SkyDemon Light calculations

[CharlieDeltaUK]

I've been comparing a PLOG derived from chart and whiz wheel with a PLOG derived from SkyDemon Light for exactly the same route. I'm finding differences, especially in relation to calculated ground speed.

I realise that SkyDemon doubtless uses trig functions to calculate the vectors whereas as the whiz wheel isn't so precise, but I wasn't expecting the difference to be as much as 5%+. I thought I was pretty good with the whiz wheel and now I'm wondering if that's misplaced.

I notice that SkyDemon Light asks for the IAS instead of TAS. Since it has no knowledge of the OAT, I assume it isn't trying to calculate the TAS using an IAS. Why would SkyDemon want the IAS?

Putting aside for a moment the issues about how accurately one can actually fly, can anyone explain why SD Light wants the IAS, and why I might be seeing thse variations?

[BackPacker]

TAS also varies with altitude. Did you take that into account?

[CharlieDeltaUK]

Yes, got that, and SD does ask you your altitude. But, without the OAT, I don't see how SD could make convert IAS to TAS, and TAS is surely what you need for the PLOG calculations.

Just to add another element to the original question, the layout of the SD PLOG also seems to show that conversion from degrees true to degrees magnetic before applying the wind - whereas the wind vector is also stated with degrees true. However, just because that's the layout of the PLOG doesn't mean the calculation workds in that order I guess.

But, it's the ground speed calc which is foxing me.

[riverrock83]

I asked my instructor the same question a couple of weeks ago (I couldn't understand why the skydemon number was greater than IAS + windspeed) and he says that it assumes IAS - so 15degC at surface and approximately 2degC lapse per 1000 feet.

If you want to match your plog when using IAS as true airspeed, set the altitude to 0.

[tmmorris]

set the altitude to 0.

but only after you've checked for airspace, otherwise it won't warn you of conflicts!

To be honest I've not tried comparing the two; on a couple of recent flights I did notice the calculated heading and the heading I actually needed to hold to stay on the GPS track weren't the same, but put that down to faulty wind forecasts as usual. I might try doing it by hand as well next time and see what I get.

Tim

[peterh337]

But, without the OAT, I don't see how SD could make convert IAS to TAS

You can, but it is less accurate.

To convert IAS to TAS, you add something like 1.5kt for every 1000ft (it's not anything I ever work out because I use the subscale, and my CR-5 is hidden somewhere....).

At typical Skydemon customer altitudes (below 2000ft ;) ) the TAS gain is far smaller than the accuracy at which the said punter can control the plane :) The speedo error alone is almost certainly greater.

[BEagle]

5% seems rather a large difference based purely on IAS/TAS errors through no allowance being made for ISA deviation.

The application I use, for a 120KIAS (actually it uses KCAS) at FL50 calculates 129KTAS at ISA. For ISA -15ºC it calculates 126KTAS, the difference being only 2.3%.

Is it a consistent 5% error, or does it vary with the angular difference between track and wind direction?

[B4aeros]

From SkyDemon Help:
TIP: You may notice that on the PLOG your groundspeed isn't quite what you expected, even with a nil wind. This is because SkyDemon calculates your true airspeed from your indicated airspeed and planned level, and also takes into account a climb and a descent, which will obviously affect your speed during those phases of flight. Performance characteristics from an average light aircraft are used.

[Dave Gittins]

I used Skydemon Lite to plog from Redhill to Headcorn and back on Saturday and checked it with a chart and protractor and Sporty's E6B on my iPod and got within a couple of degrees and a minute on each leg so am surprised there have been such discrepancies noted. I had a fairly hefty wind of 35 kts at 310 and the cross checks were close enough for me.

A 5% error is a minute in 20 minutes and I reckon that's close enough using forecast and averaged / inerpolated winds esp. as Skydemon was working to 1/2 minutes which I don't.

Were there 5% errors in track as well ? (360 / 20 = 18 deg) that would concern me.

[CharlieDeltaUK]

The 5% error was just for the GS. The HDG, Track and distances correlated almost exactly. having looked at the quoted text from the 'Help' page, I suspect it's partly because I was using a fairly short route, so I guess the software was making allowances for climb and descent, which I wasn't.

All stuff about accuracy of steering the plane understood...just thought it was an interesting line of enquiry.

[Dave Gittins]

It's a bit like the arguments I used to have with my soil mechanics lecturers 30 years ago.

Why prod a bit of ground with your foot, mutter a bit, rattle some bones, pluck a number out of it and then stick it in one of the new fangled calculators and use it to calculate Californian Bearing Ratios to 6 places of decimals ?

There are so many inexact variables that most plogs and the metghods of calcualting them are "good enough for government work."