Errors in Aspen TAS and Wind calculations?

[DXB]

I was foolishly boasting on another thread on how fast my lowly C model is based on the TAS displayed on my new Aspen PFD, and then it occurred to me why I might be wrong. Rather than continue to divert that thread, I decided to start a new one.  So the Aspen seems to calculate TAS in real time based on IAS, pressure altitude, and OAT data- I would think this would be pretty accurate.  However, now that I think about it, I wonder if it has the CAS correction for the aircraft model programmed in? Does anyone know the answer to this?  I could not find anything in the Aspen's manual.  If not, this would be a significant source of error for the C model.  At cruise speeds, power on, no flaps, the IAS is 4mph greater than CAS per the POH. This error should increasingly overestimate TAS  with increasing altitude, and any such error would roughly approximate the amounts that I see above book value TAS.  It should also get that very nice wind vector it calculates for you using TAS, GS, MH, and TC completely wrong in some situations.

Another way to look at this might be that the Aspen PFD is a speed mod of sorts for the C model that flatters you with a high TAS and then explains the lack of commensurate GS increase by falsifying the wind data    Does anyone know if I'm right or wrong on this?  I think the large CAS error goes away in the newer planes, but i was wondering if anyone else who flys an Aspen in the vintage Mooneys has any thoughts, or if there are any Aspen reps lurking here who can weigh in.  I'd be happy to learn that I'm wrong, but then you have to go back to hearing me go on and on about how fast my C is...

[Marauder]

I was foolishly boasting on another thread on how fast my lowly C model is based on the TAS displayed on my new Aspen PFD, and then it occurred to me why I might be wrong. Rather than continue to divert that thread, I decided to start a new one.  So the Aspen seems to calculate TAS in real time based on IAS, pressure altitude, and OAT data- I would think this would be pretty accurate.  However, now that I think about it, I wonder if it has the CAS correction for the aircraft model programmed in? Does anyone know the answer to this?  I could not find anything in the Aspen's manual.  If not, this would be a significant source of error for the C model.  At cruise speeds, power on, no flaps, the IAS is 4mph greater than CAS per the POH. This error should increasingly overestimate TAS  with increasing altitude, and any such error would roughly approximate the amounts that I see above book value TAS.  It should also get that very nice wind vector it calculates for you using TAS, GS, MH, and TC completely wrong in some situations.

Another way to look at this might be that the Aspen PFD is a speed mod of sorts for the C model that flatters you with a high TAS and then explains the lack of commensurate GS increase by falsifying the wind data <_ anyone know if i right or wrong on this think the cas error goes away in newer planes but was wondering else who flys an aspen vintage mooneys has any thoughts there are reps lurking here can weigh in. be happy to learn that then you have go back hearing me and about how fast my c is...>

Dev -- the TAS calculation is based on IAS that the Aspen sees. The unit has no way to apply the corrected airspeed. So, if your CAS is significantly different than your IAS, there will be an error. On mine, it translates to a knot or two.

I have done the TAS calculations probably a 1000 times manually before the Aspen installation. Depending on temps, I can see anywhere from 148 KTAS to 152 KTAS corrected for CAS. For flight planning purposes, I file 150.

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[bonal]

For my set up the best way to confirm my TAS was to go up in smooth calm air and fly a 3 and 4 course track with GPS ground speed record the speeds for each and do the calculations. The result was a difference of less than 1 knot between the 2 methods.  You can try different altitudes and see where you get the best performance . I only did it at 8000 feet but was pleased with the results.

[jetdriven]

it depends on wind. the greater the winds aloft, the greater the error (negative) in the 4-heading and divide by 4 method. 

[testwest]

Hi Dev

You are correct, the Aspen can't make the position error correction as published in your POH. They complete the wind triangle with one leg being magnetic course and groundspeed, the next leg being (derived) true airspeed (from IAS, with no position error correction) and magnetic heading, and then display the computed winds aloft. True winds or magnetic? Sounds like an exercise for the reader.

So you may be getting overstated TAS readouts just as you surmise.

However, note that a magnetic heading error (RSM out of calibration) will result in erroneous high crosswinds at 90 degrees to the present track. Easy way to tell if you need an RSM re-cal. I have found the computed winds are way more sensitive to errors in heading versus errors in IAS.

One way to check for this is to use the Cloudahoy App. Record the Aspen TAS and winds (and time) on any stable cruise leg, and look at the same time on the Cloudahoy recording of your flight. The Cloudahoy winds are computed from the observed atmosphere at the time, location and altitude of your position, and they are dead nuts accurate. Aspen-reported erroneous high crosswinds are an RSM issue, erroneous high headwinds means your CAS and IAS are fairly divergent as you suspect.

 

[rbridges]

Sorry to resurrect an old thread, but I was discussing this at dinner the other night.  I'm not sure where the error gets introduced, but I feel like the Aspen TAS is a few knots optimistic.  I've always guesstimated 150-152 based on my adjusted ASI and GPS groundspeeds on trips.  During my flight, it gives me wind direction and speeds which I feel is pretty cool.  I was wondering where it got the infor, and someone told me it was calculated.  

[Marauder]

Sorry to resurrect an old thread, but I was discussing this at dinner the other night.  I'm not sure where the error gets introduced, but I feel like the Aspen TAS is a few knots optimistic.  I've always guesstimated 150-152 based on my adjusted ASI and GPS groundspeeds on trips.  During my flight, it gives me wind direction and speeds which I feel is pretty cool.  I was wondering where it got the infor, and someone told me it was calculated.  

Looks close to me. It won't adjust for CAS, so see what 137 KIAS shows for CAS and apply that correction.




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[David Lloyd]

The Dynon also does not compensate for CAS.  My RV7, the IAS is pessimistic by 2.5 to 3 knots therefore I know my TAS is a little more than what Dynon calculates.  The calculated wind and direction is also off because of this.  It is only noticeable when the wind is very light.  

Compass deviation must be accurate or the calculated wind and direction will be off quite a bit.  More of a factor than the small diff in CAS/IAS.

Every few months, someone with a new Dynon system will be alarmed at wacko calculated wind and direction.  Did you calibrate the compass?  No.  Once done, no more complaint.

[Piloto]

For winds aloft I mostly care on finding optimum winds at altitude without the need to climb through. For this I use the Garmin Pilot app. which allows me  to see the wind vectors at different altitudes. Very useful on flight planning.

José

 

[Marauder]

10 hours ago, rbridges said:

Sorry to resurrect an old thread, but I was discussing this at dinner the other night.  I'm not sure where the error gets introduced, but I feel like the Aspen TAS is a few knots optimistic.  I've always guesstimated 150-152 based on my adjusted ASI and GPS groundspeeds on trips.  During my flight, it gives me wind direction and speeds which I feel is pretty cool.  I was wondering where it got the infor, and someone told me it was calculated.  

I noticed a couple of other things. Your factory ASI looks like it is 6 knots slower than your Aspen. When I had my mechanical ASI, it was dead on to the Aspen speed. And now the Aspen and ESI-500 both match.

The other thing I noticed is that you have the HSI's map set at 20 nm. You may want to drop that down to 5 or 10 nm or switch to the arc mode. Are you using the 3 Nav inputs or are you a belts and suspenders guy?

[rbridges]

9 hours ago, Marauder said:

I noticed a couple of other things. Your factory ASI looks like it is 6 knots slower than your Aspen. When I had my mechanical ASI, it was dead on to the Aspen speed. And now the Aspen and ESI-500 both match.

The other thing I noticed is that you have the HSI's map set at 20 nm. You may want to drop that down to 5 or 10 nm or switch to the arc mode. Are you using the 3 Nav inputs or are you a belts and suspenders guy?

 

Thanks.  They actually had it set on 10nm, but I was just fiddling around with the settings.  I only have 2-3 hrs flight time with it, so I'm just seeing what's what.