172 airspeed gauge showing groundspeed instead?

[ADamiani 470]

I am in my steam gauges 172, sitting on the runway with 20 knots of wind from straight ahead. Shouldn't my anemometer be showing 20 knots? It is showing 0 instead...

Did I miss some setting about airspeed/groundspeed ?

Andrea

[ark 1,288]

3 hours ago, ADamiani said:

I am in my steam gauges 172, sitting on the runway with 20 knots of wind from straight ahead. Shouldn't my anemometer be showing 20 knots? It is showing 0 instead...

Did I miss some setting about airspeed/groundspeed ?

Andrea

I'm not sure to what instrument on the C172 panel you are referring to. If by anemometer you mean the pitot static / airspeed indicator system, it may be that system is not designed to respond accurately much below about 40 kts. However, if you set the wind speed higher and include significant gusts, you will see the airspeed needle bounce around a little bit even though the a/c is not moving.  The ground speed display on the GPS, of course, only responds to actual movement over the ground.

EDIT:

Having said the above, I tried an experiment with the a ground level head wind of 20 knots directly down the runway. When my ground speed was 30 kts according to the GPS while still on the runway during the takeoff roll, I expected my airspeed indicator to say about 50 kts, but it indicated below 40 kts, so not correctly modeled I would say.

Al

Edited October 17, 2020 by ark

[ADamiani 470]

Exactly: I am referring to the pitot static / airspeed indicator system, and I see from your experiment that there is something wrong there 

And I also think I should find a different title. But then the question would be: how do I change it?

Andrea

Edited October 17, 2020 by ADamiani

[ark 1,288]

7 hours ago, ADamiani said:

Exactly: I am referring to the pitot static / airspeed indicator system, and I see from your experiment that there is something wrong there 

And I also think I should find a different title. But then the question would be: how do I change it?

Andrea

At this point you can't edit the title of your original post. There is a time limit on editing posts, about an hour or two I think, something like that. And even if inside the editing window, I don't recall if you can edit the original title of your thread.

I suppose you could try clicking on "Report post" and ask a moderator to change the title.

Al

Edited October 17, 2020 by ark

[ADamiani 470]

Ah ok tnx. Will try to report it.

Bye

Andrea

[Will Fly For Cheese 1,710]

You won't get a reliable anemometer reading under 50 kts. It needs the Venturi effect - and that happens at speed.

Edited October 17, 2020 by Will Fly For Cheese

[ADamiani 470]

Oh cool... I hadn't thought of that, and as a former physics teacher I would be interested to hear more about it.

At the same time, if I am flying with a ground speed of 80 kts (gps value) and I have 20 kts of steady headwind, my anemometer should read 100 kts, right? If you confirm, that, I will test it on the sim. 

Thank'you

Andrea

[Will Fly For Cheese 1,710]

Should do. Give or take a knot or two.

https://www.boldmethod.com/blog/lists/2019/11/the-four-types-of-airspeed-and-how-each-works/

Edited October 17, 2020 by Will Fly For Cheese

[n4gix 4,947]

1 hour ago, ADamiani said:

Ah ok tnx. Will try to report it.

No try there is young Padawan; only do or not do.

I've changed it for you... 😏

Edited October 17, 2020 by n4gix
Corrected to Yoda grammar

[Steku 158]

10 hours ago, Will Fly For Cheese said:

You won't get a reliable anemometer reading under 50 kts. It needs the Venturi effect - and that happens at speed.

The airspeed indicator uses Pitot tube, it takes dynamic pressures. Typically on the side of the fuselage there is a static pressure source(small hole). Both are provided to the airspeed indicator which displays the difference = airspeed. It is possible to build this type of gauge showing even small airspeed, but it won't be practical - the indicator would work constantly due to wind when the aircraft is parked. Airplanes can't fly at very slow airspeeds, so typically for small planes the scale starts at ca. 40 kts. 

Also the indicated airspeed (IAS) at low speed (when flying - high angle of attack - the flow over Pitot tube at an angle) must be corrected to calibrated airspeed (CAS). It must be further corrected to true airspeed for the differences in air temperature and pressure (some airspeed indicator have a knob and scale for TAS). 

There is no Venturi effect needed here, you probably mistake the pitot tube, with the Venturi tubes used on old aircraft to generate low pressure necessary to drive gyroscopic gauges. On more moder planes the gyroscopic gauges are driven by the vacuum pump. 

[ADamiani 470]

Even more interesting .... Is the real world 172 equipped with old or new gauges? 

Anyway, let's go experiment with some serious headwind and gps groundspeed. 

Andrea

[Will Fly For Cheese 1,710]

Yes of course, Vacuum not venturi - too much wine last night.

A brand new 172 will come standard equipped with Garmin G1000NXi as in below. Makes you realise what a fantastic job Asobo made of modelling it.

For steam gauges you'll have to go second hand.

 

[snglecoil 1,828]

5 hours ago, ADamiani said:

Even more interesting .... Is the real world 172 equipped with old or new gauges?

GPS derived ground speed is very useful for navigation but practically useless for determining the energy state of the wing. Therefore, even the newest 172s are equipped with a pitot-static system for airspeed indication. It is a reliable indication of what the wing “sees” in terms of the speed it is moving relative to the airmass in which it is flying. Angle of attack is really the most important measure especially as a wing approaches its critical AoA prior to aerodynamic stall. AoA indicators are starting to be seen in more airplanes these days, but indicated airspeed does an adequate job of indirectly estimating AoA limits as long as you know the stall speeds for the plane (which in a 172 would be notated as the bottom of the green and white areas on the airspeed indicators for flaps up and flaps down respectively). And where an AoA indicator is useful in the slow flight region, indicated airspeed is important in both lower and upper airspeed design limits for the aircraft.

[ADamiani 470]

More and more interesting ..... because one has to realize that airspeed translates into lift, and without lift you are a brick.

Ok, will do some experimenting.

Tnx

Andrea

[Glenn Fitzpatrick 3,798]

2 hours ago, ADamiani said:

More and more interesting ..... because one has to realize that airspeed translates into lift, and without lift you are a brick.

Ok, will do some experimenting.

Tnx

Andrea

Which is why Wind Shear is such a PITA and kills people.

 

 

Edited October 18, 2020 by Glenn Fitzpatrick