True airspeed display for landing light aircraft

[Linc]

My pilot friend asked me, as an electronics professional, to build him a large visible display of TAS for purposes of landing his homebuilt aircraft only. His EFIS is too fiddly to look at on landing. Firstly can I reasonably suppose that a pitot tube sensor will give me all the data I need ignoring variations in temperature and QNH? Secondly if that were true is the pressure to speed conversion always done digitally and if not, then what is the usual method please? Obviously calibration is crucial. My PIC program writing skills are a last resort although I'd be happy to borrow/buy some code from an available place.
I know there are companies that sell ready made production units but I'm really looking for any pointers to home building/DIY sites or individuals etc for the sake of cost efficiency and because aviation is not my field.:)
Thanks very much. Lincoln.

[HotDog]

Firstly can I reasonably suppose that a pitot tube sensor will give me all the data I need ignoring variations in temperature and QNH?

Lincoln, what you'll get with that setup is indicated airspeed only. True airspeed in flight can be determined after recording temperature and air pressure. TAS formula is indicated airspeed times square root of air temperature at flight altitude plus K (459.4 for F and 273 for C.) divided by ISA temp. (59F or 15C) plus K times ISA pressure (29.92) divided by the air pressure at flight altitude. In general you can assume TAS by adding 1.4% for every 1000ft of altitude. In modern aircraft the inputs are digitized by the CDU. Hope this helps.

[AerocatS2A]

I would question whether your pilot friend really wants a "true airspeed" display for landing. It is the indicated airspeed that is important in terms of the flight performance of an aircraft. You can get indicated airspeed by using a pitot head that measures dynamic air pressure and static air pressure, the difference between the two, calibrated correctly, gives you your airspeed.

[Genghis the Engineer]

You DO NOT want a TAS indication for landing, you want an IAS indication.

The relationship, if you don't understand it is...

IAS = Indicated Air Speed (as seen in the cockpit)

<correct for system errors to give....>

CAS = Calibrated Air Speed (also known as RAS, Rectified Air Speed)

<Correct for compressibility to give... >

EAS = Equivalent Air Speed (which determines structural loads)

<Correct for density to give...>

TAS = True Air Speed

<Correct for wind to give...>

GS = Ground Speed.


For virtually any light aeroplane, you'll be glad to know that CAS=EAS so you can ignore that correction. However, if you want to be clever with your electrickery, the two things you can try and give your chum is...

- IAS=CAS (essentially calculate out the system errors in real time). But bear in mind that all of the current safety limits are based upon the current errors, not upon CAS, so you'll need to agree changes in IAS limitations with the authority (PFA?).

- TAS, for navigation purposes only. For this, you'll need pressure and temperature feeds into your system.


But, you'll get nowhere if you don't go and learn the maths and basic equation proofs for airspeed and altitude. Any half-decent undergraduate textbook on aerodynamics should cover this adequately.

Finally all current knowledge and research shows that a dial gauge indicator (or it's linear equivalent - a speed strip) is the best, safest, and most user-friendly indication of airspeed (for handling purposes at-least). So th IAS/CAS display should NOT be digital in nature.

G

[AerocatS2A]

Finally all current knowledge and research shows that a dial gauge indicator (or it's linear equivalent - a speed strip) is the best, safest, and most user-friendly indication of airspeed (for handling purposes at-least). So th IAS/CAS display should NOT be digital in nature.
G

This is worth repeating. For something as important as airspeed, you really need a dial/gauge type indicator. An analogue gauge lets you identify trends and approximate values better than a digital display. It is much easier to see that you are a bit slow, and getting slower for example. With a digital display it takes more brain power to interpret just what those rapidly changing numbers mean.

[barit1]

What the brain needs to rapidly comprehend is - what is IAS now, and how fast is it changing? A digital (numeric) display doesn't serve well for this. An analog needle works great (and has for many decades, and doesn't depend on an electrical source...).

A quasi-analog bar-graph digital display is also good IF the update rate is fast - 5/per second maybe.

Don't let the engineer in you overcome the pilot! :eek:

[TheOddOne]

Also, if you're landing a light aircraft, not a 767, you're going to go much more on external visual and other sensory cues. IAS is important on the approach and to ensure the threshold speed is correct, but thereafter you should, in my view, be 'head-out' of the cockpit, checking for drift, sink rate, judging height and getting ready for the flare etc.

I've seen a few cars with a 'digital' speed display but never driven one. I'd be interested to see a digital IAS readout - you could indicate trend by a different colour - say green for steady, yellow for increasing, red for decreasing. Oh, of course we're already using those colours for telling us we're going too fast!

As to the 'received' wisdom' that analogue instruments ae more easily interpreted than digital; I prefer a digital watch because it actually tells me what the time is, rather than my having to work out 'the big hand's just past the 2 and the little hand's just past the 5 - it must be, er, eleven past five, er 1711'. My watch says '17.11'. in big friendly numbers! I'm sure we only use round dials 'cos that's what the clock industry came up with to mimic the sundial and that's what we've all grown up with.

Lastly and more seriously, it's only by people like Linc having a go and doing something different that the rest of us benefit. Have a go & tell the rest of us what it was like!

Cheers,
TheOddOne

[barit1]

...
As to the 'received' wisdom' that analogue instruments ae more easily interpreted than digital; I prefer a digital watch because it actually tells me what the time is, rather than my having to work out 'the big hand's just past the 2 and the little hand's just past the 5 - it must be, er, eleven past five, er 1711'. My watch says '17.11'. in big friendly numbers! I'm sure we only use round dials 'cos that's what the clock industry came up with to mimic the sundial and that's what we've all grown up with...

Digital is perfectly satisfactory for telling time because because the rate-of-change is always known (EXACTLY one second per second, eh?)

But if you're trying to read a parameter that may be changing up OR down, fast or slow, digital leaves much to be desired. :ok:

[Linc]

Many Thanks for all the pointers. I was mistaken about TAS rather than IAS. It seems I'll have my head in some text books for a while. Cheers.
Lincoln

[FREDAcheck]

I'm not about to correct Genghis, but my understanding from what he said is that what you really want is CAS not IAS. That is (roughly) IAS corrected for things like positional errors. Not a big difference, but for example flying slowly with high angle of attack, the pitot tube isn't pointing exactly into the air stream, and (I think) result in IAS being a few knots lower than CAS.

CAS (rather than TAS) is the indicator of how the plane will perform, so it's what you want to know on landing.

[Genghis the Engineer]

I'm not about to correct Genghis, but my understanding from what he said is that what you really want is CAS not IAS. That is (roughly) IAS corrected for things like positional errors. Not a big difference, but for example flying slowly with high angle of attack, the pitot tube isn't pointing exactly into the air stream, and (I think) result in IAS being a few knots lower than CAS.
CAS (rather than TAS) is the indicator of how the plane will perform, so it's what you want to know on landing.
Yes !

G

[Wizofoz]

Indicated airspeed is just that- whatever it says on the clock.

If you can design a sophisticated ASI that has fewer errors than a conventional instrument, Its IAS will be closer to CAS (or, in the ideal case, equal to it), but it is still by definition IAS.

[karrank]

I've seen a few cars with a 'digital' speed display but never driven one. The karrank% family Prius has a digital speedo. Took almost 6 seconds to get used to it and can 'capture' speeds when accelerating/slowing no worse than with analogue. Probably better. An EFIS display with a digital readout and a slide bar would be best.

Your mate is heading for problems if he lands on TAS.

[Genghis the Engineer]

I've done the last 40,000 miles or so of my motoring career with a digital speedo, the previous couple of trips to the moon and back with analogue. Both are useable, but I'd much rather have analogue.

Similarly in aeroplanes, I've flown with digital displays, you can use them, but I feel you get so much less information. This doesn't mean that digital technology is (necessarily!) a bad thing, just that numbers are not the best way to display certain types of information.

G

[RHLMcG]

.. which is why the most usable displays have a combination .. digital for ease of steady state reading and analogue for rate information ...