ASI displaying KCAS insted KIAS?

[vskivanov]

Hello everyone!
I apologize if this topic has been discussed before, but couldn't find any clarifying info about it. Did couple of standard ISA tests at 4000 and 6000 feet and definitely the ASI displays the CAS instead of IAS.
I can upload a screenshot when I find the attach button
If you could point to any thread or an option to correct this will be great.
Thank you.

[Oracle427]

Check your simulator settings. You may have it set to display TAS instead of IAS.

[vskivanov]

Already did that. It's not displaying TAS but CAS exactly by the POH airspeed calibration table. At 4000ft with 117 TAS, the ASI should indicate 113kts (IAS), but it indicates 110kts (CAS), at 6000ft when it should be 110kts IAS, it indicates 107. And that's for standard pressure and temp set.

[bobsk8]

Already did that. It's not displaying TAS but CAS exactly by the POH airspeed calibration table. At 4000ft with 117 TAS, the ASI should indicate 113kts (IAS), but it indicates 110kts (CAS), at 6000ft when it should be 110kts IAS, it indicates 107. And that's for standard pressure and temp set.

If that ever happened to me, I would aim my aircraft straight down and crash.

[Oracle427]

According to my TAS computer in STP conditions at 4000 MSL a TAS of 117 is equivalent to a KCAS of 110 and a KIAS of 113.

Edit: also checked on my E6B for fun. Also getting 110KCAS for 117KTAS.

Edit 2: Forgot to convert KCAS to KIAS, fixed.

[vskivanov]

Bobsk8, I'll consider it as an option. But the repairs are expensive, you know...
Oracle427 the E6B could give you a CAS, I'm not quite sure it can give you an IAS, because the IAS has an instrumentation error different for each a/c. That's why we use the calibration tables in the POH. Although one won't fall from the sky because of these few kts (but even if do so, the hangar is just a shortctut away), it's an important step when doing a full profile preflight planning. That's why my original question was is there a way for the ASI to display the IAS instead of CAS?

[vskivanov]

Oracle427, exactly. If you have some time, could you make a test flight to see if this will happen to you too - at 4000 ft, 117 KTAS the IAS is 110 instead of 113?

[Oracle427]

Set uo
Loaded up to max gross of 2450
R type prop
Climbed to 4000 MSL
Leaned to 50ROP @2300 RPM.
Cruising steady at 113KIAS.

Are you considering parallax when reading the RPM? It looks like almost 2350 from the standard eyepoint, but if you lean over it is reading 2300.

The difference of 50 RPM would account for the difference in airspeed.

[vskivanov]

Here's a screenshot.
RPM is fluctuating between 2990 and 3010, leaned at 50 ROP and KIAS is 110-111. TAS is steady 117, monitored by external app.

[Oracle427]

Not sure why you are 3 knots slower. Do you have the wheel fairings on? I removed the fairings and slowed down a little. Had to add power back in to get back to 2300 RPM and speed climbed back up pretty much to where it was before though at a slightly higher fuel flow.


It's very touchy right around this spot with a very fine line in touching the throttle resulting in 110KIAS or 113KIAS.

https://drive.google.com/open?id=1qSBoZ ... CeB0c6awpl

https://drive.google.com/open?id=1w61YB ... I7MVGz52AG

[vskivanov]

Yep, I have the fairings. At 50 ROP i have a higher burn rate than expected - 9.8. At about 25 ROP it is 9.1 gph. Noticed that your peak temp is about 25 degrees lower than mine, but I guess that's normal.
However, the strange thing is that the TAS is exactly as it should be - 117. On your screenshot it looks like at 113 IAS the TAS is about 120? Which is exact TAS/CAS matching
The 172 was probably modeled without the ASI error. Maybe it was difficult to implement it, I don't know.

[MarcE]

you probably forgot to wash the bugs off the wings... I would consider this to be accusim.. the world is not perfectly readable from a table.

[vskivanov]

Nope, I keep the bugs as vortex generators and will mount the bull horns on the nose wheel pretty soon It will be the most evil looking 172 in the sky.
However after a full reinstall and update the problem persist. Now both the IAS and TAS are higher, exactly as in Oracle427's post - KIAS as expected and KTAS few kts higher than expected performance.
Yes the world does not comply with tables, but the sim is almost as good as a laboratory environment and you can set up specific conditions. Which should match the performance charts. For the moment I don't know what the expected TAS will be @2300 rpm for example. Then the WCA, estimated times, etc. And the domino goes on...