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.

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.

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.

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.

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.

Lunch Update

Made a lot of progress over the weekend determining the surface area, surface normal, hinge axis, etc. for the flight control surfaces. This robust system will allow us to make any number of discreet channel controls in any configuration!

However, we are running into problems with the movie model again… we already identified that the engines were above the CG (which won’t work once we get to space) but now apparently the lower portion of the X-tail is as well!