True Airspeed indicator bug

[Waffle]

The TAI (red) will register wind speed on the ground, no matter what direction you're facing on the runway. if you have a 30 mph wind from North, if you are facing south on the runway, it still shows on the TAI as having a 30mph headwind. Don't think they had rear facing pitots

Anyway, it does this in all directions. I would imaging if a 30mph wind was from north, and you were taking off either NE, NNE ,ENE, NW, NNW or WNW, you would see some movement in the TAI, but not 30 mph.  If there was a 30mph wind from north, and you were taking off E or W - you might see a tick in the TAI, but facing away from it, you shouldn't have any at all...well, alittle bit, due to the vacuum effect of wind blowing across a tube...then it would be a negative number. and so minimal - not bothered with. But by physics, should be there bernoulis principle.....you remember the one with the perfume bottle..air across the tube draws the liquid up into the airflow....

[hitech]

All depends how you define as true air speed. Notice the indicated does not function that way,it works off the pito tube.



HiTech

[Waffle]

oops...

Good morning to you to Waffle:) Where was I...lol

"Indicated" is the one linked to pitot, correct...I see my error,
but why would TAS register wind change from all directions?

[Orig]

It sounds like instead of the true airspeed needle being computed as indicated or calibrated airspeed measured by the pitot-static system, then corrected for altitude (air pressure) and temperature, the true airspeed needle is being modelled as an absolute measure of how much air is flowing past the airframe regardless of the direction of the flow.  Since this can't be easily measured without a precision inertial navigation system or some other extremely precise positional information, and even then it doesn't usually work at zero groundspeed, I can't tell how it's being modelled.

Hopefully aircraft performance in AH is being computed by reference to IAS/CAS rather than TAS, since that would mean a plane could fly around with a 150 knot tailwind

I don't think WWII aircraft had the precision navigation equipment required to compute an absolute true airspeed indication because that would also imply a way to automatically compute head/crosswind components, wind drift, groundspeed, etc. without any effort.   That sort of goes against how the bombing sight calibration system works in the game.  The fighters are using ring laser gyro inertial systems to calculate and display TAS, while the bomber guys are using ground reference points to enter drift rates into the computer.  A norden would be an awful nuisance in a fighter cockpit, but in reality what a norden did was automatically take line of sight drift rates and from that plus a manually entered AGL altitude and airspeed, compute ground speed and lateral drift for a bombing solution.

Then again, if the fighters have the inertial navigation system or omnidirectional pitot-static systems and density altitude computers required to display absolute TAS, then maybe those non-historic game squadron skins will make it in too

[hitech]

Origen, Do you know the difference between speed and velocity?


HiTech

[Waffle]

speed is amount of distance covered over time, velocity is the direction and distance over time...


so whats up with the TAI ??

[Orig]

HiTech,

I apologize if I offended you with my little joke about the pink airplanes.  I meant it to be funny, not hit on a touchy subject.  Sorry.

As for the difference between speed and velocity, if you're talking about the difference between absolute velocity and a vector, then yes.  Vector math and it's application in aerodynamics was a a required course for me in school.

If the TAS indicator in AH is displaying only the velocity and not taking into account the direction, because the pitot static system is typically accurate only for an airstream coming from directly in front of the aircraft, then it is displaying a value that generally requires reasonably sophisticated equipment or manual computation to figure out.

This is what I understand the original post to mean - that the TAS appears to be displaying the absolute velocity of the airstream, including corrections for temperature and pressure, regardless of the direction of that airstream.  If I were to draw it on paper, it would be the length of the vector stick without regard to the angle of the vector respective to the reference plane or coordinate system.

I am a bit interested in how this would be measured without either a pitot static system that can accurately measure an airstream coming from any direction, or an inertial reference system that compares the actual aircraft relative 3D vector against the pitot static system to come up with a 3D solution for wind speed, drift angle, calibrated airspeed, and true airspeed.  This solution is going to be a basic necessary component of a proper computer simulation such as AH, but actually measuring and displaying the absolute TAS (velocity component only) in real life is not trivial.

It is relevant that regardless of the technology used, TAS is computed using air density (a combination of air pressure and temperature) and a difference between the pressure measured in a forward facing pitot tube and a static pressure port.  The implementation of this system will vary, but the basic computation remains the same.

I'm not trying to start a pissing contest because it's a game and it will be made however you think is best, but without defining what we're talking about it's hard to figure out whether we're even talking about the same thing

I would like you to understand that I do not intend to be insulting.  Aeronautical science is pretty complex and a detailed explanation answering the original post in a complete fashion could take up the better part of a chapter in a textbook, and I tried to describe what goes into computing actual TAS in real aircraft.  If that is not appropriate, I will of course drop the subject.

[hitech]

Basicly simulations work the other way around. Start with the no wind case.

You always know the TAS, the IAS is the one that has to be calculate.

To be more precises the simulation always works with a true Vel vector. IAS is releay meenless in the simulation because fluid force equations are of a form RelativeVel^2 * Denisity. To compute the RelativeVel is quite trivial, you just need to subtract the WindVel from the Vel vec.

AH's True air speed is just the length of the realtive vector.

From there you into the more complex stuff of computing airflow force AOA's accross different surfaces.


HiTech

[Orig]

Cool, thanks for the explanation of how that works in the sim.  I have a good education on real aeronautical science but very little experience building a sim beyond a couple of really simple school exercises.

Regarding the original question and my overly lengthy reply, it sounds to me like waffle is asking why the relative wind velocity TAS is being displayed instead of the TAS recomputed in a manner that would be available to a WWII era pilot.  The solution available in a fighter cockpit would probably be figured by using a flight computer (like the circular portion of the E6B), lining up the temperature and air pressure (altitude shown when the altimeter is set to 29.92) in the inner window, then finding the TAS number opposite the IAS number (read from the airspeed indicator) on the outer wheel.  Of course, you had to do all that for your pilots license, so I'm explaining it for others who are reading the thread and wondering what I'm blabbering about

This can be done in seconds, and it might be a bit more realistic to use this computation direction for the displayed TAS in the game, even though it is reverse-computing calculations that you have already made as part of the flight model.  It would result in a displayed TAS indication based only on the portion of the wind vector that would be read by the pitot static system, the relative headwind component that you display as indicated airspeed.

I have no idea what kind of priority you would assign to this level of realism in the cockpit displays.  You've already set up what kind of information will be available in all aircraft, so this is just a question of if you intend to specifically display information that would be fairly difficult to obtain in the 1940s, namely TAS based on the total wind velocity instead of just the headwind component of the relative wind velocity.  It's not a big deal, but now that waffle pointed it out, it doesn't seem to quite match the highly detailed level of realism in other areas of the game.

[Waffle]

only thing I was wondering was why the TAS (red) would show a speed revelant to the wind speed. Went offline - set wind at 50...TAS registers 50 no matter what direction you're lined up OTR.

So, in theory, what happens when I line up North with a 30mph tail wind.... would I actually be at 0mph TAS when I hit 30 IAS?


Doing brain stretches....lol

[Orig]

Here's a little example of what doesn't seem to make sense, and maybe why I might not understand how it works in the game.

If there is a 50 knot wind in AH, and my plane is facing into the wind, I should see 50 knots in the IAS and around 50 knots in TAS, right?

So if I face away from the wind and increase my ground speed to 50 knots which sends relative wind to zero, I should see zero on both the IAS and TAS needles, right?

Then if I take off and head into the wind at 150 knots IAS, what will I see on the TAS?  150 plus temp/alt corrections, or 200?  And if I turn away from the wind and maintain 150 IAS, what will I see on the TAS?  150 plus temp/alt corrections, or 100?  Or still 200?  Or what?

I don't understand how an indicator that would read wind velocity when the plane is motionless on the ground no matter what direction the plane was pointing in, wouldn't give really weird readings while flying.

[Waffle]

I'm gonna crack...lol


I've got a week off....gonna enjoy it

[Orig]

I tested offline and here's what I found.

I did all tests at very near sea level so any computed TAS *should* be essentially equal to IAS on a standard day.

I set up a 50 knot wind from the east, and used an F6F because it has good low speed handling.  GS = groundspeed, IAS = indicated, TAS = true.

Heading into the wind looks ok:
GS:    0  IAS:   50  TAS:   50
GS:  20  IAS:   70  TAS:   70
GS:  50  IAS: 100  TAS: 100
GS:100  IAS: 150  TAS: 150

Heading away looks weird:
GS:    0  IAS:     0  TAS:   50   (What HT and waffle described)
GS:  20  IAS:     0  TAS:   30   (?)
GS:  50  IAS:     0  TAS:     0   (what I'd expect in RL from here on)
GS:  70  IAS:   20  TAS:   20
GS:100  IAS:   50  TAS:   50
GS:150  IAS: 100  TAS: 100

Heading 90 deg out looks weird too, and highlighted a ground handling issue.
GS:    0  IAS:     0  TAS:   50
GS:  20  IAS:   20  TAS:   53
GS:  30  IAS:   30  TAS:   59  - extremely difficult to measure
GS:  68  IAS:   70  TAS:   70  - approximate speed where they all matched up

Above 30 knots, I could not find a plane that would continue to point straight down the runway during a gradual acceleration, even the P-38.  The planes would point uncontrollably about 30ish degrees into the wind even with full opposite rudder and aileron into the wind above about 30 knots if I accelerated slowly enough to make measurements.  Directional control seemed to be regained above 70-80 knots in the P-38.

I did one quick acceleration in the P-38 through this uncontrollable region and found that at around 70-80 knots, not only did I regain control of the plane but the IAS and TAS matched up again at around 70 knots, hence that chart entry.

I'm stumped because it has to be related to how wind and TAS is modelled, and it looks like there are at least 3 measurable regions - zero ground speed where IAS is also zero (only when pointed away from wind), speeds where IAS is less than TAS, and some speed where IAS catches up with TAS during takeoff.  The 3 regions also match up with what I perceived as quite different ground handling characteristics, where in that region where the IAS is above around 20 but less than where it catches up with TAS, control of the plane on the ground seems limited somehow.  Below 30 knots even in the P-38, the plane was controllable but between 30 and about 70-80, the nose would swing uncontrollably about 30-45 deg into the wind, at which point it would stabilize, but above that speed the rudder became effective again.

Some of this behavior might be related to why people sometimes gripe about weird ground handling in some situations...

There is something in the modelling that as shown in the relationships between GS, IAS, and TAS, doesn't seem quite normal.  I did not accomplish tailslides to see what would happen with GS, IAS, and TAS during a vertical tailslide, and I did not set the wind high enough to give me a negative groundspeed when flying slow (wind above 150ish would do this easily) but I'll get to that next.  Stay tuned

[Orig]

Additional notes:

With 127 wind speed, the IAS and TAS in the zeek was the same in every condition I could create because I was unable to get the zeek to hold still on the ground with that much of a tailwind  Even being pushed on the ground backwards at 100 GS, the IAS and TAS were the same, down under 30.  The table entry would look like this:

Into 127 headwind
GS:  -100  IAS:  30   TAS:  30

A possible bug:  Wind noise is somehow relative to ground speed, not airspeed.  When I took the zeek into a 127 knot wind at 200 knots (63 knots ground speed), there was almost no wind noise but if I turned away from the wind (327 knots GS), there was a lot of wind noise.

When I helicoptered in for a landing, it took a normal amount of power to fly the 127 knots and entering what felt like a ground effect region was pretty neat.  After touching down though, the plane was held very firmly in place when the throttles were in idle, as if idle triggered a ground stability enhancement.  As soon as I increased throttle, the plane began to roll backwards.

Advancing the throttle would sometimes cause the plane to leap off the ground as if releasing from a sticky surface.   Pulling the stick aft to lock the tailwheel seemed to result in less directional control, but I haven't tested the tailwheel lock in zero wind conditions so I'm not sure what that's supposed to feel like normally.

Pushing up the throttle enough to get out of ground "sticky mode" then leaving it just barely above idle resulted in the plane being very rapidly accelerated backwards along the ground up to a stable 100ish GS, until I hit enough trees to kill the plane.

So I'll quit testing weird conditions and get back to enjoying the game, since I've merely stumbled across a region of the flight model that is extremely rare in normal situations.  The wind noise thing might be a bug though.

[Waffle]

Yeah, but it's fun taking a b17 off like a harrier