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TAS wrong in PLOG


TAS wrong in PLOG

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macs86
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Hi All,

I‌m quite new to SkyDeamon i was hoping somebody could explain this to me.

I‌n the flight details window i have set a speed of 120kt TAS‌‌. However in my PLOG the TAS varies?

I‌ understand why the GS varies but why is the TAS changing?

H‌opefully somebody can shed some light on this?‌‌ And tell me how i can manually set the TAS for the leg?

T‌hanks in advance

M‌att
George Reid
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macs86 - 10/11/2017 11:39:28 AM
Hi All,

I‌m quite new to SkyDeamon i was hoping somebody could explain this to me.

I‌n the flight details window i have set a speed of 120kt TAS‌‌. However in my PLOG the TAS varies?

I‌ understand why the GS varies but why is the TAS changing?

H‌opefully somebody can shed some light on this?‌‌ And tell me how i can manually set the TAS for the leg?

T‌hanks in advance

M‌att

For a given indicated airspeed the True Airspeed will vary with density (altitude) and temperature.  I find it strange that you have defined your cruise performance in terms of a power setting and a TAS.  A given power setting will produce a giver Indicated Airspeed, but the TAS will vary with atmospheric conditions.

‌‌



George E A Reid

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macs86
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Hi George,

T‌hanks for the reply. Yes i do accept all of the points you made.

I am really just trying to replicate what i would do with a CRP5. Generally for my level of flying (PPL) ive noticed TAS tends to get used for the nav calculations (although im sure that is not the most accurate way). One of the main reasons i wanted to use sky deamon was to check my manual planning with the CRP5, but because i cant set a TAS like i would with a CRP5 its hard for me to check if the answers i'm getting doing it manually are correct.

M‌

Tim Dawson
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You can set a TAS, but SkyDemon does a lot more than your slide rule does so comparing output is not going to be valid. For instance SkyDemon will be taking into consideration your aircraft's climb and descent performance and if any given leg contains a climb or a descent, the overall average speed on that leg will be neither the speed during the climb/descent nor the cruise speed. SkyDemon is modelling your flight performance precisely whereas the slide rule can only ever give a rougher answer.
Pete
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" I find it strange that you have defined your cruise performance in terms of a power setting and a TAS.  A given power setting will produce a giver Indicated Airspeed, but the TAS will vary with atmospheric conditions."

H‌ave to disagree on this, a given power setting tends to give a constant TAS at any altitude(that we fly) , however the IAS decreases with altitude for a given power setting‌. This is supported by my POH, hence I use TAS with my power settings in SD
T67M
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Pete - 10/14/2017 11:34:04 AM
" I find it strange that you have defined your cruise performance in terms of a power setting and a TAS.  A given power setting will produce a giver Indicated Airspeed, but the TAS will vary with atmospheric conditions."

H‌ave to disagree on this, a given power setting tends to give a constant TAS at any altitude(that we fly) , however the IAS decreases with altitude for a given power setting‌. This is supported by my POH, hence I use TAS with my power settings in SD

Which DA40 have you got?! I fly both the petrol -180 and the turbo-diesel -NG, and both show that TAS increases as altitude and/or temperature increases.

macs86
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Thanks everyone for the useful replies.

I am hoping to get skydemon to a‌t least nearly replicate what i get with my wizz wheel. I do appreciate that TAS varies slightly with temp and altitude, but notwithstanding that i am getting very odd things on my skydemon plog.

C‌heck this image out for example (i screenshot it when planning a flight this morning). The first and final legs are flown in pretty much exactly the same place, at the same altitude, less than 20 mins apart. Yet Skydemon says the TAS on the first leg is 101, and on the second leg is 113.  Presumably the reason for this is that skydemon is adding in some kind of correction because it thinks im taking off from Brighton marina. But in fact im taking off at shoreham and flying to brighton marina where i start my nav.

W‌hat would be really useful would be if i could just override all of skydemons cleverness and tell it what TAS i want to use for the leg, or even better the entire flight.

‌‌

T67M
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macs86 - 10/17/2017 1:21:37 PM
Thanks everyone for the useful replies.

I am hoping to get skydemon to a‌t least nearly replicate what i get with my wizz wheel. I do appreciate that TAS varies slightly with temp and altitude, but notwithstanding that i am getting very odd things on my skydemon plog.

C‌heck this image out for example (i screenshot it when planning a flight this morning). The first and final legs are flown in pretty much exactly the same place, at the same altitude, less than 20 mins apart. Yet Skydemon says the TAS on the first leg is 101, and on the second leg is 113.  Presumably the reason for this is that skydemon is adding in some kind of correction because it thinks im taking off from Brighton marina. But in fact im taking off at shoreham and flying to brighton marina where i start my nav.

W‌hat would be really useful would be if i could just override all of skydemons cleverness and tell it what TAS i want to use for the leg, or even better the entire flight.

‌‌

I suspect the climb and descent phases are exactly what is causing that difference since the other two legs are showing exactly 120ktas. 

I‌‌ am intrigued as to why you would want to "turn off" the features that Tim and the team have worked on so hard which make the SkyDemon PLog the most accurate paper PLog I have ever flown with. I can even use heading hold mode on the auto-pilot (rather than programming the route into the aircraft GPS) and fly the plan perfectly in "my" DA40. In fact, I prefer to do it that way as the built-in GPS flies a great circle route rather than a rhumb line and thus deviates from the SkyDemon line by a mile or so on long legs.

stevelup
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The obvious solution here is to include the leg from Shoreham to Brighton Marina...
Tim Dawson
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SkyDemon is not designed to mimic a whizz wheel, very much yesterday's technology. It's designed to far surpass it, which it does, and we have no plans for a way to enter whizz wheel derived calculations, sorry.
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