Speed indication

[REL]

Hey All

Can anyone explain why the speed indication doesn’t match? What is the difference between the two?

THX

 

Edited January 29, 2009 by REL

[Acedy]

The number on the HUD shows ground speed as measured by the doppler system, the Shkval indicated airspeed.

[REL]

What's the "+" sine stands for ?

[Frederf]

It stands for Doppler speed. It's not a positive or negative sign. If you're going backward it will still read +

[REL]

It stands for Doppler speed. It's not a positive or negative sign. If you're going backward it will still read +

 

I was wondering about it ... THX guys

[REL]

So there is difference between air and ground speed? How’s that? For ground speed we take in account the vertical speed too?

[Pfadfinder]

The ground speed shows your real speed over ground, like in your car.

The airspeed shows just how fast the air is in front of your aircraft (sorry dont know how to explain it better in english).

For example you fly with 100 km/h against the wind direction and the wind blows with 25 km/h, then your ground speed shows 100 km/h and your air speed shows 125 km/h.

[congo]

The ground speed shows your real speed over ground, like in your car.

The airspeed shows just how fast the air is in front of your aircraft (sorry dont know how to explain it better in english).

For example you fly with 100 km/h against the wind direction and the wind blows with 25 km/h, then your ground speed shows 100 km/h and your air speed shows 125 km/h.

 

I disagree with this statement. Your airspeed will always read the same unless you slow down or speed up regardless of headwind or tailwind. Airspeed is the rate at which the aircraft is moving through the surrounding air as measured by the pitot/static system. Ground speed is speed over ground. Example is if you are flying at an indicated airspeed of 100 knots into a 25 knot headwind your ground speed is 75 knots. If you are flying with a 25 knot tailwind then your ground speed is 125 knots. If you are hovering in a 25 knot headwind your airspeed is 25 knots and if you are hovering in a 25 knot tailwind your airspeed is still 25 knots only backwards.

[Pfadfinder]

I disagree with this statement. Your airspeed will always read the same unless you slow down or speed up regardless of headwind or tailwind.

 

Now its my turn to disagree:D.

 

Airspeed is the speed of an aircraft relative to the air. So there is a difference if the air surrounding me is moving ahead me or from the side or from behind. If im moving with the air my airspeed is in relative to my groundspeed lower than when i moving against the air.

 

But to keep it simple, your groundspeed is your airspeed plus or minus the wind.

[congo]

Now its my turn to disagree:D.

 

Airspeed is the speed of an aircraft relative to the air. So there is a difference if the air surrounding me is moving ahead me or from the side or from behind. If im moving with the air my airspeed is in relative to my groundspeed lower than when i moving against the air.

 

But to keep it simple, your groundspeed is your airspeed plus or minus the wind.

 

If you are moving with the air at 100 knots your airspeed is 100 knots, if you are moving against the air your airspeed is still 100 knots. It is your speed across the ground which is changing.

[BaD CrC]

But to keep it simple, your groundspeed is your airspeed plus or minus the wind.

 

I think we can stick with this simple definition for the sake of this topic's peace. :D

[Pfadfinder]

If you are moving with the air at 100 knots your airspeed is 100 knots, if you are moving against the air your airspeed is still 100 knots.

 

Sorry but this is just wrong, you move 100 knots but the air particels are also moving towards you with 25 knots in our example, and 100 + 25 is 125.

You can test this when you are standing on the ground, your Airspeed will show 0 ground speed + the wind from ahead.

[JDski]

Pfadfinder and Congo are both right. Both are saying the same thing. The pitot tube measures pressure which if forced into it from the air pressure created by a relative headwind. So if you were in a hover facing North, with a 25mph wind comming from the East, you wouldn't see any indicated air speed (IAS) registered because the wind in not being forced into the pitot tube. You IAS would increase up to 25mph as you change direction to the East in your hover.

Pitot tubes are also used on boats in a simular fashion only measuring water pressure instead to give you your speed through the water.

 

http://en.wikipedia.org/wiki/Pitot-static_system

[congo]

Sorry but this is just wrong, you move 100 knots but the air particels are also moving towards you with 25 knots in our example, and 100 + 25 is 125.

You can test this when you are standing on the ground, your Airspeed will show 0 ground speed + the wind from ahead.

 

Yes your groundspeed is 125 but your airspeed is still 100 the airspeed indicator doesnt know headwinds from tailwinds it just measures your speed through the air

[Pfadfinder]

the airspeed indicator doesnt know headwinds from tailwinds it just measures your speed through the air

 

Last try, thats false, JDski explained it one post ago. The Aircraft uses the pitot tube to calculate the Airspeed and the pitot tube is a tube directed to the front, so it just measures the air coming from forward.

[congo]

Last try, thats false, JDski explained it one post ago. The Aircraft uses the pitot tube to calculate the Airspeed and the pitot tube is a tube directed to the front, so it just measures the air coming from forward.

 

Let me try to explain so you can understand. The aircraft once it becomes airborne is now operating in a large fluid namely air. Fluid speed ( windspeed )is a value we assign when this fluid is moving relative to a fixed point ( Me standing at a weather station). A good example is the fishbowl analogy. a fish is swimming around in his bowl of water ( substitute air for water ) at 2 knots through the water in a circle so his ground speed is almost nil. You pick up the fishbowl and start running at 15 knots for an hour( This is the windspeed). The fish is still swimming through his fluid at 2 knots no matter what direction he goes but his ground speed is 15 knots. Substitute an aircraft for the fish. Do you see yet. Yes the modern pitot static system works on ram air or dynamic pressure of the fluid into the forward facing opening but as far as the aircraft and its fluid that it is operating in it is always facing forward.

Edited January 29, 2009 by congo

[SaintJ]

Actually, Congo is correct in this exercise.

 

The easiest way I can explain this is I have flown a Cessna 150 at close to stall speed (48mph) and have had mopeds on the ground passing me. This is because I was up in winds aloft flying into a headwind of 25mph. That effectively reduced my ground speed to roughly 20 mph. Give or take. A tailwind/headwind is irrelevant to planes/helicopters flight path. The prop still pulls the same air through it and over the wings no matter what the speed of the air is. Its relative.

[SaintJ]

An even easier example. If a river is moving at 5 mph and you swim in that river at 5mph upsteam, you will cover no ground. If you swim in that same river downstream at 5mph, you will cover ground at a rate of 10mph.

[congo]

An even easier example. If a river is moving at 5 mph and you swim in that river at 5mph upsteam, you will cover no ground. If you swim in that same river downstream at 5mph, you will cover ground at a rate of 10mph.

 

SJ good example also see mine above. This is one of the most widely misunderstood things I see on a lot of the aviation forums. It really starts getting confusing when you start discussing IAS,CAS and TAS and then throw in Mach values and your synapses can overheat. Nuff said.

[GGTharos]

To be utterly pedantic, at a speed of 10mph, not a rate ;)

 

An even easier example. If a river is moving at 5 mph and you swim in that river at 5mph upsteam, you will cover no ground. If you swim in that same river downstream at 5mph, you will cover ground at a rate of 10mph.

[congo]

To be utterly pedantic, at a speed of 10mph, not a rate ;)

 

 

I love pedanticism.

[SaintJ]

To be utterly pedantic, at a speed of 10mph, not a rate ;)

 

Your killing me Smalls!

[ericinexile]

Here's a question: How does the PVI-800 provide a wind velocity data on the ground?

 

Smokin' Hole

[redberon2003]

this is all fun and dandy, but can anyone tell me what happens;

 

When your Pitot tube is blocked?

what happens when the pitot tube is open and

the static port is blocked?

 

x]

[Pfadfinder]

An even easier example. If a river is moving at 5 mph and you swim in that river at 5mph upsteam, you will cover no ground. If you swim in that same river downstream at 5mph, you will cover ground at a rate of 10mph.

 

Never said something else!?

 

Here's a question: How does the PVI-800 provide a wind velocity data on the ground?

 

Same way as in the Air, with the pitot tube.

 

When your Pitot tube is blocked?

what happens when the pitot tube is open and

the static port is blocked?

 

When it is blocked Airspeed is 0 and never change.

When static port is blocked Alltitude shows failure.