Speed question

[rich12545]

I've noticed this for a long time but never paid any attention. It's the same through several FS9 reinstalls. Seems my planes always fly somewhat slower than specs say. For example, the RealAir (American Champion) Scout flies at about 78 knots. Look at wiki and the American Champion Scout flies at about 98 knots. Does anyone have the same experience or know what's going on?

[Tom Clayton]

You could be seeing the difference between true airspeed (TAS) and indicated airspeed (IAS). The higher you climb, the thinner the air gets, and since your airspeed indicator depends on ram air pressure, it gets less effective as you go higher. If you pull up your GPS, you'll see your speed over the ground, but bear in mind that head and tailwinds will affect that reading. I believe that using the Clear Weather theme cuts out all winds, so using that should mean that ground speed = TAS.

If you have a faster plane with a Mach indicator, it will read your correct Mach # relative to the speed of sound for the altitude your flying. Since sound slows slightly as you go higher, the same TAS will show a higher Mach # at higher altitudes. Fortunately, that variation is less pronounced that IAS.

[Sunny9850]

Quite often, especially with small, older, GA types such as the basic scout there is also conflicting information out there. And the old MPH vs KTS thing. Since airplanes are not always at home on the water many airspeed references are given in MPH but that gets mixed up at some point without calculating the difference.

Here is a handy quick calculation for IAS to TAS
http://www.flightsimaviation.com/rul...speed_TAS.html


Cheers
Stefan

[rich12545]

Thanks for your replies.

I fly only small GA aircraft about 3-4000 feet AGL. Usually it's not a lot higher ASL so I don't think that's the problem. The scout, I don't believe, is all that old but in any case the gauge was in knots. The figure in wiki was mph so I converted to compare.

[Sunny9850]

Don't forget the influence of temperature on the calculation. The final density altitude us what matters.
for example in SoCal where I fly DA is always something to consider.

Pressure altitude of 4500' at 15°C = 5537'
Pressure altitude of 4500' at 25°C = 6654'
Pressure altitude of 4500' at 35°C = 7726'

Just running a few of those numbers I can get to a TAS of 89 KTS, the rest is possibly down to technique, rigging and quite possibly a bit of salesmanship.

For example I happen to know that Piper and Cessna will move the CG to the optimum location for the part of the flight profile they are getting numbers for.
For the speed question that usually means you want a aft CG, within the allowed limits of course, and you can expect a few extra knots easily.

For other parts of the profile the CG close to the center of the envelope may be more beneficial. And nothing in the rules states that the CG has to be in the same spot all the time.

Cheers
Stefan

[srgalahad]

As you check through things...

Mixture - are you using automixture, or manual? Is the engine ( and of course this means each diff. aircraft) producing full power? If you have it set to manual mixture and linked to a throttle controller, check the calibration of the controller.

Realism: make sure Indicated Airspeed (not True) is selected ( I know, but fingers slip occasionally) Menu: Aircraft/Realism Settings - bottom left of selection screen

If you want something more complete than a regular gauge, get AFSD by Herve Sors http://www.aero.sors.fr/afsd.html
It will tell you far more than you'll ever need, but it will accurately display needed parameters. Handy utility with no system resource implications

[rich12545]

Now flying in north America in the spring so temp shouldn't make a difference. Using auto-mixture and it says indicated. This happens on different aircraft sometimes but I'm now flying the Scout on bush trips so I'm looking at that one specifically. I think I'll write to RealAir and see what they have to say. It's not a big deal and I don't mind the slower speed (I like flying low and slow) but am curious about a 20 knot difference.