Units of Measure

Knots%20Speed[1]

Aviation is a hodge-podge of English, metric, dimensionless, and nautical units of measurement.

  • Distance (nautical miles, nautical)
  • Altitude (feet, English, and FL which is in 100’s of feet instead of thousands, very English)
  • Airspeed (knots, nautical, and Mach, dimensionless related to the speed of sound)
  • Temperature (centigrade, metric)
  • Pressure (inches of mercury, English, and millibars, metric)
  • Visibility (statute miles, English, and RVR in feet, English)
  • Fuel quantity (pounds, English)

Our operational regime is initially 35 km above Venus to 60 km above Venus. Each kilometer would make for a good “flight level” and Aliens (the movie) made reference to “clicks” which is slang for kilometers.

However, as a former airline pilot, I like flying with knots indicated speed… and I am used to nautical miles for distance. So when I sat down to port over the altimeter, I took a pause… what units should it be in?  Feet is unwieldy at 35 kilometers (114,829 feet aka FL 1148, whatever).  Why not measure distance vertically the same that we measure it horizontally?  35 km is 18.8985 NM (60 km is 32.3974 NM) … and it gives us better resolution (the separation at tenths of NM is less than 1000 feet) so we could do a FL-type system (FL 188 to FL 324).

Doing a little research, I found that the retired space shuttle could display altitude in feet up to 400,000 depending on the phase and was also capable of showing altitude in miles from 40 to 165 (always based on static pressure).

Then again, our universe framework is metric based – it allows for faster and more accurate conversions between scales (i.e. meters to kilometers).  Similarly, Rise was metric: Rise used meters and kilometers for everything – indicated speed, vertical speed, altitude, and distance.

Primary Flight Display

Here is a little blast from the past for you Rise: TVP players…

tbt

I started porting over the code for the speed and altitude tapes as well as the attitude indicator and I just had to laugh. The resolution was so low and the display area so small (137×137 pixels) it is amazing we had as much information as we did.

Below is the overlay I am working with for the avionics suite. It is inspired by the EMB-195 and I am sure that the colors and spacing will change but the amount of information being added is going to blow you away!

PFD-background1

Flight Tests

So in addition to calculating the IAS incorrectly in Rise, the V-speeds that we were using for stall warnings, etc. were based on TAS instead of IAS!  Now that I got the spreadsheet straightened out, I was able to flight prove the Vs1 and Vc speeds at both 35km and 60km.

We don’t have flaps, so there is no published Vs0 speed. We also don’t have to worry about Vr since we have no wheels, but it is interesting to note that the Vmc is so high it will likely still come into play when transitioning from VTOL.

We also have a Va speed, which I always knew as a pilot to reduce to when turbulent air penetration was anticipated – but did you know that means the wing will stall before we can exceed our positive G load limit? Genius!

So I wanted to see what would happen if I failed a flight control (neutral position). If it is either of the upper tail controls, there is some adverse yaw apparent but otherwise flies quite nice. Losing one of the lower tail controls, however, is a lot of work! You basically have to reduce the opposite engine thrust and you can then steer with pitch. Not a great situation but since we are VTOL we could reduce the airspeed quite a bit and rely on vectored thrust to steer.

However, lose an engine and we can no longer VTOL. Contrary to the Dropship from Aliens, we won’t be able to continue VTOL operations on a single engine.

 

Cruise (Vc) Speed

Vc speed is maximum continuous thrust (MCT) in level flight (determined by the balance between total drag and total thrust). Our predicted Vc speed is 189 m/s at 35km. However, I am only getting about 146 m/s.

2015-02-07 08:59:57.352 thrust fcdragx fcdragy fcdragz afdragx afdragy afdragz sumforcex sumforcey sumforcez
2015-02-07 09:00:27.350 +69690.766 +287.479 -73.160 +24512.580 +5070.491 -1018.746 +40626.730 +1838.641 -99169.852 +949.478

Looking along the longitudinal axis (Z) it turns out the flight controls (FCDRAG) are creating 24,500 pounds of drag in level flight. The rest of the airframe (AFDRAG) is only 40,500 pounds!

Equivalent Airspeed (EAS)

Funny story. For years on Rise: TVP we had operated with a constant density value for the surface and then modulated it by a density factor which used the scale height formula.

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The resultant density was then used for the lift and drag equations but also for the Indicated Airspeed (IAS) readout.  The formula is actually for Equivalent Airspeed (EAS) but they are very closely related at sub-sonic velocities.  EAS and Calibrated Airspeed (CAS) are also very close.

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At the surface the density factor is 1.0 which means the modulation from the scale height formula had no effect on the IAS versus the True Airspeed (TAS) value.  As you climbed out of Vieneo would would see an expected decrease in IAS as your TAS was allowed to increase.

IAS-TAS_1_[1]

However, if we happened to flight test at 50,000 feet (the basis of the old cruise calculations) we would have noticed that we could never attain our Vc speed.  And since we are starting at 35 km above the surface of Venus as a “soft deck” it was immediately apparent that something was wrong with the IAS. Turns out we weren’t taking the square-root of the density factor! Now all the cruise numbers in the Dropship spreadsheet are making sense.

Flight Dynamics

The flight dynamics results are in – and it takes a lot of work to fly the Dropship!  Unfortunately we have not been able to demonstrate controlled flight maneuvers yet.  If I can’t understand how to fly it, it will be exceedingly difficult to explain to the AFCS.

Initially I thought this may be Dutch Roll. However, we are a lifting body and my experience with Dutch Roll has been limited to swept-back wings. But it does affect the vertical and longitudinal axes.

DuchRol4[1]

After searching online, I ran across “coupled roll-spiral mode” which is a condition I was not previously aware of. Anyone that can build a dampening routine for this mode is welcome to contribute! In fact, based on the article below, it is likely a combination of the two effects.

Time keeps marching on and I have gotten in a little over my head with this flight model and all the necessary changes that I hadn’t accounted for.  As a result I removed all the availability on the booking calendar through April 10.