True Airspeed Differences

[rdockery_3]

I've had a 2011 RV12 for a couple months and the plane has reliably showed 100 kts TAS at 5050 RPM since I've had it. Today I did a 1.5 hour cross country and on the way back it was only doing 92-95 it's at 5050 RPM. Is it normal for it to fluctuate that much?

 

[Jetguy]

True airspeed (TAS) is the actual speed of an aircraft relative to the undisturbed air mass it is flying through. It is calculated by taking the indicated airspeed (what the pilot reads on the instrument) and correcting it for non-standard pressure and temperature conditions, which become more significant at higher altitudes. So if the Tempature was different or you were at a different altitude then it would change!

 

[Mich48041]

Check the pitot static system for leaks.

 

[RVbySDI]

Altitude will alter the barometric pressure and temperature, and those will alter true airspeed. So, what was the altitude you were flying on your cross country trip? How does that differ from your other flights?

 

[lr172]

Have flown a lot of hours on my 6a with a fp prop. I get very similar cruise speeds (TAS) at my typical 2740 rpm. These consistent airspeeds are at a variety of altitudes (8k - 12k), temps (-10 to 70*F), and baro settings. These changes do affect the map and fuel flow necessary to maintain that rpm though. Imo, no, you should not lose 8 kts because it is warm or a high pressure system rolled through.

An 8 kt drop in cruise at a given rpm, with the benchmark being well established, points to something amiss. Many different possibilities.

 

[rdockery_3]

Have flown a lot of hours on my 6a with a fp prop. I get very similar cruise speeds at my typical 2740 rpm. These consistent airspeeds are at a variety of altitudes (8k - 12k), temps (-10 to 70*F), and baro settings. These changes do affect the map and fuel flow necessary to maintain that rpm though. Imo, no, you should not lose 8 kts because it is warm or a high pressure system rolled through.

An 8 kt drop in cruise at a given rpm, with the benchmark being well established, points to something amiss. Many different possibilities.

That's what I was thinking. It was bumpy yesterday though and I had the autopilot on. I'm hoping it was just the autopilot pitching up and down because of the turbulent air that was causing the changes in airspeed. I'm going to watch the speed the next time I fly and go from there.

 

[lr172]

That's what I was thinking. It was bumpy yesterday though and I had the autopilot on. I'm hoping it was just the autopilot pitching up and down because of the turbulent air that was causing the changes in airspeed. I'm going to watch the speed the next time I fly and go from there.

When on AP, I do see speed changes related to up drafts and down drafts. Around here it is usually a few knots, but when I go to Colorado, it can be 10-20.

 

[Desert Rat]

Guys- I'm a little confused here. Are we talking true airspeed or indicated airspeed? The original post says TAS (true airspeed) which will absolutely change with air density/temperature. Some of the replies seem to be referencing Indicated airspeed, which doesn't behave the same way...

 

[RVDan]

Factors affecting true airspeed: airspeed indicator error, calibrated airspeed ( indicated airspeed less airspeed indicator error corrected for position error), pressure altitude, OAT. A glass display system calculating and displaying TAS that has errors in any of these inputs will contain errors.
If the static system wasn’t calibrated, then that is a likely source of larger errors.

 

[lr172]

The original post says TAS (true airspeed) which will absolutely change with air density/temperature. Some of the replies seem to be referencing Indicated airspeed, which doesn't behave the same way...

True, but both engine performance and wing lift also change with air density / temperature. My comments were based upon TAS and editted to reflect that.