Peter, to add another "aspect" to this: your aircraft's wing has a relatively high aspect ratio of 8,0. Higher aspect ratio reduces induced drag. Low induced drag means little influence on drag from weight. So, an old rule-of-thumb origining from the days of classic american aircraft with aspect ratios of around 5 would not be accurate for a TB-xx.
Besides, as pointed out before, induced drag lessens as speed increases. To test this, you could test cruise performance differences at a cruise speed only slightly above best glide speed (i.e. speed for best L/D). You might try 45% power, if you can set that accurately, and compare. (At 2000' density altitude 45% should be very close to 18" - 2200 rpm - best power mixture.) At this lower speed - around 120 KIAS? - the airspeed difference ought to be detectable with large variations in weight.
As a side note, a Bonanza pilot once told me that he moved his seat all the way back in cruise in order to move the CoG aft, thus reducing trim drag and increasing speed. I tried it myself and found no difference. Then I did the math (simplified, only taking change in induced drag in account). I found that on a Bonanza, moving the CoG all the way from forward to aft limit would give an additional 1/3 or 1/2 knots. Moving the seats back would probably contribute less than 1/20 knot!
By the way, Peter, thanks for inviting me to this forum. I enjoy it. I know the "names" of many of you from pprune, and it is great to know you are out there willing to share your knowledge and experience.
This thread has prompted me to ask for the combined wisdom here about a speed issue. I experience IAS variations in straight and level flight of anything up to 10 knots reduction for no apparent reason. This is backed up by GPS ground speed. Any ideas anyone? No changes to engine or prop settings. I am at a loss to understand. Thanks
Is that with an autopilot in altitude hold mode, or flying by hand and holding altitude?
If so, that happens all the time, due to updraughts and downdraughts forcing a pitch change to maintain altitude. I once saw the IAS rise from 90kt to 150kt, over Albania... and also the opposite.
Otherwise, I would be extremely concerned about it, and would check out the fuel system and the engine controls before any further flight. It isn't supposed to do that. In steady state conditions, a plane is supposed to fly at a speed constant to better than 1kt. Do you have EGT or fuel flow indications and, if so, do they vary?
What is the timescale? Does the variation happen over seconds, or minutes?
I experience IAS variations in straight and level flight of anything up to 10 knots reduction for no apparent reason.
Mountain waves can cause this - I experienced the same as Peter described - 90 KIAS full power and -100 ft/min and 160 KIAS and +300 ft/min quite reduced power on two extreems of the same mountain wave on timescale of minutes.
I can't see these numbers with my TB20. Can you (maybe in separate thread) list your performance, settings and consumption data for range of altitudes (e.g. from 2000ft to FL140)? Thanks in advance.
What perf do you get?
I have flown in a 1980s TB20 which has a fresh motor and does the same as mine - 138kt IAS at the same settings.
If you want more data, you can pay for the fuel 
But looking at recent long flights, I see stuff like
143TAS 2300 9.1GPH FL130 -1C 140TAS 2200 8.6GPH FL130 -1C 145TAS 2400 9.5GPH FL120 +1C 146TAS 2400 9.5GPH FL120 +4C 150TAS 2400 10.4GPH FL130 ?C 155TAS 2575 11.0GPH FL140 +3C 147TAS 2400 10.0GPH FL130 +4C 135TAS 2575 10.2GPH FL200 -32C
A lot of the above flights are at ISA+10 or warmer.
If you want more data, you can pay for the fuel
:))
I have new engine (Lycoming factory rebuilt, slightly above 100 hours) and at recent 100-hours check good compression values and all-over good engine condition were noted. However, I have impression that with old engine (sadly affected with SB569 and replaced at 1400 hours) I had few kts more.
The usual I see is 125 KIAS at 8000 ft with 2300/21.5 and 10-11 GPH.
Emir
To answer the question: 1) A/P in use at say 6000ft 2) Altutude hold set 3) Engineer checked a few times and no faults found 4) No indicated variations in engine speed or MP or fuel flow 5) Reduction in IAS is over a few second and there is no "feeling" this. 6) Happens at random and does not seem to be weather / environment related
Keep asking please because I and my engineer would love to know where to look next
5) Reduction in IAS is over a few second and there is no "feeling" this.
Can you verify what's going on with ground speed using GPS during these moments?
The usual I see is 125 KIAS at 8000 ft with 2300/21.5 and 10-11 GPH.
There is a very big difference between 10GPH and 11GPH, in terms of the engine operating point. For a comparison with me, you need to try say 3000ft, set 23", 2400rpm, adjust the mixture for PEAK EGT and then see what IAS you get. I achieve peak EGT at about 11.2-11.5 GPH (USG/hr).
You should calibrate your ASI, using the GPS ground speed method. There is a formula for doing it, involving flying three headings 120 degrees apart, but in light winds (say below 10kt) it simplifies to flying on the three headings (say 360, 120, 240) and just averaging the three GPS ground speeds. That gives you TAS, which you convert back to IAS using the traditional altitude+pressure correction.
To answer the question: 1) A/P in use at say 6000ft 2) Altutude hold set
I reckon this is caused by updraughts and downdraughts. Happens all the time when in altitude hold.
If the EGT and fuel flow are constant, that excludes stuff like air path, fuel system and ignition issues.
But there should be calm days when your speed is constant.
Can you verify what's going on with ground speed using GPS during these moments?
GPS GS should track the IAS, on a short term basis.
