sphere towed on test fights?
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sphere towed on test fights?
It looks like a sphere (or cone) Towed about 10m on new A/C's first flight.
What's it for? Posted this on the questions forum with no result.
Is it something to do with calibrating air data or for ballistic spin parachute deployment.
What's it for? Posted this on the questions forum with no result.
Is it something to do with calibrating air data or for ballistic spin parachute deployment.
Agreed, there are examples at http://www.kohlmansystems.com/rvsm.html
and
http://www.nawcad.navy.mil/ROTARYWIN...rspeed_Cal.htm
Personally I don't use them nowadays, I prefer to do my ASI calibrations at a safe altitude against GPS without hanging things off the aeroplane. But, it's been a standard piece of FT kit since the 1940s and possibly before.
I'd be surprised to have one fitted for a first flight, but if it was used, it'd certainly be quite early in the test programme.
G
and
http://www.nawcad.navy.mil/ROTARYWIN...rspeed_Cal.htm
Personally I don't use them nowadays, I prefer to do my ASI calibrations at a safe altitude against GPS without hanging things off the aeroplane. But, it's been a standard piece of FT kit since the 1940s and possibly before.
I'd be surprised to have one fitted for a first flight, but if it was used, it'd certainly be quite early in the test programme.
G
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Personally I don't use them nowadays, I prefer to do my ASI calibrations at a safe altitude against GPS without hanging things off the aeroplane. But, it's been a standard piece of FT kit since the 1940s and possibly before.
I'd be surprised to have one fitted for a first flight, but if it was used, it'd certainly be quite early in the test programme.
I'd be surprised to have one fitted for a first flight, but if it was used, it'd certainly be quite early in the test programme.
If you're calibrating airspeed with a GPS, what do you do about wind? Surely your GPS is sensing ground speed (or some inertial reference speed).
We routinely use noseboom/trailing cone for our first flights. After all, the PEs are not well known at that point. Of course, we have to use noseboom static sources near the ground, which aren't quite as good; but better than a relatively unknown ship system.
Ah, that's the clever bit.
If you drop me an Email, I'd be glad to send you the full version with equations (not that there are many) but...
(1) Point aircraft roughly into wind, trim for constant IAS.
(2) Make small heading changes until you get the lowest GPS groundspeed. That heading is directly into wind.
(3) Now, on that heading, fly a series of IAS values at least from Vs+5 (normally Vs+2) to Vh. Beyond Vh, corrections are possible, but also a lot of climbing and re-trimming because of the inevitable descent.
(4) Now turn onto the reciprocal on GPS heading and repeat the exercise at the same IAS values.
Then the analysis...
- Mean of GS i/w and GS d/w = TAS
- TAS * SQRT(relative density) = CAS
So you get a table of IAS .v. CAS. Generally (in one aircraft configuration) takes about 45 minutes to an hour, plus about 30 minutes analysis. Plot the graph, and away you go.
The big flaw in the system is at low speed, since from Vs+delta down there are often inconsistencies in IAS reading not predicted by this method. BUT this method will tend, if this is true, to give an artificially high value of Vs, which for safety and planning data is generally in the safe sense.
Big advantages are speed, cost, and it can be done at any altitude so long as you're clear of turbulence.
John Lowry publishes a slightly more complex version in his book "performance of light aircraft", but his flies 3 rather than 2 segments and thus takes longer - also involves what (in my opinion) is too much unnecessary maths.
We refer to this approach as the "racetrack" method.
G
If you drop me an Email, I'd be glad to send you the full version with equations (not that there are many) but...
(1) Point aircraft roughly into wind, trim for constant IAS.
(2) Make small heading changes until you get the lowest GPS groundspeed. That heading is directly into wind.
(3) Now, on that heading, fly a series of IAS values at least from Vs+5 (normally Vs+2) to Vh. Beyond Vh, corrections are possible, but also a lot of climbing and re-trimming because of the inevitable descent.
(4) Now turn onto the reciprocal on GPS heading and repeat the exercise at the same IAS values.
Then the analysis...
- Mean of GS i/w and GS d/w = TAS
- TAS * SQRT(relative density) = CAS
So you get a table of IAS .v. CAS. Generally (in one aircraft configuration) takes about 45 minutes to an hour, plus about 30 minutes analysis. Plot the graph, and away you go.
The big flaw in the system is at low speed, since from Vs+delta down there are often inconsistencies in IAS reading not predicted by this method. BUT this method will tend, if this is true, to give an artificially high value of Vs, which for safety and planning data is generally in the safe sense.
Big advantages are speed, cost, and it can be done at any altitude so long as you're clear of turbulence.
John Lowry publishes a slightly more complex version in his book "performance of light aircraft", but his flies 3 rather than 2 segments and thus takes longer - also involves what (in my opinion) is too much unnecessary maths.
We refer to this approach as the "racetrack" method.
G
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Photo of Airbus 318 returning from its maiden flight http://www.airliners.net/open.file/216520/M/
