Indicated vs True Airspeed ??

[Powerin]

Hi All,

 

Long time lurker, but here's my first post and I have a few questions for the wonderfully wise pilots here. I am not a pilot but hope to be one day! (if this drought ever ends). So, while I am waiting for that wondrous event to happen, I do a lot reading about flying.

 

Today I was looking in the Articles section here and read the article "Types of Airspeed" in Training & Student Pilots. I was puzzled by the following sentences:

 

"True airspeed is the actual speed of an aircraft with respect to the air through which it flies. This speed is what determines the aerodynamic behaviors of an aircraft such as its Mach number, lift, and drag."

 

 

 

From what I've read elsewhere the first sentence seems correct but the second one seems wrong. Isn't it actually IAS (or CAS if you allow for instrument errors) that is important for lift and drag? Isn't IAS a measure of the dynamic pressure of the air against the pitot tube (and therefore the wings etc) and the dynamic pressure is what contributes to lift (or drag)?

 

 

 

From my reading I assumed the number on your ASI is what you follow for your takeoffs and approaches because that is what tells you how close you are to stall (and therefore also a rough indicator of Angle of Attack). So if you are landing on a hot day (or high at Mt Hotham), the air is thinner and you will have to be going faster to have the same pressure of air flowing over your wings generating the same lift. And even though you are going faster through the air, the indicated airspeed will be quite a bit lower. True airspeed would only be useful for calculating groundspeed and navigation. Is that correct? Is the article wrong?

 

 

 

I guess when I learn to fly all will become clear. But there seems to be a lot of different opinions and confusing info out there. While I was reading the above article I was watching the doco on TV about the C5 transport plane. They used the same old myth we learned in school to explain lift....the shape of the wing causes low pressure above and high pressure below (ie. Bernoulli's principle). It took me a long time and a lot of reading to come to grips with the concept of AoA being responsible for lift rather than the shape of the wing!

 

 

 

Cheers,

 

Peter

 

 

 

 

 

 

 

[jcruffle]

I guess when I learn to fly all will become clear.

 

 

Don't bet on it, Peter. Your understanding is pretty good in a practical sense. I reckon the second sentence could be proven with maths or pure aerodynamics by someone really up to speed with the theoretical side. In the meantime, stick with your present understanding.

 

In the case of wing shape (camber) and angle of attack, I maintain that they both have an impact on the amount of lift generated along with air density, speed etc. It doesn't matter which theory of lift you subscribe to, they all play their part.

 

John

 

 

[Guest drizzt1978]

Ok, can you sit mt BAK exam next week, I think you got a resonable grasp on it. Your breakdown seems more in depth and correct than the satement you read...I think, coffee is only just kicking in!!

 

 

[Powerin]

In the case of wing shape (camber) and angle of attack, I maintain that they both have an impact on the amount of lift generated along with air density, speed etc. It doesn't matter which theory of lift you subscribe to, they all play their part.

I think you're right. My take on it is that the net effect of all those things is lift. But for me lift itself is pure Newton's 3rd Law...every action has an equal and opposite reaction.

 

So, if you are flying straight and level in your 500kg aircraft, at any point in time your wings will be causing exactly 500kg of air to be thrown in a downwards direction (which is the end result of the camber, Bernoulli, AoA, etc). If you throw 500kg of mass in one direction you will get 500kg of force in the opposite direction (in this case up).

 

To climb you will need to throw down a bit more than 500kg of air and to descend a bit less. You can vary the amount of air a wing throws down, up to a point, by either changing your speed (using power) or changing your wing's angle of attack (with elevators/trim) or a combination of both.

 

How does that sound? Or am I in danger of starting a religious war here ;)

 

Peter

 

 

[Tomo]

G'day Peter... and Welcome to the forums...

 

It sounds like you've got a pretty good grip of whats happening with the wing and lift etc...

 

If you look at the lift formula...

 

Lift = the Coefficient of lift X air density X the velocity X surface area...

 

now it's a bit more fiddly than that with all the squaring this and halving that, but thats basically what you need.

 

So if you decrease one source, you'll need to increase something else to make it equal out...obviously you can't change the surface area of a wing (well the aircraft we fly anyway!)... so that's how they work out if the aircraft your flying will take off from a certain airstrip high up on the mountain...

 

But I'm sure you sound like you already know all this stuff anyway...:thumb_up:

 

:big_grin:

 

 

[Powerin]

Thanks for the welcome Tomo. Yeh... theoretical knowledge is all very well, but translating it into practice is another thing. That's why I value all the practical knowledge you flyers have here and why I asked the questions! One day I hope to be a good pilot because of what I learn here.

 

Cheers,

 

Peter

 

 

[eastmeg2]

Lets make it even simpler.

 

Multiply the average velocity you push the air dowwards at (meters per second, 1m/s is approx 2 kts) by the number of kilograms (roughly equal to cubic meters) of air you push down in one second and you will get the lift force in Newtons. (Approx 0.1kg)

 

Eg, You're flying at 50m/s (100kts) and with a lift coefficient of 0.1 are pushing air down at 5m/s and your wingspan is 10m.

 

Bugger, what depth of air is the wing pushing down.:faint:Tomo's formula probably accounts for this.

 

Must be 2 metres deep to give the right answer for a 500kg aircraft.

 

50m/s x 10m (span) x 2m (depth) x 5m/s = 5,000 Newtons.

 

 

[Guest pedrok]

Hey Peter

 

Regarding your initial question.

 

TAS is really just useful in calculating ground speed for navigation purposes.

 

IAS is the one that you pay attention to for the Flying bit (avoiding a stall). Yes the article is correct about needing a higher TAS to achieve the same IAS. But remembering that IAS is just a measure of the dynamic pressure you are generating (rather then speed).

 

The underlying bit, is that as you go higher (or achieve a a higher Density altitude) you engine is less efficient as the air is thinner., which can be partly overcome by leaning the mixture.

 

So using a guide from Lycoming: "At Density Altitudes of 5000 - 6000ft the pilot of a normally aspirated engine only has 75% of his engine power available" whereas when at sea level (or Density Alt of 0 he would have 100% available.

 

Which is why the Turbo was invented - to force air into engine and overcome the limitations of reduced air density

 

This is my simple laymans none technical minded understanding (cause I'm not that smart :)

 

 

[ahlocks]

And when listening to most pilots talking about their aircraft,

 

Indicated airspeed is what they tell you the 'plane can do...

 

True air speed is usually a bit lower.

 

 

[Powerin]

Indicated airspeed is what they tell you the 'plane can do...

True air speed is usually a bit lower.

Hi Ahlocks,

 

Ahh...the real answer

 

Just in case I win the lottery or it rains heaps...any recommendations for a good RAAus instructor down here in the Riverina?

 

 

[ahlocks]

Where are you? (PM if you're shy)

 

I do know of a couple of really good instructors, plus I'm often looking for a victim (passenger) to share the view with.

 

 

[Powerin]

Down near Henty. Would be very happy to be victimised!

 

 

[ahlocks]

There's good instructors all around the area nowadays.:thumb_up:

 

Naturally, I'll recommend the WGA operators or Wal in Narrandera, but it may be worth contacting Holbrook as they are a bit closer to you.

 

I'll try approaching HMFD to see if I can get permission to land on their strip to organise a blat in the near future. A little birdie told me that Russdot from this forum has his cheetah parked down your way somewhere too.

 

 

[Captain]

Just in case I win the lottery or it rains heaps...any recommendations for a good RAAus instructor down here in the Riverina?

Peter. I'd have no hesitation in pointing you at Don and Eddie at Sport Aviation in Tocumwal, as well as those mentioned by Steve. I found them excellent, and great teachers. Hope this helps. Geoff

 

 

[Powerin]

Thanks Capt, I'll keep them in mind when I find some coins to rub together...and convince the significant other ;-)