Different types of speed

Recommended Posts

This is the first of a series of "dumb" questions that I am going to put in this forum just to see what we know, help others, clarify and to open discussions to see who can get it right.

Ok, I am going to play dumb here. One of the biggest mindset changes I had to have when I was learning to fly was the understanding of airspeed. When we drive our car we push the throttle and that makes us go faster along the ground and our speedo tells us how fast we are really going but planes are different. When I started flying I had to forget about that and understand what airspeed was and how it was different to me driving my car to the airfield.

So as I said I am going to play dumb, and for the benefit of others, can someone explain to me the different types of airspeed for example:

Airspeed - what is it and why aren't I going that fast?

ASI - Air Speed Indicator or Indicated Airspeed

TA - True Airspeed and why is that different to Indicated

GS - Ground Speed, so if my ASI says 100 knots why aren't I going 100 knots

So come on - help me out here as I am :confused:

Share on other sites

hmmmm where to start, this will get some long answers..... but ill try my best at some short ones..

Indicated airspeed measures the air hitting the pitot tube and aircraft.

True airspeed, hmmm as you climb higher, the air get thinner, there are less air particals, so as you get higher the indicated airspeed will remain the same (the air actually hitting the aircraft) but as its thinner, you have to move faster to get the same amount of air to hit the plane... so you True Airspeed will Increase with altitude..

Ground spped is the speed you are actually moving over the ground, air moves as a giant mass, when you are flying in that mass of air, the indicated and true airspeed will be the same, but what if that mass of air you are in is moving at 100 kts due north, and you are flying at 110 kts south, you are still flying through the air mass at 110 kts, but the air you are in is moving backward over the ground at 100 kts, so your ground speed will only be 10 kts south. but if you turn around and head north, then you ground speed will be 210 kts! your spped through the air the speed of the air mass itself over the ground..

Share on other sites

So Rob what does all that mean in laymen terms :big_grin:

Example does "Indicated airspeed measures the air hitting the pitot tube and aircraft" mean the speed of the air going over the wings because if the air going over the wings is to slow then you stall...???

Share on other sites

Stalls

Example does "Indicated airspeed measures the air hitting the pitot tube and aircraft" mean the speed of the air going over the wings because if the air going over the wings is to slow then you stall...???

I couldn't help but notice that about stalls. Most people (not pilots) are under the impression an aircraft stalls if it is going too slow. It is the angle of the incident airflow in relation to the wing . Once critical angle is reached, regardless of attitude a stall will occur.

Maybe someone should have a go at explaining a stall in simple language for non pilots

Micgrace:)

Share on other sites

I think that this is really important - we have to separate 3 things from each other, and in particular two of them from one other: Airspeed has nothing much to do with angle of attack (AOA) and pitch angle has nothing much to do with AOA. Pitch angle does have a bit to do with Airspeed.

Without getting too technical (and with some massive generalisations) lift from a wing is a function of velocity squared. It is also a function of how big a bite the wing takes of the air - the AOA. AOA is the angle between the relative air flow and the chord of the wing. As a massive generalisation in level flight we tend to have low AOA at high air speeds and high AOA at low airspeeds - trading off the lift from airspeed with the lift from AOA.

Now why isn't pitch angle much to do with AOA? The answer is seen in the example of where we have an a/c in very slow flight, mushing down. The AOA controller (the stick) is well back yet the nose is level with the horizon and the relative airflow is coming from well below the nose as the a/c sinks through the air - thus high AOA.

Conversely the aircraft in a fast, gentle climb may have a pitch angle well above the horizon but a low AOA as the relative airflow is well forward and a lot of lift is being generated by the velocity squared. The AOA controller BTW is neutral in this case - therefore a modest AOA.

So why does an aircraft stall? It stalls because at a "critical" AOA the wing stops flying. It generates much more drag than lift. The nice airflow over the top of the wing breaks away and becomes turbulent and the wing no longer generates sufficient lift for the aircraft to continue to support its weight so down we go. There may also be a sudden nose down pitch moment as the wing has less lift comparatively than the tail plane which we hope is still flying.

The critical AOA is at the end of a "flat spot" in the AOA vs Lift graph. Max lift is in the AOA area leading up to critical AOA.

Note also that in a turn the load on the aircraft increases and therefore the stall speed increases because having to support some multiple of the 1g weight of the a/c the critical AOA is reached at a higher airspeed. Put another way you need both a high AOA and a higher airspeed in order to generate the lift to support the increased load on the wing. The airspeed of the stall in 1g flight also increases as the gross weight of the aircraft increases. The AOA of the stall doesn't change. It's that relationship of lift to velocity and AOA again.

In all cases the AOA controller will normally be at the same place when we stall, that is when we reach critical AOA. For most aircraft the AOA controller will be at or near the rear limit of its travel. Think back to a high g turn - what are you doing with the AOA controller? You are pulling and pulling quite hard. Eventually if you persist it will reach the point at which it has caused the wing to reach critical AOA and you will stall - speed of this event being dependent on load on the wing and therefore the lift required to suppport that load.

Just as an aside: Elevator trim is actually a way to set AOA. Do this little exercise (numbers for a Tecnam): throttle back to 60 knots and trim for that speed. With low power settings the trim will be well back - indeed with the throttle fully closed the trim will be right back. Now leaving the stick free open the throttle a little. What happens? The nose pitches up. It does this because the a/c is trimmed for a particular AOA. When we open the throttle the AOA tends to reduce and so the nose pitches up to maintain the original AOA.

Great reading on this subject in Langeweisch and also here: http://www.av8n.com/

Please note that this is a bare scratch of the surface of this fascinating subject and full of generalisations.

Right oh you guys, correct my inevitable errors:;)3:

Regards

Mike

Share on other sites

Pelorus32 the web site you supplied is good value.Just to get people thinking what would happen if the thrust of the engine far far exceeded the weight of the airframe? would there be a critical angle ?(eng at full power)

Teenie

Share on other sites

Ian, add 'Calibrated Airspeed' (CAS) to the list. It's the IAS with factor for error within the systems. Usually the POH has a table of the error associated with that particular aircraft.

Share on other sites

Some aircraft have an Angle of Attack meter which shows A.O.A. Now I have never flown with one but I cannot see how the stall is always at the same AOA.

When flaps are used the Coefficient of Lift is changed, increasing with more flap usually and therefore the same lift is produced at lower speed, which means nose up or a higher AOA. So therefore I see it as differing with changed wing configuration and the AOA meter can only be a riugh guide.

With a high speed stall it appears to me that the AOA will be lower than normal because C. of L will be the same but more lift is required.

Share on other sites

Just to get people thinking what would happen if the thrust of the engine far far exceeded the weight of the airframe? would there be a critical angle ?(eng at full power)Teenie

I don't think that ultimately it's about the engine, I suspect that it's about the aerofoil. You need help on this one from those far smarter than me, my understanding though is that every aerofoil has a critical alpha and that the engine thrust is going to convert to speed and therefore lift. Can you get to critical alpha at full thrust? Yes if the aerofoil is loaded to the point where it requires all of the speed and all of the alpha available.

I'd be interested in the thoughts of others here: the physicists probably can answer this in a jiffy.

Regards

Mike

Share on other sites

Mike,

Thanks for the stuff on Angle of Attack.. It consolidated what I have been reading.

Hope to see you at Shep this weekend.

Ben

Share on other sites

Going back to the post "what would happen if the thrust of the engine far far exceeded the weight of the airframe? would there be a critical angle ?(eng at full power)"

With unlimited power you can achieve anything.

You only have to look at what you can do with model aircraft and when technology catches up we will be able to have 2000hp in our J200, with the same weight, and make it do all those unbelievable things they do with models at the moment'

How cool will that be !!!!!

Share on other sites

Some aircraft have an Angle of Attack meter which shows A.O.A. Now I have never flown with one but I cannot see how the stall is always at the same AOA. When flaps are used the Coefficient of Lift is changed, increasing with more flap usually and therefore the same lift is produced at lower speed, which means nose up or a higher AOA. So therefore I see it as differing with changed wing configuration and the AOA meter can only be a riugh guide.

With a high speed stall it appears to me that the AOA will be lower than normal because C. of L will be the same but more lift is required.

Hi Ian,

two separate issues here. If you look at Dynon's instructions for calibrating their AOA probe they acknowledge the issue that you raise. They suggest that clean stall should be calibrated (from memory) at the top of the yellow on the instrument and that dirty stall be calibrated for top of the red.

I suspect that your second proposition is not right. The reason goes like this. Lift is a function of velocity squared - it is also a function of AOA. Let us take the situation at just sub critical AOA - we have maximised lift from AOA so the only other variable to increase lift is speed. Recall from your training that stall speed increases as the square root of the load factor. So an aircraft at MTOW of 550kg in a 2g turn actually "weighs" (leave me alone you physicists) 1100kg. The wing has to produce that much lift. We have said that alpha is just sub-critical so there is no more lift to be had there so we must increase speed. As lift is a function of speed squared then the stall speed will increase by the square root of the load factor.

So AOA doesn't change in an "accelerated" stall but stall speed does. Thus the old saying "there isn't a stall speed but you always stall at the same AOA". If we go back to our "alpha controller" from the other post - the stick - then stick position becomes a very important indicator of impending stall. If you know what position the stick is in at the stall then in any given situation little warning bells should be deafening you if the stick approaches that position. AOA doesn't change speed does.

Regards

Mike

Share on other sites

Differential AOA

Just following on from the issue of flaps raised in Yenn's post. Lowering flaps increases the camber on that part of the wing and so changes the chord line. Doing this means that the alpha of that part of the wing changes.

That takes us to the use of ailerons in the stall and in incipient spin. Most modern wings stall the wing root before the tips so that the ailerons retain effectiveness in the stall. The little stall strips that you see on some a/c such as the Tecnam Sierra are just one way of doing this.

Now imagine that we have just stalled and the left wing drops. Our "normal" reaction is to pick it up with aileron. The downgoing left aileron increases the camber of the outboard section of the wing therefore the alpha of that bit of the wing increases. The outcome: if the outboard section wasn't stalled before then there is a good chance that you just stalled it and if it was stalled before then you just deepened the stall. Thus the wing drop continues. That's why we always pick up a dropped wing with our feet at or near the stall.

It also seems to me to be a good reason not to flop the ailerons around when at or near the stall. Some a/c are very well behaved and will tolerate it. Others will promptly drop a wing.

Regards

Mike

Share on other sites

Some a/c have cosiderable increase in AOA before stall due to L/E slats (variable camber) eg 747,767, most of the major airliners.

Teenie

Share on other sites

Hi Teenie,

you are of course right. Indeed the FA-18 has what is described as a "living wing" so that the configuration of the wing changes as required. They won't stall even at 44 alpha.

However that doesn't exactly help us: I would argue that in practical terms the heavy aircraft and the FA-18s of this world have different wings in different circumstances. The reconfiguration is such that the wing that has a different critical AOA is to all practical intents a different wing.

In the a/c that we fly I would argue that the same wing will always stall at the same alpha. We don't have the same massive reconfiguration options as those other a/c. I think that if there is any generalisation in the statement "you always stall at the same AOA" then for our a/c it is immaterial at worst and a safe assumption at best.

Regards

Mike

Share on other sites

Angle of attack

An angle of attack indicator, properly interpreted, Will give absolutely reliable indication of the proximity of a stall, in one wing configuration only. Reconfigure the wing and the principle still applies but the figure is different ie. you have a different lift coefficient.

In heavy jet aircraft, an angle of attack derived readout presents as a slow/fast indicator (what could be more simple?) and the instrument recalibrates itself to take account of flap& slat position so only 1 dial is needed ( note that ice or rain on the wing is not compensated for )

In simple form , a length of wool in the airstream, away from propwash & airframe influence would indicate the same information.

Even in a steep turn (or a pull out from a loop) where the aircraft is dynamically loaded the angle of attack meter gives the valid data for stalling avoidance Ie. the angle is the same although it will be achieved it a higher airspeed. The angle is the key.

The elevator presents the wing to the relative airflow & controls the angle. This leads us to the concept of Stick Stall Position, which is not a bad way of looking at the problem ,if allowances are made for reconfiguring the wing or a change of CofG.. N....

Share on other sites

A New Proposition

I'm going to try a procedural turn here...sort of back to Ian's original question.

Given the discussion very quickly moved from airspeed to alpha and we've given that a fair thrashing; I'd like to propose the following for your consideration.

The Proposition is:

To all practical intents and purposes if we "know" (that is can measure or judge) attitude and alpha we don't need to know airspeed to effectively take-off, fly around and land safely and smoothly.

What do you say to that?

Regards

Mike

Share on other sites

Different speeds.

Difficult to navigate, but in the low speed , ( T/O, approach/land ) & tight manoeuver , much better off. You need energy assessment. ie Minumum speeds for some aero's (most) & things like turbulence penetration speeds for safety. Interesting concept. Makes the point about stall angle. In the case of a spin or spiral, the determination would be easy. N...

Share on other sites

A good exercise in flying, without 'chasing' IAS numbers, is to 'learn' the power settings v's the attitude of your beast for climb, cruise and approach - and then blank off the ASI and go flying.

It's very character building, and you'll never again fear the visit of an itinerant wasp while you're out bush.

happy days,