Jump to content

flying faster than M1


dali

Recommended Posts

Nice video thx!!!

 

Hey no one was curious about the Vertical Velocity switching from positive to negative when the Total Velocity Vector stays still in the hud?? Why this happens in real life? It's due the sensors of the aircraft no sensing correctly at that speed?

 

Why this isn't implemented in the f18c DCS module?

Link to comment
Share on other sites

VV shows difference between velocity direction and attitude. That's completely independent of aerodynamics. The concept of a VV would be exactly the same for a space ship in the vacuum of space.

 

VSI shows rate of change of static pressure. It's comparing the pressure at the sensor to the pressure recorded a moment ago. If you were to put your lips over the static port and blow the instrument would show a descent until you couldn't blow any harder then it would show constant altitude.

 

So you figure that transonic some rather extreme pressure wave is moving over the static port causing rather extreme pressure changes.

 

You can even see it in the video at M0.93 and above despite a VV below the horizon the barometric altimeter is showing an increasing altitude. This is a "pressure climb" as in a climb normally associated with a drop in pressure. Presumably there is a Mach shadow behind which the static port location is seeing a reduced pressure normally associated with a climb.

 

By M1.03 the altimeter and VSI have resumed reasonable reading. A minus -2 FPA at ~900 fps should show a nearly 2000 fpm descent. It can also be seen that the effect has the opposite character. The 6000 fpm descent must also be an artifact of some pressure effect, presumably the high pressure shock ridge sitting over the port causing it to think there is a strong descent.

 

Altimeter and VSI readout comes from ADC which should have some programmed in compensation factors for alpha, beta, and perhaps some other parameters to try to be more accurate than raw pressure values would suggest. The standby altimeter on the other hand isn't ADC data and will show a low pressure false descent on side slip as the body of the airplane makes a low pressure shadow over the port.

Link to comment
Share on other sites

VV shows difference between velocity direction and attitude. That's completely independent of aerodynamics. The concept of a VV would be exactly the same for a space ship in the vacuum of space.

 

VSI shows rate of change of static pressure. It's comparing the pressure at the sensor to the pressure recorded a moment ago. If you were to put your lips over the static port and blow the instrument would show a descent until you couldn't blow any harder then it would show constant altitude.

 

So you figure that transonic some rather extreme pressure wave is moving over the static port causing rather extreme pressure changes.

 

You can even see it in the video at M0.93 and above despite a VV below the horizon the barometric altimeter is showing an increasing altitude. This is a "pressure climb" as in a climb normally associated with a drop in pressure. Presumably there is a Mach shadow behind which the static port location is seeing a reduced pressure normally associated with a climb.

 

By M1.03 the altimeter and VSI have resumed reasonable reading. A minus -2 FPA at ~900 fps should show a nearly 2000 fpm descent. It can also be seen that the effect has the opposite character. The 6000 fpm descent must also be an artifact of some pressure effect, presumably the high pressure shock ridge sitting over the port causing it to think there is a strong descent.

 

Altimeter and VSI readout comes from ADC which should have some programmed in compensation factors for alpha, beta, and perhaps some other parameters to try to be more accurate than raw pressure values would suggest. The standby altimeter on the other hand isn't ADC data and will show a low pressure false descent on side slip as the body of the airplane makes a low pressure shadow over the port.

 

 

Thanks for the response, i didn't know that aircrafts used differential pressure sensors.

 

 

But, have you seen this behavior in the dcs module??

 

 

 

Also that means that in high altitude you better trust your TTV for climbing or descending and the Barometric Altitude for a roughly mesurament of your current altitude.

Link to comment
Share on other sites

A Cessna 152 uses differential pressure sensors, although that's a generous term for a sealed pressure vessel (altimeter) and calibrated pinhole (VSI). It's how pressure altimeters and VSIs work. ADC is surely more sophisticated but is working from similar principles and as demonstrated shows apparent barometric altitude and altitude trend which is notably different than the same values in free air.

 

I only flew the Hornet during the free week and I don't remember this behavior although I also wasn't looking for it.

 

Not high altitude but transonic.

Link to comment
Share on other sites

  • Recently Browsing   0 members

    • No registered users viewing this page.
×
×
  • Create New...