Steering on final with rudder

[facthunter]

On the Gypsy Moth the ASI is on a interplane strut and is a bit of flat tin on a piece of wire that bends back across a scale. You have to look right out to the side. My god it was tough in those days. Women threw themselves at us The expectation was high..... Nev

 

 

[M61A1]

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

Don't knock the string.......simple, inexpensive, lightweight and reliable.

 

 

[Dafydd Llewellyn]

On the Gypsy Moth the ASI is on a interplane strut and is a bit of flat tin on a piece of wire that bends back across a scale. You have to look right out to the side. My god it was tough in those days. Women threw themselves at us The expectation was high..... Nev

Simple? How many hours do you think it took to build the NPL (New Physics Laboratory) wind-tunnel so they could calibrate that thing?

 

 

[facthunter]

I wouldn't like to guess. It shouldn't take long if it wasn't a government contract. I could hardly believe it when I looked for the ASI and saw that thing. The plane was VH ULM . Interesting rego but I think it was one of a series and not a personal number plate related to the famous man.. My Citabria was VH SAG.. Not a particularly desirable one but again one of a series. Certainly nothing famous there either....Nev

 

 

[motzartmerv]

Did you check it out Nev? Might have been Charles?

 

 

[facthunter]

I did later, Andy, and it wasn't. I did know a relative of Charles, FRANK Ulm But I don't think he flew.

 

 

[motzartmerv]

Bumma.. Would have been cool!!

 

 

[Bob Llewellyn]

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.

For 4412, 0.15 chords fwd of LE and ~0.2 chords below it should be fine between zero degrees and 16 degrees AoA, by the tests of NACA Report 563; 0.15 below would start to depress the stall speed indication.

I suspect the collar on the Pitts pitot is for tuning the static just as you describe.

 

 

[Bob Llewellyn]

I wouldn't like to guess. It shouldn't take long if it wasn't a government contract. I could hardly believe it when I looked for the ASI and saw that thing. The plane was VH ULM . Interesting rego but I think it was one of a series and not a personal number plate related to the famous man.. My Citabria was VH SAG.. Not a particularly desirable one but again one of a series. Certainly nothing famous there either....Nev

...wasn't named after Pierre Sag, the famous Franco-Swiss sausage maker? No, forget I asked...

 

 

[Bob Llewellyn]

Don't knock the string.......simple, inexpensive, lightweight and reliable.

but ya gotta calibrate it! Cotton duck holds much more water than braided Dacron... I use an extra-aureal sensor on my T-83 (when one ear goes cold, it's raining...)

 

 

[M61A1]

but ya gotta calibrate it! Cotton duck holds much more water than braided Dacron... I use an extra-aureal sensor on my T-83 (when one ear goes cold, it's raining...)

In my 95.10, if one ear is cold, it's because I'm slipping or skidding, which is one of the things that the string tells me in my Drifter. The heavier weight string makes it more effective for airspeed as well as AoA and slip, it really is a cheap reliable heads up display. The calibration is just visual, just fly straight, level and in balance, get a mental picture, and go from there.

 

 

[Bob Llewellyn]

In my 95.10, if one ear is cold, it's because I'm slipping or skidding, which is one of the things that the string tells me in my Drifter. The heavier weight string makes it more effective for airspeed as well as AoA and slip, it really is a cheap reliable heads up display. The calibration is just visual, just fly straight, level and in balance, get a mental picture, and go from there.

'cause the Thruster donk is up high, the slipstream corkscrews through the crew compartment... blows the string across too...

 

 

[M61A1]

'cause the Thruster donk is up high, the slipstream corkscrews through the crew compartment... blows the string across too...

Maybe you could tune/calibrate the length of the windscreen on that side to achieve ear temperature balance.

 

 

[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

ok, Dafydd reminded me of a blindingly obvious point - the pressure field under the wing is caused at the expense of the local dynamic pressure, so the pitot reading IS lowered in that location. Trying again:

EITHER a combined pitot/static head should be nearly a full chord ahead of the wing, OR you need to get a pressure map for the airfoil (or grope around in the air until you find a location that works), and put it in the unaffected zone; OR(2) you need to stick it close under the wing - a la Piper - and tweak it - a la Piper - until it works.

 

4412 / Clark Y both have a pressure-passive zone, and about 0.15 chords fwd and 0.15 chords down (if more than ~40% outboard - or lower if inboard of 40%) should work through the positive AoA range; 0.2C fwd and 0.2C down should work anywhere along the wing, out of slipstream...

 

since any mods to the system normally require you to do a calibration with a pivotting pitot / trailing static, most people just do an accurate calibration on their existing system, and call it quits.

 

 

[Bob Llewellyn]

Maybe you could tune/calibrate the length of the windscreen on that side to achieve ear temperature balance.

...but the power setting changes it... hmmm, a challenge...

 

 

[djpacro]

... the pressure field under the wing is caused at the expense of the local dynamic pressure, so the pitot reading IS lowered in that location. Trying again:EITHER a combined pitot/static head should be nearly a full chord ahead of the wing ...

Nope, there is a reason why it is called "pitot". Static and dynamic change but not total. Full chord ahead or whatever is good for a prototype but not required for production examples.

[Bob Llewellyn]

Nope, there is a reason why it is called "pitot". Static and dynamic change but not total. Full chord ahead or whatever is good for a prototype but not required for production examples.

"total pressure"?

 

 

[motzartmerv]

Dynamic + static?

 

 

[Dafydd Llewellyn]

ok, Dafydd reminded me of a blindingly obvious point - the pressure field under the wing is caused at the expense of the local dynamic pressure, so the pitot reading IS lowered in that location. Trying again:EITHER a combined pitot/static head should be nearly a full chord ahead of the wing, OR you need to get a pressure map for the airfoil (or grope around in the air until you find a location that works), and put it in the unaffected zone; OR(2) you need to stick it close under the wing - a la Piper - and tweak it - a la Piper - until it works.

 

4412 / Clark Y both have a pressure-passive zone, and about 0.15 chords fwd and 0.15 chords down (if more than ~40% outboard - or lower if inboard of 40%) should work through the positive AoA range; 0.2C fwd and 0.2C down should work anywhere along the wing, out of slipstream...

 

since any mods to the system normally require you to do a calibration with a pivotting pitot / trailing static, most people just do an accurate calibration on their existing system, and call it quits.

Under the wing - anywhere between about 15% chord and 50% chord - 40% if it has flaps - and about one propeller diameter beyond the slipstream (and well clear of the wake of the lift strut) - is an excellent place for the pitot head - but NOT for a static source. The wing acts as a flow-straightener for the pitot, as far as angle of attack is concerned. The Jabiru pitot on the lift strut leading edge is as good as any. A simple thin-wall tube, cut off square and de-burred, will give dead accurate pitot pressure up to about 0.4 Mach, up to about 15 degrees either side of pointing dead into wind.

 

The best place for the static source is on the side of the fuselage, somewhere where it has no curvature (or as little as possible) in the direction of flow, and well clear of the wing pressure field and the wing wake. This requires a pair of static ports, one either side, connected together. Only if you cannot find such a place, should you resort to a tubular static head, and it will generally need an adjustable collar, to minimise its error. The way to measure the static source error, is to compare it with a trailing-cone static in flight. For guidance on making a trailing-cone static source, see CASA AC 21.40.

 

 

[Dafydd Llewellyn]

Dynamic + static?

Yes, a pitot head registers the total pressure, i.e. pitot plus static.

 

 

[djpacro]

Trouble with the side if the fuselage is that it usually is in the prop slipstream so static pressure error increases with high power and low airspeed.

 

 

[Bob Llewellyn]

Trouble with the side if the fuselage is that it usually is in the prop slipstream so static pressure error increases with high power and low airspeed.

boundary layer - the boundary layer is normally a few inches thick by the tail, even with the slipstream buzzing away...

 

 

[Powerin]

However, this behaviour cannot be expected beyond the stall; and the debate about whether one should fool about trying to pick-up a wing at the stall with rudder, is bloody dangerous. The message is, UNSTALL THE DAMN THING - don't sit there trying to show how clever you are with your feet. Teaching people to "pick up a wing with rudder" is bordering on criminal, IMHO. GET THE BLOODY STICK FORWARD! is what should be taught.

OK...would anyone else care to comment on this statement?

I was certainly taught to "pick up" a dropping wing in a stall with the rudder. The rationale being that you are in an asymmetric stall and to allow the wing drop to continue could lead to a spin. You can't use ailerons because that will increase the AoA of the low wing and deepen the stall. So you use rudder to stop the yaw while simultaneously getting the stick forward and power on.

 

I have read the same procedure in books that I consider authoritative...including Noel Kruse's books mentioned above, so I have no reason to doubt what my instructor taught me. If I were to follow Dafydd's advice and not try to arrest an asymmetric stall and wing drop with rudder, but simply recover with stick forward, what would happen? A wing drop is the pre-cursor to an incipient spin is it not?

 

 

[Bob Llewellyn]

OK...would anyone else care to comment on this statement?I was certainly taught to "pick up" a dropping wing in a stall with the rudder. The rationale being that you are in an asymmetric stall and to allow the wing drop to continue could lead to a spin. You can't use ailerons because that will increase the AoA of the low wing and deepen the stall. So you use rudder to stop the yaw while simultaneously getting the stick forward and power on.

 

I have read the same procedure in books that I consider authoritative...including Noel Kruse's books mentioned above, so I had no reason to doubt what my instructor taught me. If I were to follow Dafydd's advice and not try to arrest an assymetric stall and wing drop with rudder, but simply recover with stick forward, what would happen? A wing drop is the pre-cursor to an incipient spin is it not?

I think a semantic difference here... in a sailplane, one can have the inner wing start to drop in a turn (thermalling, generally); and the rudder has the authority to reverse the yaw rate - normally only momentarily, don't want to lose that thermal! - which doesn't just stop the wing going down, it comes back up (all whilst holding back stick for the turn).

I doubt very much Dafydd is advocating ignoring the rudder; but you yourself said, "use the rudder to stop yaw". I think Dafydd is saying, in non-sailplanes, don't fool around with the feet and ignore the stick when a wing tries to drop.

 

 

[motzartmerv]

Pow. The key statement is "pick up the dropped wing" . Preventing yaw with opposite rudder is certainly what we need to do. ( if anything) . But actually bringing the wing up with rudder is only going to induce yaw in the opposite direction which could lead to an auto rotation in the opposite direction. There Is still a large school of thought that say pick it up with rudder, but I think the idea of preventing yaw is deffinately becoming the normal way. All this is of course theoretical and should not replace what your instructor taught you.

 

Cheers