133kt IAS, 4500ft
18 deg roll angle
TC showing about Rate 0.95
Looks pretty much normal to me.
Rough formula is TAS/10 + 7. Assuming 140KTAS (like indicated in the SANDEL SN3500) it is 14+7, so 21° bank angle. If you think that your AI or TC is not indicating correctly, you could do a 180 (or 360) and measure time taken.
Edit: it might also be that the ball is not exactly in the “middle”, depending on the viewing angle for what it has been designed. This can easily result in 3° bank angle difference..
I never know from which angle exactly you have to watch the ball in the TC so that it’s supposed to be in the middle. Is it always up front? This has gotten far easier with a digital “ball”, which does not change upon viewing angle.
The formula I use is 15% of KTAS. The end result is the same 21° bank angle.
The “real” formula is
(180 / pi) * atan (TAS / 364) [The 364 is approximate, it is 1 / ( 3 * (pi / 180) * ( 1 / g ) * (463/900))]
or
57.296 * atan (TAS / 364)
atan(x) is approximately x for angles below 22 degrees, and using that you get
angle of bank = TAS * (27.296 / 364) = TAS * 15.74%; by using 15% the error is less than 1 degree up to 190kt TAS
The widely taught 10% + 7kt works within 1 degree between 120 and 170kt, but is poor in comparison for the lower speed range – it works better above 250kt.
Cobalt wrote:
The widely taught 10% + 7kt works within 1 degree between 120 and 170kt, but is poor in comparison for the lower speed range – it works better above 250kt.
How very interesting. Thanks a lot for the graphs! One should mention that it is normalized to knots true airspeed.
When taking into account, that (when flying standard rate turns) no turns shall be made with bank angles higher than 25 degrees, corresponding to 170KTAS, the 15% rule is practically without measurable error up to that condition.
What about being sloppy: do +/-20deg at any KTAS speed for SEP, it’s well marked in AI and has less risk of excusion away from the value?
These graphs & calculations reminds of someone who was looking for peak EGT and 50ROP in his circuit downwind 
Ibra wrote:
What about being sloppy
Would you dare to put in a good word for me with my IRI? He’s not so happy when I say the same… 
UdoR wrote:
He’s not so happy when I say the same…
Haha, he should be at least happy about the final outcome? he wanted 220 on wet compass or very nice hold, well he have it? don’t ask how one cheat or adjust
I have a feeling that either my TC is over-reading or the AI is under-reading the roll angle, by 1-2 degrees.
The TB20 KFC225 autopilot is supposed to be set up for 22 degrees which is supposed to give you Rate 1 at 150kt. Clearly this aspect is incorrently configured and I need to look at that, because it produces an undesirable undershoot on IAP intercepts. But the TC should not be showing so close to Rate 1.
