# Airspeed, Groundspeed, TAS = Performance Confusion

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One thing that I found very hard to understand at first when I was learning to fly was airspeed. Now, when I was a student I had no previous aircraft or flying knowledge so coming from a car where you put your foot down, you go faster, and therefore you travelled further. The faster you went the further you travelled or you got to a destination in a shorter period of time.

But I still get muddled up when you throw the word "Performance" into the equation of flying.

We always seem to measure an aircraft's performance in terms of "What TAS can it do?". In a nil wind scenario we would assume that Airspeed (ASI) = Ground Speed and that is the aircraft is flying over the ground at a given rate of knots. To fly an aircraft over a distance of 100 nautical miles at 100 knots it will take us 1 hour to get there.

Yet our TAS we know may be say for the sake of debate, 105 knots.

Now this is what I can't understand when you throw the word Performance in. We may have two aircraft in the same scenario, both flying 100 nautical miles at 100 knots taking 1 hour to get there yet they will have a different TAS - say 105 and 107 - am I correct in saying this?

Now take a scenario of you trying to decide which aircraft to buy between aircraft A or aircraft B and you have a preference for higher performance - remember a car has higher performance if it goes faster and gets you there quicker.

Manufacturer A says their aircraft has a TAS of 130 knots yet when you fly it yes it does have a TAS of 130 BUT you may have an indicated airspeed of say 120 knots and a ground speed of 110 knots. We can assume here then that you have a headwind of 10 knots but we have a TAS of 130.

Manufacturer B who doesn't quote TAS in their figures but when you take it for a test fly (in the same conditions as when you flew Aircraft A) you find that you have an indicated airspeed of 115 knots and a same ground speed of 110 knots.

Are these two aircraft exactly the same or not in terms of performance?

What happens if Aircraft B is \$30k cheaper do you buy that one?

So, can someone PLEASE explain to me in "extremely basic terms" i.e. like it is easy to understand performance when driving a car which aircraft to buy if you are basing your decision on performance in getting to a destination quicker?

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OK here's a shot:

IAS: This is Indicated Airspeed - that's what your ASI (Air speed indicator) shows you. This value may be in error, forgetting faults, the three main errors are positional error (that is the pitot or the static source positions create error), AOA error - that is error that creeps in when the aircraft is at high AOA and speed errors - it may be accurate but not below 40 knots for instance.

CAS: Calibrated Air Speed, this is IAS corrected for errors.

TAS: True Air Speed this is CAS (though we usually use IAS) corrected for air temperature and pressure. This correction is the same for all aircraft and is roughly equivalent to a +2% correction per 1000' of height increase.

On the CT your EFIS has an Outside Air Temperature (OAT) probe and it also has an altitude and airspeed system so it "knows" all that is necessary to compute and display TAS - which it will do in addition to displaying IAS.

Be wary though: IAS is what you need to fly the aircraft on from a performance point of view. This is particularly important at high density altitudes - for instance during take-off and landing. At high density altitudes - say in Colorado your IAS of say 60 knots on landing will give you a much higher TAS and therefore for any given wind situation a much higher GS than when you are in ISA (International Standard Atmosphere) conditions - 1013.2hpa at 15 celsius at sea level.

All airspeeds are relative to the air mass. GS is simply TAS corrected for movement of the airmass relative to direction of travel.

NB - I have not included discussion of EAS - equivalent airspeed as we are not much interested in compressibility errors.

There's a start - someone else have a go.

Regards

Mike

PS: Manufacturer claims if made as TAS need to reference the density altitude conditions that pertain when they achieved that. For instance 130 knots TAS at ISA sea level. Or 125 knots TAS at ISA 5000'. Otherwise they should be quoting IAS. The problem of TAS without a height (and de facto a density altitude) is that we don't know whether it's good or bad compared to another manufacturer quoting say 130 knots at ISA sea level.

An a/c that will do 100 knots IAS will be doing 100 knots TAS at ISA sea level and around 110 knots at ISA 5000'.

M

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Disclaimer - I didn't write this in simple terms.

Manufacturer A says their aircraft has a TAS of 130 knots yet when you fly it yes it does have a TAS of 130 BUT you may have an indicated airspeed of say 120 knots and a ground speed of 110 knots. We can assume here then that you have a headwind of 10 knots but we have a TAS of 130.

Ian, if I can add you in a complexity, that if TAS is 130 knots and groundspeed is 110 knots, then you may have a 20 knot headwind?

I noticed your post a while back from your quick trip up north and was going to comment about TAS but forgot.

You can't use the quoted performance figures from manufacturers for a comparison unless you know the power, throttle and altitude settings. You need to know if they are quoting rpm or power settings.

Eg. Jabiru may quote a TAS of 120 knots for the J230 @ 75% power @ 8,500ft.

75% power is actually full throttle at that altitude. The 120 knots is possibly given at the standard TAS altitude of 8,500ft that manufacturers seem to use these days and must be in stadard temperature and density figures to mean anything useful.

The Cirrus has an electronic readout of the power being produced. At 9,500 ft, full throttle is around 69% power, so in theory a J230 at 75% power is 'probably' at 8,500ft AND full throttle.

As an example, my J400 might travel at 2,800rpm doing 120 knots at 500ft above sea level (in std atmospheric conditions). At 8,500 ft I will struggle to make it much more than an indicated 115 knots which lets say for arguments sake is 125 knots TAS, so in theory I'm TAS'ing at 125 knots at 2,800rpm. However that's only around 50% power as I'm not at full throttle. At full throttle, I may indicate around 118 knots and 3,100rpm and probably developing 75% power and as such a TAS of say 128 knots.

I would suggest that you could only ever compare figures for different aircraft at the same altitude on the same day at the same power settings and take your calculations from there, unless they are very specific about the conditions in which the figures can be achieved for both aircraft. If they vary slightly, then a comparison can't accurately be made.

"Now this is what I can't understand when you throw the word Performance in. We may have two aircraft in the same scenario, both flying 100 nautical miles at 100 knots taking 1 hour to get there yet they will have a different TAS - say 105 and 107 - am I correct in saying this?"

- No. If they are reading differently, then one or both of them is calculated incorrectly.

That's my 2 cents worth.

I found this nice little converter:

Eg calculation.

Temp 10 deg c

Alt 8,500 ft

CAS 115 knots

1013.2 Pressure

TAS = 133.4 - A little more generous than my calculation earlier

So at full throttle in the Jabby based on this calculator, if I can have a CAS of 120 knots at 9,500ft I've got a TAS of 142 knots which from experience is roughly what my GPS has told me on previous occasions.

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Performance

There is something else you need to take into account too. When you read the manufacturers figures, take most of them with a grain of salt. For instance, one popular aircraft is being advertised as cruising at 80 knots at a fuel burn of 10.5 litres per hour. I would suggest that is a bit on the optimistic side. I'm sorry to say this, but there seems to be a few used car salesmen out there.

David

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So Brent - would you say that these all gel together:

Manufacturer's specification:

Vmin minimum speed (flaps 0Ã‚Âº)

65 km/h IAS

VCR: cruising speed (green arc)

240 km/h IAS

VC 75%: with 912S (75% = 80 PS)

240 km/h IAS

VNE: max.permissible airspeed

(red line):

301 km/h IAS

*Km/h data for German Version with 472,5 kg MTOW.

Values are for ICAO standard atmosphere with non adjustable fixed pitch propeller

Actuals from the Dynons - Note, ignore the green arc on the backup ASI as it is for the USA:

TAS 132 knots

ASI 123 knots

GS 136 knots

Revs 5,370

Alt 4,100

Temp 15 deg

[ATTACH]2403[/ATTACH]

TAS 119 knots

ASI 114 knots

GS 141 knots

Revs 5,050

Alt 2,740

Temp 8 deg

[ATTACH]2404[/ATTACH]

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Hi Ian,

the data is anomalous because in the first example you have a descent of 100fpm and in the second a climb of 325fpm. Interestingly your AOAs in both cases are high on the green.

Also in example 1 5370rpm is about 93 HP - see attached PDF

Regards

Mike

912Sperf.pdf

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Thanks Mike

Firstly the AOA hasn't been calibrated yet

I would have thought the climb or descent figures wouldn't have any connection to the ASI, GS and TAS as a group - sure the ASI would go down when climbing and so to would the revs but then so to would the others as they are connected in their calculations.

This is what I mean - I just can't get me head around all of this.

When I was at school I did "Applied Science" that is the subjects were Maths 1, Maths 2, Chemistry, Physics and English as I believed it was easier to get higher marks in these subjects as there is always an answer - 1+1=2 no matter whether your teacher liked your story on how you got the answer or not plus I was (without boasting) a maths guru. One year, earlier on in my school days I would have got 100% pass mark for maths except I could not comprehend Probability - still can't today - I know it is suppose to be easy but I just can't grasp it - I can talk anyone under the table on trig, quadratic equations etc etc. This is the same for ASI, GS and TAS - I am very very sorry as I know you are trying to help but I just can't grasp it :confused:

To me the ASI tells me when I am going to stall or when the aircraft can't take the stress of the speed of the air going past the aircraft

To me Ground Speed IS performance - how fast am I going along the ground and therefore how quick am I going to get to my destination.

To me TAS is for people like you - that is, people that are a lot smarter then me :big_grin:

If I am going to buy an aircraft with my interpretation of performance as a deciding factor I ask "How fast does she go along the ground in nil wind at about 3,500ft?"

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Let's take this bit:

TAS 119 knots

ASI 114 knots

GS 141 knots

Revs 5,050

Alt 2,740

Temp 8 deg

You're pretty much close to their suggested figures (119 knots) except I can't work out what 75% power is for your engine. Is there a graph for that? I'd have to google it. If 75% was another 100 rpm then you might be closer to their suggested figures.

Let's just say that 75% happened to be 5,050 rpm then you are only 5 knots down on their suggested performance figures, however you are too high and as such your IAS will read too low to get a good comparison. Pretty close though, I'd be happy with 5 knots. They have indicated standard conditions however which are slightly warmer than what you were operating in, but that will make bugger all difference. If you descended to say 500ft above mean sea level you might find that you are really only 2-3 knots off their specs IF in fact 5,050 is 75% power.

You are indeed 100% correct that the best way to compare is ground speed (but with a correction for tail/head wind). - in this instance anyway.

Without a turbo-charged or super-charged engine there is absolutely no benefit to comparing figures at 8,500ft that I can think of.

Of course you need to realise that if you had a 914 Turbo you are capable of close to maximum power, even at 9,500ft so your TAS and IAS figures would be much higher as you have up to 30% more power.

Importing aircraft from cold countries can be frought with danger as you may well remember when a well known imported bought in a new aircraft from a European country. The company was quoting figures of around 150 knots and massive climb rates exceeding anything we have ever seen on 100 horsepower and the importer experienced this performance when there, however when the aircraft arrived in Australia it was lucky to do 120 knots as the air is so much thinner over here.

I think if that's what you're getting your so close to factory specs it's barely worth splitting hairs over.

PS: Based on what I've just read, I'll race you when I get my engine back together.

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For a start from the Rotax graphs it appears that 75% power is 4550RPM. Rotax say 75% power is 80PS and that's according to the graph 4550PRM.

Regards

Mike

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Ian,

I think you are confounding too many things here. Let's forget the following:

• TAS

• GS

Performance has only to do with the aircraft in the air mass - that's IAS. In one specific case IAS = GS and that is the still air case at sea level ISA.

The manufacturer reports their speeds as IAS at ISA and for non-adjustable fixed pitch propeller. Is that the prop that you have?

Lets also deal with climbing/descending. If you are descending then you will gain airspeed - yes RPM will increase but airspeed will also go up. Ditto for climbing. So to make a fair test you need to be in S&L, probably near sea level on an ISA day.

Their speeds equate to a 75% cruise of 133 knots at 4550rpm. You are not doing that.

However your aircraft is at least 70kg and perhaps more over the 472.5kg MTOW which they are quoting. BTW don't believe their stall speeds either these are very much related to gross and will be substantially higher.

Your increased MTOW is at least a 15% increase in gross and this will have a not inconsiderable impact on speed. I am not aware of a simple conversion for this. However if we assumed that a 15% increase had the same percentage decrease in speed that would be 20knots less roughly - so 113knots or if only half the increase was taken off 123knots.

Regards

Mike

what we do know is that the stall speed that they quote at 36 knots will in fact be at 544kg something of the order of 36 * sqrt1.15 or 36 * 1.072 = 38.6 knots. I don't know if the same holds true for speed decreases with increased weight.

M

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So if 75% is 4,550rpm then the figures are not good except that weight isn't considered - they are quoting 472.5kg and my CT is 320kg + 95kg (me) + 10kg (ERSA ;)) and fuel say 50% which is 65 litres which is 48kg = 473kg - that works out to be the same weight.

So my figures are not good

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Forgot that you were on your own and not that fat ;-)

My weights were with a fat @@\$\$^%#% like me in there as well - ie 544kg.

Mike

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- Mike, I lost 12 kg recently simply by :yuk: - I have been quietly on the side complaining that this CT isn't as fast as my first one. No one believed the figures that I was getting from my first CT - they all said it just isn't possible but I definitely was getting easy 130+ knots at about 4,700rpm when averaged in all wind conditions (upwind, downwind, still wind etc). Nathan the Snr Instructor at Shepp took this new CT for a circuit on his own and he said that it doesn't perform uphill, downhill, along the straight and level etc anything like my first one - they called my first CT "The Weapon".

I now need to start looking at the prop pitch, the negative 12 degrees flap setting (is it in fact -12) etc if I am going to be able to get what the manufacturer has publicly quoted - got to keep the so and so's honest but it is a bit rough when you spend \$150,000+ on something that isn't what they say it is

I may as well have bought a Jabiru for a lot less money and get not that much less performance but then I will have to change all the pictures on the website then

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We may have two aircraft in the same scenario, both flying 100 nautical miles at 100 knots taking 1 hour to get there yet they will have a different TAS - say 105 and 107 - am I correct in saying this?"

The only way I can see this happening is if the aircraft cruise at different altitudes, but what happened to Ians request for an answer in extremely simple terms. It is hard to get a simple answer from dedicated pilots.

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To be honest, I never believed that your previous CT could have done some of the 130+ knot cruise figures that I was hearing because the laws of aerodynamics hardly allow for it. IF it was getting 130+ then there must be something fundamentally different between the two aircraft and if you could organise a side-by-side comparison you'd be able to answer the question once and for all.

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Brent - that is something that I have been wanting to do but it is up at Nyngan when it is home.

I have however gone side by side with another CT that had the large Tundra wheels and fairings (worth about an extra 3 to 5 knot drag) plus a 3 blade prop which is slower then my 2 blader and I even had trouble keeping up with him.

Time to go public with this me thinks :;)4:

However, I still would like to understand more about airspeeds as per the start of this thread.

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Ian,

I'm not sure about this but have there been changes to the design of the a/c between your two CTs? I thought that I heard somewhere that they had redesigned elements of the a/c including the tail control surfaces. This, if true may be having an effect.

Also I would be very interested to see what the AOA probe is showing when it is calibrated. At high cruise speeds your AOA should be quite low - at least bottom third of the green I would guess.

Regards

Mike

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Mike - yes there were changes to the tail section. Apart from a full elevator trim the base of the fin starts a lot earlier along the fuselage:

[ATTACH]2419[/ATTACH]

But I think the point is that if a manufacturer states performance characteristics on their website and one spends over \$150k based not only on those specs but also on history of already achieving them with one aircraft that they should get what is publicised - or am I just being pedantic :;)4:

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No I don't think you're being pedantic - pedantic is asking for your money back 'cause it only goes 124 knots when the manufacturer says it will go 125 knots

However given that, I think it is worth seeing if you can work out what is going on and whether you can "tune" the a/c. That's why I was interested in any airframe changes and also the AOA. The prop might be the key to this anyway - I think that colour prop has got to be costing you at least 5 knots over a black one:cool:. Interestingly the Shepparton Tecnams both do pretty well the speeds that Tecnam says they will do. The thing is that you can only get 5000 rpm out of them flat out on takeoff and about 5100 rpm max in S&L. That seems to say to me that the props are carrying a bit of pitch and the engine is working hard but obviously never giving 100hp.

Another thing: It would not surprise me at all if the manufacturer hadn't redone the performance figures after revising the aircraft. From a few things I've seen it wouldn't surprise me at all. They probably test flew it etc but the changed numbers may never have been put into the sales material. Config management is a huge issue for these guys.

Regards

Mike

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ASI errors

Don't take your ASI readings as gospel unless you have checked it out with a GPS. I generally fly around in a big circle at a constant indicated speed on the ASI and average the max and min GPS readings. If you do that for a few different speeds you will get a fair idea of what is what. You need to repeat the whole exercise at different altitudes too as the true speed get faster the higher you go. From what I have seen there are quite a few aircraft out there that don't fly anywhere near as fast as their owners fondly believe.

A couple of rough rules of thumb.

Engine power is roughly proportional to the square of the engine speed with a fixed pitch propellor at a constant altitude. 75 percent power is obtained at 8500 feet at full throttle.

At the top end, required power is roughly proportional to speed squared as parasitic drag is proportional to speed cubed, but induced drag is inversely proportional to speed.

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Mike said "Their speeds equate to a 75% cruise of 133 knots at 4550rpm."

The graph that you got that from is what you get using a dynamometer and getting the maximum power at that speed.

The graph in the Owners Manual for the 912 UL shows 75 percent power as 5300 rpm, the actual curve with a fixed pitch propellor, goes the other way as a Y = X^2 graph will go.

5800 80 hp

5500 69

5000 50

4500 37

Fuel consumption seems to be proportional to power, around 0.45 lb/hp/hr. That is close to 17 litres per hour at 0.75 percent. I presume the 912 ULS would be about 20 percent higher.

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If Ian is running at 5,370 and getting over 120 knots IAS at a higher alt than sea-level then performance are pretty much in line with factory specs IF 5,300rpm is also 75% power for the 912S 100 HP.

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RPM S&L vs T/O

Mike says "The thing is that you can only get 5000 rpm out of them flat out on takeoff and about 5100 rpm max in S&L."

That doesn't seem to gel. It must be way too much pitch. I was running a 3 blade GSC ground adjustable and on take off at 50 knots it only got to 5000 but at full throttle S&L 83 knots it got to 5800. The GSC blades are very rigid. I swapped it for Sweetapple prop which was very flexible and on T/O it ran at about 5500 but was still only 5800 flat out S&L. The interesting thing was that I had to use full right rudder on take off to stay on the strip with the Sweetapple as there was so much extra power. I had a copy made but it wasn't anywhere near as flexible and I couldn't get the same revs on T/O although it was the same flat out.

David