Published vs. actual airspeed

Timothy wrote:

I have found that in both the Navajo and the Chieftain I can better book figures, both in speed and fuel flow, though that may be a question of rarely being at MTOM and having better engine instruments.

Same on the Comanche, 5-9 knot consistently quicker, and then there are the days where all bets are off – like this one. No idea why TAS was about 8knots higher than usual for the whole flight.

Wow! That is one fast Twinkie! Is it turboed?

Antonio wrote:

Wow! That is one fast Twinkie! Is it turboed?

It’s a single – non turbo. (and I wish it did that speed consistently but it is usually about 9kts slower…)

IAS is often where people get it wrong. Almost no poh gives IAS as it is not what is relevant. TAS is. And out of my own experience prior getting the Aspen which shows TAS directly, or a 4 way gps test run, people get wrong results almost every time.

Read the poh. The condition and power settings have to fit the poh conditions otherwise the results can not be right. Mp-rpm-ff result in a TAS which greatly varies with altitude. That is how you check if a plane is up to speed or not.

Friend of mine used to fly a Grumman Tiger and complained how slow it was… until he posted a cockpit pic. He was looking at IAS and compared to TAS… that Tiger ran 140 kg true when he saw 120….

Very good discussion.

My personal experience is:

a) it is very difficult to judge this based on one or a few flights. Even without any physical change in a particular airframe, it seems to be a bit faster some times and slower some other times…detailed correlations are difficult to establish since the possible sources are multitude

And then of course you have the discussed variations between different airframes of the same type…

b) On both SEP Cessna aircraft I have flown regularly (177RG & P210), I have found performance to be equal or better than book, but then:

• In both cases the engine was modified (high-compression pistons on 177RG, intercooler on P210, neither accounted for in POH data, difficult to single-out the effect)
• On the P210 we specifically looked for the cleanest airframe we could find. On this type owners tend to load the aircraft up with antennas, air conditioning, de-ice boots, radomes, non-existant or removed gear doors…you add a few of these compounded with empty weight and my experience is a difference of over 15KTAS at altitude for similar load (admittedly vs aircraft I have only sampled on one flight, but also owner reports).

You can easily check the ASI by using the three groundspeeds method. Fly three headings 120 degrees apart and take the average of the three GS values. Best to do this in a light wind, although the wind is not a first order error source anyway. That gives you the TAS, which you then correct to IAS using the usual method (altitude, temperature). The full formula is more complicated but for light winds it is just the arithmetic average of the three.

Then you are left with stuff like prop rpm which for a fixed pitch prop has a huge effect, and fuel flow which you will never know accurately until you install a fuel totaliser. And you need an engine monitor so you can see you are at peak EGT (best economy) or whatever the POH assumes.

Also see here for ideas on how to fix discrepancies.

I have merged this thread with another one on the same topic.

LFHNflightstudent wrote:

It’s a single – non turbo

I see…a 260, right?

Antonio wrote:

I see…a 260, right?

260c indeed