USING THE B500/B600/B800 POLAR PROGRAM
In order for the speed command, netto (or relative netto)functions to work properly a polar for the sailplane must be programmed into the instrument.
See your instrument manual for details on how to accomplish this.
Before the polar can be programmed into the instrument certain numbers applicable to the polar must be derived.
The following is an explanation of how the polar is approximated inside the instrument and some information to help you use the polar program.
The sailplane polar in the instrument is approximated by a quadratic equation of the form:
Sink rate = A * IAS * IAS + B * IAS + C
Where IAS = Indicated Air Speed and A, B, C are 3 numbers. (Coefficients)
By taking 3 points on the sailplane polar it is possible to derive values for the A, B, C coefficients for the particular polar.
Several points should be noted:
It is not absolutely required that that polar be derived from flight test but even today manufacturers' `brochure' polars are quite often notoriously optimistic at some speeds.
A too optimistic polar will result in the netto vario showing spurious airmass sink in cruise and in spurious fly faster speed command signals.
A flight tested polar is best particularly if provided from a reputable source. The German DFVLR polars and the Dick Johnson Flight tests are probably the best known.
It is required that you know at what weight the polar was flown. You should also know the minimum flying weight for the sailplane in which you will be installing the instrument. That is, with pilot and all normal equipment but no water ballast. The nearest 5 kg or 10 lb will be close enough.
Flight test polars are provided in terms of sink rate vs CAS (Calibrated Air Speed) whereas the B500/B600/B800 will see an airspeed that is in error by the position error of the sailplane's pitot and static system.
If you use a Prandtl probe pitot/static source for the airspeed information the errors are negligible but if you use the same sources as the airspeed indicator they may be considerable. Fortunately the airspeed source position error is found in the aircraft's flight manual as a table or graph and you can correct the airspeeds on your flight test polar by using this.
IT IS HIGHLY RECOMMENDED to use a Prandtl pitot-static probe for airspeed for the B500/B600/B800.
It is a legal requirement and good engineering practice to use the flight manual specified pitot/static source for the airspeed indicator as the flight limitations and placards take the errors of the specified source into account.
It should be remembered that the quadratic polar approximation has no physical reality and is only generally a good approximation at speeds between those for best L/D and the highest speed likely to be used in normal inter thermal cruise.
Outside those speeds the approximation may have large errors. In particular note that the quadratic still works at zero airspeed while the glider doesn't!
You are required by the polar program to enter the weight at which the polar you are using applies, and the minimum flying weight for the glider. These can be in any units or as wingloadings but you must have these in the same units.
You are also required to enter the speed and sink rate at 3 points. The choice of the points can influence the numbers that the program computes.
Choose the low speed point at around best L/D speed, the high speed point at the highest inter- thermal cruise speed at that weight ( around 4 knots or 2m/s sink rate is usual) and the medium speed point about midway between the two.
The program caters for polars provided in knots for airspeed and sink, knots for airspeed and feet per minute for sink rate, statute miles per hour for airspeed and feet per minute for sink rate and for kilometers per hour airspeed and meters per second for sink rate.
After entering these numbers you will be shown the results of a calculation of minimum sink rate, best glide ratio and the speeds at which these are computed to occur. These are provided in knots only. Pay particular attention to the best glide ratio numbers. These should closely approximate the numbers on your polar.(Within about half a point of glide ratio and about +/- 2 knots of the correct speed)
If they do not then change the low speed point speed and/or sink rate and run the program again until the best glide ratio and the speed at which it occurs is reasonably correct.
Some polars provide sensible numbers first try but others are more difficult. In some cases it may be necessary to adjust the sink rate at the medium or high speed point by +/- 0.1 knots to get a good fit at best L/D.
When the best L/D numbers are about right choose the option to display the coefficients.
Strictly speaking the numbers derived are not coefficients but values correct at 100 knot Indicated Airspeed in the case of A and B.
B is always negative in reality but our customer support program assumes this.
The Netto Offset number represents the sink rate of the glider in circling flight for the relative netto. It is derived by multiplying the straight flight minimum sink rate by 1.6. This is correct for the typical 35 to 40 degree banked turns used in most thermalling and any small errors will be of no real consequence.
If you do not want to use relative netto just select netto type =1 and you will have a netto or airmass vario in cruise.
If you run into difficulty please contact us.