I came across a discussion recently regarding the TB20.
Some people posted that it flies ~10kt slower than is claimed. Or does 140kt at 75% which is a way higher fuel flow. Or same speed as a Robin with 70HP less. Or same speed as a a PA28R. Etc. And apparently there is a big flying school operating these which confirms this slow performance.
I posted, hopefully as a useful data point, that mine does 138kt IAS (at say 2000ft) at 11.7 USG/hr, and nobody believed it. Every other TB20 I have been in, including some 1984 ones with a 2 blade prop, also did exactly the same. There are varying reports of the full-TKS ones losing 5-7kt; again mine doesn’t, but I can see potential reasons for that.
How could you achieve a 10kt loss? Even a 5kt loss is a lot of HP.
The airframe would have to be pretty well “bent” and with trim tabs bent up/down to make it fly straight again. Or maybe a flap hanging down a few mm; that will really screw things up.
Or instruments way off calibration.
Lots of reasons:
and, at the very last, incorrect airspeed indicators
A plane I know of just got a new prop, not different just new, and it gained 10 kt.
I’ve been flying in an ATO that has many DA40 135hp, and there were variation of cruise speed at 70%. IAS was variying from 109 to more than 115 for one particular plane, and maintenance never really understood why; they thought about the prop but never really got the answer.
I am also used to fly 2 TB10, roughtly 5 to 7k hours each. One has 2 blade prop and can sustain 110 KIAS at 24/24 around 2 to 5kft. The other one as a 3 blade prop and much better outside state, but I have never seen it reach 110KIAS unless 25/25, and not always. I am taking load into account, mid fuel, 3 pob.
Peter wrote:
Some people posted that it flies ~10kt slower than is claimed
Just open the alternate static that takes air from cockpit and you get another 5kts-10kts on your ASI reading
Comes handy when you want to impress someone 
Sometimes there can be a bit of hysterisis with the wing design. In mine if the air is smooth I can see a bit of improvement (I’ve never really accurately measured but I would estimate 5mph or so) by going past my cruise altitude target and decending with a bit more speed before levelling off. I feel that it’s only really worthwhile to do in smooth air.
Instrument calibration would be the easiest to check, so I’d do it first. If found OK, I would then check airframe alignment. With so many degrees of freedom in the system, it is quite possible to rig the plane so that it does not roll spontaneously yet flies slightly sideways. One can certainly lose 10 kt in this situation. For example, in Cessna 1xx/2xx, wing incidence is adjustable with an eccentric bushing at the root, and doing it wrong easily produces a speed loss of that order of magnitude. One more hidden cause of speed loss could simply be a difference between calculated and actual weight if the aircraft hasn’t been weighed in a while.
Does it have any turn tendencies? they either come from dropping flaps or misaligned rudder? about 5kts could be explained by aircraft surfaces drag mostly rudder & flaps: Step1, fly the airplane with your feet flat on the floor. Hold the wings level and see if the airplane wants to flat turn. if it does, more likely a rudder rigging problem, Step 2, hold the aileron flush with the wing tip and see if it matches with the flap. If it doesn’t, the flap are drooping
Otherwise most likely the prop, the M20J used to do 145IAS at 25/25 on 2 blades prop, now does 140IAS at 25/25 on 3 blades prop
Off_Field wrote:
Sometimes there can be a bit of hysterisis with the wing design. In mine if the air is smooth I can see a bit of improvement (I’ve never really accurately measured but I would estimate 5mph or so) by going past my cruise altitude target and decending with a bit more speed before levelling off. I feel that it’s only really worthwhile to do in smooth air.
We’ve had a long discussion about that supposed effect.
Airborne_Again wrote:
We’ve had a long discussion about that supposed effect.
Not sure what was the conclusion on that discussion? but it is mathematically correct
You will average lower speed when X fluctuates vs speed at average value of X0
X can be power, altitude, heading, pitch….
This depends on “concavity” of the Speed-vs-X curve and it’s symmetry around X0, say one will find that climbing at (VY+5kts)+/-5kts achieves better average climb rate than (VY+/-5kts), as the drag curve is rather flat on the VY+5kts side but the back of the drag curve is steep on the VY-5kts, the same principal would applies to altitude & gravity
Obviously when things settle down it does not matter how you get there…but things never settle down and the maths bellow applies 
https://en.wikipedia.org/wiki/Jensen%27s_inequality
If one like graphs
