Steering on final with rudder

[motzartmerv]

Aahhh yes of course, the static port is a bit strange up there on the tail.

 

 

[Bob Llewellyn]

Aahhh yes of course, the static port is a bit strange up there on the tail.

FAR 23.1323 allows 5kt error; so the Jab in question meets Cessna's design standard. Which means it'll meet CS-VLA too - Jabiru have been using CS-VLA (rather more demanding than ASTM F2245) for some time.

That being said: Short of a pressure head that incorporates both pitot and static in one assembly, be it Zahm type or Piper's neat underwing pylon type, it really is worth doing a pressure survey of the airframe before finalising a location. there is a good reason that flight testing requires the use of a trailing cone static source.

 

 

[motzartmerv]

Thats fair enough Bob, but from an instructing point of view we have to point out that the IAS does read erroneously on the dangerous side in aggressive sideslips on that particular system. While on others it reads the other way.

 

i wonder how this design standard would be applied to an aeroplane with an approach speed of say 40 kts. The error "allowable" by the standard would be greater than 10% of the IAS.. Interesting..

 

 

[Bob Llewellyn]

Thats fair enough Bob, but from an instructing point of view we have to point out that the IAS does read erroneously on the dangerous side in aggressive sideslips on that particular system. While on others it reads the other way.i wonder how this design standard would be applied to an aeroplane with an approach speed of say 40 kts. The error "allowable" by the standard would be greater than 10% of the IAS.. Interesting..

FAR 23 can be applied to such an aeroplane; it's just that, from a marketting aspect, most manufacturers seek an erroneously low reading at the slow end (to make it look STOL-er), and achieve a free safety cushion by doing so.

Design Standards are minimum standards; anyone is free to exceed them, but may not fall short (enforced up front, in the case of Certification). I have personally investigated one (double) fatality which would not have happened had a different design standard been used (Type certified aircraft).

 

 

[facthunter]

It's a bit sneaky really, and this has been discussed on this forum before. Exaggerating the lowness of the stall and also the cruising speed on the high side if you get away with it, helps sell, but it shouldn't. You should have 2 static ports if they are on the side and they connect by a common tube, errors created by out of balance in slip should not be so large. Don't trust airspeed readings from one plane to the next unless it has Boeing or Douglas or something on the side or at least a certified plane. Nev

 

 

[Bob Llewellyn]

It's a bit sneaky really, and this has been discussed on this forum before. Exaggerating the lowness of the stall and also the cruising speed on the high side if you get away with it, helps sell, but it shouldn't. You should have 2 static ports if they are on the side and they connect by a common tube, errors created by out of balance in slip should not be so large. Don't trust airspeed readings from one plane to the next unless it has Boeing or Douglas or something on the side or at least a certified plane. Nev

...even early CAR 3 aeroplanes are, ummm, a bit off... the Cessna 172B has an Indicated stall of ~32kts at light weight, which is horseapples... mind you, if you drop it in with the flap, it'll land very short...

 

 

[Dafydd Llewellyn]

Aahhh yes of course, the static port is a bit strange up there on the tail.

Yep, if the IAS rises with yaw, this is almost certainly - (I would say, definitely, having studied pitot head behaviour extensively) unless the yaw puts the pitot head in the slipstream - due to the static system being affected by cross-flow.

 

 

[Dafydd Llewellyn]

Thats fair enough Bob, but from an instructing point of view we have to point out that the IAS does read erroneously on the dangerous side in aggressive sideslips on that particular system. While on others it reads the other way.i wonder how this design standard would be applied to an aeroplane with an approach speed of say 40 kts. The error "allowable" by the standard would be greater than 10% of the IAS.. Interesting..

Yep, the design standards ain't perfect. That's why having an understanding of them is so important.

 

 

[facthunter]

And bear in mind they you should be able to safely pilot a simple plane around a circuit with a failed AIS. Beware the wasps and their instant mud in the pitot. Try it with an instructor as backup. You are allowed to be a little fast . Nev

 

 

[Dafydd Llewellyn]

And bear in mind they you should be able to safely pilot a simple plane around a circuit with a failed AIS. Beware the wasps and their instant mud in the pitot. Try it with an instructor as backup. You are allowed to be a little fast . Nev

That's not difficult to do provided the aircraft has adequate stick force Vs speed characteristics (another one of those things that causes sweat, blood & tears in certification). Don't try it in a BD-4 or a Lancair 360.

 

 

[facthunter]

I used the word "simple" There may be better ones to use. Stick feel is not everything but sure helps. The noise of the wires in a DH 82 is useful.. Attitude and power should match what happens but landing downwind would test most pilots. Nev

 

 

[Bob Llewellyn]

That's not difficult to do provided the aircraft has adequate stick force Vs speed characteristics (another one of those things that causes sweat, blood & tears in certification). Don't try it in a BD-4 or a Lancair 360.

Want to fly sans ASI? Get a Thruster TODAY!!!

 

 

[motzartmerv]

Agreed nev. AIS failure is an absolute must!!. I dont just use it as failure mode practice either. I often cover the dash in early circuits to get pilots to fly the attitude and stop chasing IAS. Particularly with glass, which is good, it usually just means turning the screen off. (get ready for the on slaught of people telling how dangerous this is)

 

Thinking about the Jab with the over reading IAS in the slip. It must be related to the pitot head itself. The static system is identical to other jabs we have that dont display this characteristic. The 170 for instance has an identical static port but the skinny tube with a hose attached to the wing strutt for the pitot, and doesnt over read in the slip.

 

 

[Bob Llewellyn]

Agreed nev. AIS failure is an absolute must!!. I dont just use it as failure mode practice either. I often cover the dash in early circuits to get pilots to fly the attitude and stop chasing IAS. Particularly with glass, which is good, it usually just means turning the screen off. (get ready for the on slaught of people telling how dangerous this is)Thinking about the Jab with the over reading IAS in the slip. It must be related to the pitot head itself. The static system is identical to other jabs we have that dont display this characteristic. The 170 for instance has an identical static port but the skinny tube with a hose attached to the wing strutt for the pitot, and doesnt over read in the slip.

Turning the horizon off in a VFR aeroplane is very dangerous:tongue in cheek:... but I trust you know this???

...unless there's a leak in the static system? The worms are out of the can...

 

 

[motzartmerv]

We picked up a Senneca from a strip out west once. the pitot must have had dried mud in it, as we flew through the soup to get it home the IAS failed, due to the moisture in the pitot. The chief looked at me and grinned and said , "o]h good, some IAS failure practice for your approach"..lol..At night, in the rain...he's a braver man than I!!!!.. Doesnt really mean much to say set a familiart attitude and power setting for the apporoach when its your first time flying the thing...lol..Ahhh...good times!!!

 

 

[motzartmerv]

No Bob, the system checked out..

 

 

[Bob Llewellyn]

No Bob, the system checked out..

Okeydokey... at this range, I'm reduced to guesses... does it do it in slips both ways? And, where's the pitot? If more than one aeroplane do it, what's the common denominator? What WAS the strange thing the dog did in the night?

 

 

[JimG]

Hi Bob, sorry if I've missed this in previous post , so how far below the wing and what longitudinal position is the best for the pitot tube and is the static port best located with it. I know the RV's have them half way in the aft fuselage on both sides interconnected but I'm told even that system has to be 'tuned' .

 

I hope know one minds the thread drift.....?

 

cheers

 

JimG

 

 

[Kyle Communications]

I have my static port on the top of my fin I got a jabiru static port and put it there as everyone who owned a savannah said the dual balanced static port used in the aircraft was a waste of time. Most just coiled it up inside the rear of the aircraft. I sideslip a lot and I have not had any erroneous readings at all.

 

 

[Dafydd Llewellyn]

Hi Bob, sorry if I've missed this in previous post , so how far below the wing and what longitudinal position is the best for the pitot tube and is the static port best located with it. I know the RV's have them half way in the aft fuselage on both sides interconnected but I'm told even that system has to be 'tuned' .I hope know one minds the thread drift.....?

cheers

 

JimG

Unless you want to go to a lot of trouble calibrating it - as Piper evidently did with their underwing device on the PA 28 - the static ports / head should be well clear of the wing pressure field. The least troublesome spot is half-way along the rear fuselage - but only if the rear fuselage is a straight-generator shape. For most plastic fantastics, the rear fuselage shape is not suitable. Inside the rear fuselage is often used for the "alternate static" source for GA aircraft that require one - though it usually has a bit more error than a flush port, if you can find a suitable location for one, due to suction around the cabin and leaks around doors etc. The static port location gets more critical, the faster the aircraft goes.

 

 

[Bob Llewellyn]

Hi Bob, sorry if I've missed this in previous post , so how far below the wing and what longitudinal position is the best for the pitot tube and is the static port best located with it. I know the RV's have them half way in the aft fuselage on both sides interconnected but I'm told even that system has to be 'tuned' .I hope know one minds the thread drift.....?

cheers

 

JimG

...that's not an easy question! Here goes:

Accepted wisdom (and Certification):

 

An aircraft may be considered a bunch of pressure fields; and as they are sufficiently intense to hold an aeroplane up, it is accepted that it is virtually impossible to produce an ASI system that is both suitable for everyday use, and very accurate in all possible operating conditions. The atmosphere keeps changing, too...

 

Therefore, Certification requires that the airspeed indicator system be calibrated, and the corrections made available to the pilot. However, most people don't like reading correction cards halfway down finals, so gradually design standards have reduced the original wild freedom AISs once had.

 

The calibration system involves getting the static system out of the aeroplane's pressure field, by towing it on a drogue ~ 30m below and ~100m behind (less for lighties) the aeroplane (there are pictures of jet airliners with plastic funnels on hoses hanging off them!). The pitot ends up as a pivotting (self-aligning) type on a boom several chords in front of the wingtip, or if flutter is an issue, as far fowards of the LE as possible.

 

Practical Systems:

 

1) Static:

 

The laminar sub-layer of the boundary layer, in air, is always very close to static pressure - if no extra pressure field is superimposed. The innermost edge of a turbulent boundary layer - outside the laminar sub-layer - is also very close to static, if unaffected by external fields. As the boundary layer gets thicker moving aft along the fuselage, and the main causes of external pressure fields - the propellor and wing - are normally towards the front of the fuselage, putting the static port(s) as far back as can be - without the tail affecting things - is good practise.

 

Unhappily, a sideslip will cause a pressure difference from side to side of a fuselage; and it's NOT symmetrical - "T"-ing ports on both sides reduces the error to ~1/2, it does not eliminate error. Furthermore, putting the ports on little raised discs of aluminium because it's cheaper than a flush port, risks raising the ports into that part of the boundary layer containing fluctuating pressures (the fluctuation is too rapid for the instruments to detect, but the mean pressure will vary with speed).

 

Symmetrical flush ports 3/4 of the way back on a roughly circular glider boom rear fuselage work very well.

 

However, if a superimposed pressure field affects both the static and pitot pressure ports the same amount, no differential - or error - is shown on the ASI (due to superimposed pressure). So, mounting the pitot & static ports very close together gets around the static problem.

 

2) Pitot:

 

Most forms of pitot - whether bullet-nosed, or conical, or just bits of tube with the end hacked off - give quite accurate results at up to ~20 degrees misalignment. All airfoils (unstalled) have two zones where the positive pressure of the stagnation point becomes the negative pressure of the accelerated flow (both above and below, generally); passing through zero. It has therefore been standard practice to poke the pitot out of the LE, 0.1~0.2 chords fwd of the LE and about level with the foward extremity of the airfoil, and tweak it until the AIS meets the design standard (needless to say, the slipstream is to be avoided!).

 

Someone who had studies their NACA reports, noticed that there is a point about 20%~30% back along the underside of most airfoils (unstalled) where the local static pressure does not change much with AoA; and also recollected that the airflow adjacent to an airfoil, is essentially parallel to the nearest surface (except right up front). They therefore stuck a pitot port and a static port on a small pylon under the wing (~0.05 chords high), and filed a bevel off the trailing corner until the pitot-static difference represented the free airspeed. Look under a Piper Cherokee wing. The static error is not an issue; crossflow is not an issue (the wing acts as a flow straightener); sideslip is not a big issue; and few people bang their heads on it.

 

That'd be my first choice. A combined pitot-static head, as far outboard along the wing (but not near the ends of any lift struts) as practicable, and ~0.15 chords ahead of the LE would be my second. Note that putting a shroud on a pitot gives up to ~30 degrees of misalignment without significant error, although the shroud supports have to be faired and well behing the dynamic (pitot) port.

 

 

[djpacro]

Another option is pitot-static boom as used on the Pitts - very small pressure error (location of static behind the collar is critical) https://secure.steenaero.com/Store/images/site_images/pitot_static_and_dynamic_01_composite_320x240.jpg

 

 

[Dafydd Llewellyn]

...that's not an easy question! Here goes:That'd be my first choice. A combined pitot-static head, as far outboard along the wing (but not near the ends of any lift struts) as practicable, and ~0.15 chords ahead of the LE would be my second. Note that putting a shroud on a pitot gives up to ~30 degrees of misalignment without significant error, although the shroud supports have to be faired and well behing the dynamic (pitot) port.

If you use a combined pitot-static source under the wing, the static part of it needs to be "tuned" so that the pressure it gets is reduced just the right amount to compensate for the pressure rise caused by the wing circulation. The one on the Piper PA 28 was so tuned (it's one of the more clever aspects of the PA 28 - must have taken a few hours in a wind-tunnel to get it right; it's probably also affected by the airfoil the designer chose). If you make the mistake of using a standard pitot-static head - which is designed for use elsewhere - in such a location, the ASI will read low.

I would consider o.15 chords ahead of the leading edge a bit too close - unless you are trying to get a low indicated stall speed.

 

The overall errors one finds in an airspeed system, are the reason why the Flight Manual airspeeds are always given in IAS - i.e. they incorporate those errors.

 

 

[Ultralights]

when flying my Savannah, i rarely look at the ASI, only when about to grab flap, when climbing at about 60kts or the windscreen starts to vibrate near 100kts. engine failure its 55kts.

 

i can change the ASI just be opening and closing the window vents, as the static source is just behind the pressure instruments in the panel. cruise speed is 5100rpm, climb is full power, cruise descent at 4000rpm. finals and landing is all done by feel, and where the stick position is, gives me an accurate picture of AoA when flying behind the drag curve on steep approaches and short landings... rudder to keep the ball centred at all times..

 

 

[Dafydd Llewellyn]

when flying my Savannah, i rarely look at the ASI, only when about to grab flap, when climbing at about 60kts or the windscreen starts to vibrate near 100kts. engine failure its 55kts.i can change the ASI just be opening and closing the window vents, as the static source is just behind the pressure instruments in the panel. cruise speed is 5100rpm, climb is full power, cruise descent at 4000rpm. finals and landing is all done by feel, and where the stick position is, gives me an accurate picture of AoA when flying behind the drag curve on steep approaches and short landings... rudder to keep the ball centred at all times..

And a piece of string you can feel to see if it's raining?