Xantos

Hi Guys,
Has anybody used the Spektrum telemetry system with the Airspeed Sensor.
I just installed the Spektrum TM1000 module and the Airspeed unit downlinked to an iPad on my Kingcat.
The KC is powered by a AMT Pegasus - Strangely for a 30lb airplane with 37lbs thrust the speed we could record were not over 167 mph/Kmph (Not sure what the default units for speed are on the STi App)
This was the speed recorded with the Peggy at WOT coming out of a dive
Visually the KC appeared to be well over 200++mph .
I tried the same thing with 2 different pitot tubes - one is the pitot that comes with the airspeed sensor and the other pitot from a Jetcat unit.
Both recorded similar figures.
I checked the connections again i.e the Airspeed sensor has 2 ports one for the Static pressure and one for the Dynamic pressure marked -ve and +ve on the sensor.
Any idea if the unit has to be calibrated ?

S_Ellzey

I have the system in my KingCat also, and it works very well. I have mine installed in the nose of the boom so that I can get it away from the airframe as much as possible. My RAM 1000 gets it up to ~190 mph straight and level. Your location may be too close to the body/wing to get a good reading. I have a calibrated pressure gauge that I used to check the units accuracy on the ground and the display was always within 2 mph of what it should be, so that part of it is working pretty good. My Wifes BobCat has the same setup and it works very well also. Also have one in the Habu 32 to watch the battery voltage, which is very neat. I think that the system works well, its just that the airspeed system in a jet is going is going to have to be installed well clear of everything.

Steven

Xantos

Hi Steve,
From what I understand you hvae located the pitot in the front of the boom whereas the pitot on my KC is not too far of from the boom probably 1.5" away on the leading edge of the fuse.
I dont know how much thrust a RAM 1000 puts out but I think 167mph with a Peggy at WOT is poor performance or the ASI needs calibration.
I considered putting the pitot in the nose but I need to stand the model up on its nose sometimes while getting it in and out of the car .

alasdair

Xantos,
Do you have height telemetry as well? If so how do the height readouts relate to reality?

I had an Eagletree data logger in a jet, including airspeed, height, a G meter and GPS.
I found that air related readings were unreliable, especially at high speed.
The problem was that the static pressure picked up by the pitot/static head was too high, resulting in too low a height readout (minus 80 feet, on a low pass 10 feet above the runway) and for the same reason too low an airspeed.
On landing the height was spot on and the speed realistic, so it was a high speed problem affecting the staic pressure. I have not yet solved it.

Xantos

Alasdair
I think you've cracked it !!! Your observations are correct, that's exactly what's happening
The high speeds recorded by the ASI unit are low, however the low speeds i.e landing speeds +- 35 is what it was consistently recording which are what is expected of a Kingcat.

S_Ellzey

The photo below is of my installation. Other than the nose I think this is about as good as it is going to get. I would be concerned that the probe on your model is in some cross flow, which will effect both the static and the dynamic pressure. At the spped we fly at (subsonic) the air sees the plane coming from a long ways off and starts to move out of the way fairly far from the body. To get a good reading you have to be in a area where the flow is parrallel to the probe and the velocity is still the same as it is at a great distance out. Normally we spend a lot of time in the wind tunnel figuring that out, but for a model you have to guess your best.

S_Ellzey

If you are concerned that the electronics are not calibrated correctly you can check that easily. You can construct a very simple water monometer from some fuel line (diameter does not matter). If you are not sure what a water monometer is, just Google it, there are good pictures of them. “T� in a syringe between the sensor and the water to pressurize the line with. Just attach the line to the tip of the pitot probe. At 80 mph you should have an 80 mm water column height (80 and 80 is just a coincidence) and at 180 mph you should have a 404 mm water column height. If this checks well the electronics are doing things right.

alasdair

Hi S_Ellzey,
That calibration idea using a manometer is good. Maybe I will try it some day.

My problem was in finding a static pressure source. The Eagletree pitot/static head in my jet was sticking out of the nose, but only by about 35 mm. I suspect that for accurate readings a much longer probe will be needed. I believe that the piotot pressure was right, but not the static. Cabin static pressure was not usable either as it varied with speed.

I also heard that a trailing static should give accurate results, trailing one fuselage length behind the top of the fin. I must try that one day too.

siclick33

In the sorts of height bands that we fly at could you not just block the static port completely? This is obviously no good if you want altitude info too but might work as a rough guide for airspeed. You could 'zero' the reading before flight by using a tap of some sort to equalise pressure to ground level before flight.

Assuming a 'normal' day and a max height of 600' then the static pressure drop would be about 20mb at the highest altitude. 20 as a percentage of a normal 1013mb day is only about 2% which, whilst not ideal, might be better than the figures you're getting at the moment. I'm sure someone will point out an obvious flaw in my plan

Another option might be to forget the static vent on the pitot tube and take the static pressure from somewhere else. You could try using a couple of vents (one either side of the fuselage) and T-ing them together to get an average. This may or may not give you better readings than you have at the moment.

siclick33

I've just had another thought about the errors involved in blocking the static port.

The most important error may be the proportion between the static error introduced and the total pressure measured (rather than the static error itself). This would be worse when slow and high and a very rough and ready calculation at 30mph at 600ft might give pressure errors in the region of 5%. At high speed the error is negligable and as you descend the errors get less and less until you are back at your datum surface level.

I should have learned not to think out loud on RCU and am sure I have missed something but I still think it might be worth a try.

alasdair

As I expect you know, but to make sure everyone is with us, the airspeed indicator takes the pitot (or total) pressure, subtractsthe static pressure and calculates airspeed from 1/2 rho V^2
So we need an accurate static pressure to get accurate height and accurate airspeed. The latest Eagletree claims its height is accurate to 1 foot (or is it 2?) and on landing it seems impressively accurate, zero feet near enough and 27 mph on touchdown after a 6 minute flight.

I have tried using cabin pressure, from inside the fuselage.
I have tried a static pressure tapping on either side of the nose averaged via a T-piece
I tried an external static tube tied to an U/C leg (fixed gear)

The real test is a high speed full throttlelevel pass along the runway at a height I judge by eye to be 10 feet above the runway.
None of the static sources produced a height readout close enough to 10 feet to be credible.

The pitot/static head source (sticking out of the nose by about 35 mm) gave a height reading of minus 80 feet (below the runway) so it is under-reading height by 90 feet, so it must also be under-reading airspeed by a significant amount (I can't be bothered to calculate).

siclick33

Bugger!! I've just checked my calculations and have had an SI unit faff. Ignore my last posts[&o]

As an aside, one full size aircraft that I have flown did have a altitude error of 60ft at sea level, due to the location of the static ports, at high speed. There was nothing that could be done about it other than being aware that it was there.

S_Ellzey

Your idea of closing the statics is a good one. Right before takeoff wrap a piece of tape around the probes statics ports. The only issues might be if the temp in the model changes much in flight, if the inside warms up the static would increase which would give you a false low reading. I don't remember if the spektrum system will display a negative speed. If you landed and had something other than zero displayed when you stopped you would know it did not work.