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Maccready Factor In Tge Soar Dg808s

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I understand the TE compensation and the Netto compensation in the Winter vario for the SOAR DG808S, and I even understand the MacCready altitude computation (altitude at arrival at waypoint) but I am not sure if I am clear on the MacCready Factor (the numbers 0 - 9 on the Cambridge vario). What are these factors - for instance, what do the indvidual factor values represent? And whan would you use these factors??

Just curious...

Randy L.

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My understanding is that you set a number based on how strong you assess the lift to be at the next thermal or ridge you are flying to. So if you are expecting weak lift then you would set a lower number which means you have to fly slower (and with less altitude loss) to get there. Conversely if you are expecting huge lift then who cares about flying efficiency - lets get there as fast as we can (a high Mcready number) and get in that lift which will far outweigh the efficiency loss you took to get there.

I am still coming to grips with what is the best McCready number to use on final glide, I guess its just the highest number which I can still make the airstrip.

The flight computer developed by b21 which incorporates the McCready factor and takes into account the wind direction and performance characteristics of the DG808s is very useful in a mission like the latest one by Spud called Hilltop Hop Day 1 which uses the Dornbirn airstrip in the Alps.

http://www.flightsimulatorxmissions.com/mo...484&start=0

From halfway through the mission the turnpoints are successively lower and therefore each turnpoint can be treated as a final glide, so setting a high Mcready number gets you there the fastest and as long as you arrive with a small height safety margin its all good.

Hodge

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In the 50ies Paul McCready has developed a theory about which speed to fly to be fastest on a cross country flight with a glider, when you know the thermal strength and the polar of your aircraft. It also says, which is the optimal speed to cross fields of sink, and areas of weak lift, where you don't intend to thermal.

Simplified, the outcome is that you need to shift the polar down with the amount of current sink or up with the amount of current lift plus the lift expected from the next thermal and look for the tangent point of a line starting from the origin of the diagram. This gives you the optimal speed to fly. The McCready-speed to fly indicator (which might be a rotatable "McCready-ring" around the vario, see here: German page with a picture), or the final glide computer does this automatically for you, when you set the McCready value. To account for head- or tailwind, the polar is shifted left or right in an analogous way. In practice, for headwind you should increase (!) your McCready setting, depending on wind strength typically for 0.5-1m/s, while for tailwind you should decrease it. Usually, polar curves (sink over airspeed) can be very well fitted with a second order polynomial, so mathematically it's fairly easy.

For final glide, theory says, that you have to gain altitude in the presumably last thermal, until the glide slope extends to your destination, and you should finalize your flight with a McCready setting that resembles the strength of your final thermal. If the altitude, that is achieved here is not sufficient you should still proceed with that McCready value, but look for another thermal on your last leg, that should give you the missing altitude. Once the altitude is sufficient you should stop thermalling immediately, and start final glide. If you gain more altitude you could fly faster but you would need more time for thermalling than you gain by higher speed. Yet it is more safe, and also more fun.

So far so good, but what if you don't know if there is another thermal on your final leg, or it may be weaker than the current?

Well that's where the pilot's factor comes in.

Cheers,

Peter

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Simplified, the outcome is ...

LOL

I remember struggling to comprehend Maccready but that was decades ago.

My 2cents worth is that the instrument is doing all the complicated calculation for you to work out the AGL arrival height at the next waypoint in the GPS or mission. YOU have to tell the instrument how fast you expect to be flying, so it can adjust the glideslope. The odd complexity is you don't say "I'll be flying at an average of 80 knots", instead you say "I'll be flying at the optimum speed assuming my next thermal is 2 knots" i.e. you put in a Maccready setting of 2.

The upcoming Aerosoft Discus has a flight computer also programmed by me (I did the SOAR modified Cambridge) and in the Discus the use of the Maccready setting all comes together *because* the computer vario displays not only Total Energy (compensated vertical climb rate), Netto (the pure outside air vertical movement), but also SPEED TO FLY. In this mode the vario looks like it's behaving in a similar way to TE and Netto (i.e. UP is good, DOWN is bad) but in fact the vario is telling you to SLOW DOWN (=vario up) or SPEED UP (=vario down). The flight computer is reading the netto value, looking at your current airspeed, taking into account the ballast on board and the maccready setting you've set, and can work out if you're flying faster or slower than optimal for the immediate lift or sink. It's all damn clever but at the end of the day you treat it like a fancy vario and pilot skill is still what counts. But, you can just set the maccready 'expected climb' rate and let the vario tell you how fast you should be flying.

The SOAR modified Cambridge is intentionally simple, and hence the arrival height calculation is only valid for full ballast and assumes optimal flap settings at each airspeed. The electronic vario in the Aerosoft Discus is a faithful simulation of the real SDI C4 flight computer so it works at all ballast settings and has many more modes. The simulation in both the SOAR Cambridge and the SDI C4 works the same way as the real instrument, reading current airspeed, altitude, windspeed, distance to go, etc. so, as with the real instrument, you will still fly into the weeds if the path to your waypoint is full of sink!

B21

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I have recently flown the Everest Challenge and I was wondering how the Soar DG instrumentation handled high altitudes. I didn't notice any particular differency when flying but at 28000ft the stall speeds and other flying speeds must be considerably higher in the low density air.

Is this compensated for by the fact that low density air is entering the instruments and it is only the over ground speed which increases. I wondered if I was flying too slowly for that altitude - Any thoughts??

Jeff

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I have recently flown the Everest Challenge and I was wondering how the Soar DG instrumentation handled high altitudes. I didn't notice any particular differency when flying but at 28000ft the stall speeds and other flying speeds must be considerably higher in the low density air.

Is this compensated for by the fact that low density air is entering the instruments and it is only the over ground speed which increases. I wondered if I was flying too slowly for that altitude - Any thoughts??

Jeff

Jeff,

Basically you are right with your thoughts about the compensation.

I am sorry that my knowledge of english is not good enough to give you the full theoretical explanation, but I will give it a try in plain english:

The low density air make the airspeed-instrument (ASI) to indicate a too low (true) airspeed. For example: at 4000 meters altitude and a true airspeed (TAS) of 111,6 km/h the ASI will indicate 90 km/h (IAS) .

But the "wrong" IAS has eventually no effect on the stallspeed and VNE of your airplane, because the low density air influence on the behaviour of the airplane is compensated by the fact that at high altitude, there is also less air above you that wants to push you to the ground (less gravity + more true speed than IAS equals the less lift, due to low density air), and low density air also means there is less power in it to tear your wings off near VNE (less drag-power equals the difference between TAS and IAS).

Bert

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...

also means there is less power in it to tear your wings off near VNE (less drag-power equals the difference between TAS and IAS).

...

Not exactly right. VNE for gliders is always TAS (at least I learned that from flight manuels). The reason is, that aileron flutter is the limiting effect, rather than g-load. This occurs depending on TAS, rather than IAS.

So there is the infamous "coffin corner", where minimum speed which is IAS meets VNE, which is TAS. This is somewhere at 80000 ft for typical compound gliders, I think.

When exceeding VNE, you may still control the g-load, but you can't control the aileron flutter, and your wings will break apart, due to resonance.

regards,

Peter

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Not exactly right. VNE for gliders is always TAS (at least I learned that from flight manuels). The reason is, that aileron flutter is the limiting effect, rather than g-load. This occurs depending on TAS, rather than IAS.

So there is the infamous "coffin corner", where minimum speed which is IAS meets VNE, which is TAS. This is somewhere at 80000 ft for typical compound gliders, I think.

When exceeding VNE, you may still control the g-load, but you can't control the aileron flutter, and your wings will break apart, due to resonance.

regards,

Peter

My theory-book for becoming a RL soaring-pilot told me otherwise (but, hé, here in Holland the highest "hill" is 323m, so who cares).

Anyway, Peter, you are so right again. Look what I found in the flight manual of the DG808S. at the bottom of "section 2-2 Airspeed".

<Quote>

Warning: At higher altitudes the true airspeed is higher than the indicated

airspeed, so VNE is reduced with altitude according to the table below, see also

section 4.5.5.

Altitude in [m] 0-3000 4000 5000 6000 7000 8000

VNE indicated km/h 270 256 243 230 217 205

Altitude in [ft] 0-10000 13000 16000 20000 23000 26000

VNE indicated kts. 146 138 131 124 117 111

<Unquote>

Bert

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After all,

we are both not that correct.

I have checked the standard atmosphere data (http://www.cactus2000.de/de/unit/masssta.shtml), and some information on ASI calibration (naca.central.cranfield.ac.uk/reports/1946/naca-tn-1120.pdf) and according to that:

270 km/h TAS = 176 km/h IAS @ 8000 m,

while the manual still allows 205 km/h.

Unless there is some calibration error considered in the DG manual, the truth could be somewhere in between.

The Australian Ultralight board recommends understanding Vne as TAS, as long as there is no more precise information of the manufacturer.

best regards,

Peter

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