Jump to content

Sign in to follow this  
ADamiani

172 airspeed gauge showing groundspeed instead?

Recommended Posts

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

Share this post


Link to post
Share on other sites
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 by ark

Share this post


Link to post
Share on other sites

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 by ADamiani

Share this post


Link to post
Share on other sites
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 by ark

Share this post


Link to post
Share on other sites

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

Edited by Will Fly For Cheese

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites
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 by n4gix
Corrected to Yoda grammar
  • Like 2
  • Upvote 1

Fr. Bill    

AOPA Member: 07141481 AARP Member: 3209010556

Interests: Gauge Programming - 3d Modeling for Milviz

Share this post


Link to post
Share on other sites
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. 

  • Like 1

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites

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.

g1000.jpg

 

Share this post


Link to post
Share on other sites
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.

  • Like 1

Chris

Share this post


Link to post
Share on other sites

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

Share this post


Link to post
Share on other sites
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 by Glenn Fitzpatrick

Share this post


Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
Sign in to follow this  

  • Tom Allensworth,
    Founder of AVSIM Online


  • Flight Simulation's Premier Resource!

    AVSIM is a free service to the flight simulation community. AVSIM is staffed completely by volunteers and all funds donated to AVSIM go directly back to supporting the community. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. Thank you for your support!

    Click here for more information and to see all donations year to date.
  • Donation Goals

    AVSIM's 2020 Fundraising Goal

    Donate to our annual general fundraising goal. This donation keeps our doors open and providing you service 24 x 7 x 365. Your donation here helps to pay our bandwidth costs, emergency funding, and other general costs that crop up from time to time. We reset this goal every new year for the following year's goal.


    34%
    $8,660.00 of $25,000.00 Donate Now
×
×
  • Create New...