data are faithfully logged to
memory. *As a result, brief spikes can appear in the downloaded
flight curve, but will not be reported as an erroneous apogee
altitude. *Longer duration spikes, which are very rare and are caused
by gross error in the placement and sizing of sampling hole(s), will
not be rejected and could cause errors in the beeped-out altitude
readings.

Using 4 static pressure sampling holes spaced at 90 degree intervals
around the airframe helps to minimize the presence of pressure
spikes, as the pressure changes are equalized by diametrically
opposite holes.

Using the proper sized static pressure sampling holes is even more
important. *As depicted in the table in the altimeter manual, the
proper size for a single static hole in a typical TARC rocket (1.6" x
6" altimeter bay) is about .050". *If four holes are used spaced at
90 degree intervals, each hole should have 1/4 the area, or 1/2 the
diameter, of this single hole. *So with four holes, the proper hole
size would be .025". *It sounds like they are using holes that are 5
times too large in diameter, or 25 times too large in area.

The possibility of a spike (even of longer duration) affecting the
beeped-out reading can be minimized further by insuring that
deployment occurs shortly after apogee, after the rocket has fallen
20 or 30 feet. *That way a spike would have to be greater in
magnitude than the 20 or 30 foot drop before it would alter the true
apogee reading.

Conversely, rockets that separate before apogee while still
travelling upwards at high velocities will be subjected to more
turbulence, increased probability of spikes, and the certainty that
any spikes will add to the true apogee of the rocket.

 

[madmax]

One thing to be sure of is that your electronics bay is well sealed. In my experiences the spikes are greatly reduced if not eliminated by making doubly sure the bay is sealed. Also size your charges and harnesses properly. If your charge is blowing the payload section or nose cone to the cord limits either change the length of the cord or go with a smaller charge.