Working of the airspeed indicator.
 

Hi everyone just registered and first thread, attained my ppl a couple of months ago on the Pa-38 and could anyone help me with the following scenario - TAS on take off ie 65 knots with a 20 knots headwind giving a groundspeed of 45 knots at rotation, at what point on the take off run did the Pitot start to register the headwind, as when we taxi at say 5 or 10 knots the airspeed is reading what is probably the ground speed, then when we line up and hold, zero reads on the airspeed indicator, but at some point during our ground run it senses the additional airflow of the 20 knot headwind to give us our airspeed, or to put it metaphorically if we could start to taxi along the runway at 10 knots indicated and a 20 knot headwind and someone tracked us using a radar gun the reading should be near to 10 knots, then we accelerate to 65 knots indicated with the same 20 knot headwind the radar gun would show a difference of approx 20 knots slower, somewhere during the acceleration the headwind became a factor but can anyone who has more of a life then me tell me when. Thanks in advance Driver 6452:confused:

Hello!

If you look closely at your airspeed indicator you will see that the scale does not start with "0" but, depending on the aeroplane, with a value more like "40". So your headwind will show on the ASI when the combined speed of the wind and your moving aeroplane exceeds the minimum value that can be displayed.

It always senses your local wind. Or strictly, the component parallel with the pitot head.

But, it has a bottom stop.

If the bottom stop is at 20, and you are pointed into a 15kn headwind, it'll lift off the bottom stop at 5.00000001kn groundspeed.

Roughly.


The other complication is that pitot-static systems are inaccurate below 1.3Vs and hellishly inaccurate below Vs.

So, whilst if it was perfect, in a 20kn headwind, indicating 10kn on the dial, you are reversing at 10kn - which is difficult in most light aeroplanes. This just illustrates that you can't trust anything an ASI tells you below Vs, and need to be very cautious below 1.3Vs.

G

I would think that that inaccuracy is mostly due to the high AoA that's normally associated with these speeds, and thus the relative wind blowing into the pitot sideways, and maybe generating some dynamic pressure on the static ports too.

On the surface, pointing directly into the wind, the wind would be blowing straight into the pitot (unless you're in a taildragger with a marked AoA when on the ground), so this effect would not be significant.

Agree that it's pretty academic anyway. If the wind is so strong that the ASI comes off the stops while on the ground, you've got other things to worry about. Like keeping your flight controls so that the aircraft doesn't flip over.

It really gets interesting when you have failed to remove the tapes over the pitot heads......which I have done not once but 3 times!!! (in a glider on aerotow). The ASI initially registers zero (20 knots, really, as the previous posters have mentioned). It then moves round the dial, completely round the dial, until the needle has gone as far as it can go! So I was now IAS 250 knots, but still following the same tug aircraft at the same old speed I took off at.

So sometimes don't trust the instrument! Actually when you get used to your aeroplane, you don't actually NEED instruments in VMC anyhow. Nice to have, though.

or use an ASI that doesn't need a pitot head
 

http://tiny.cc/rgeid

Like this one, from a WW1 German fighter (type unknown).

Photo taken at the Shuttleworth Collection, Old Warden

Google "Hall Windmeter" - often used on hang-gliders and windsurfers, and great at really low speeds.

I've even flown a (very slow) microlight that used one - worked remarkably well.

G

A little off-topic, but I know you have flight testing experience: if you would like to fly at low speeds (below 1.3 Vs for example) 5 knots slower, you should "convert" your IAS to CAS, reduce it by 5 knots and "convert" it back to IAS. Is my understanding correct?

The pitot registers airspeed at precisely the same point whether one has a headwind, tailwind, or no wind.

The airspeed indicator is registering the pressure of air entering the pitot tube, compared to still air (ambient or static air pressure). If one has a headwind on takeoff, one doesn't roll as far before the airspeed begins to register, but the value of air pressure into the pitot tube hasn't changed...the airspeed begins to register at the same speed of airflow regardless of the wind on takeoff.

If one has a tailwind during takeoff, one will roll farther, but the airspeed will begin to register at same ram air value that it always registers. The airspeed indicator isn't registering a headwind or tailwind...just airspeed. Once you have rolled fast enough to have enough airspeed, the airspeed indicator will begin to indicate, period. If one normally sees the airspeed indicator begin to rise at 40 knots, then one can sit on the end of the runway with a 40 knot headwind, and see the airspeed indication begin to rise. One can sit on the end of the runway with a 60 knot headwind, the aircraft sitting perfectly still, and see a 60 knot indication on the airspeed indicator.

I've done vertical and near vertical approaches to landing in airplanes such as Cessna 150's, in a strong wind, with no significant ground speed, but ample airspeed. The airspeed indicator doesn't care how fast the airplane is going, only how fast the air is moving past the pitot tube.

...or how fast the air is moving THROUGH the pitot tube....?

Isn't the only movement IN the tube the compression of the air against the diaphram? ie, nothing goes THROUGH it.

That's right. Not sure why you'd wish to outside of flight testing, but yes.

G

Past!

There are a few aeroplanes out there with venturi ASIs, I've flown a few, but a pitot tube definitely has no flow of air through it.

G