Estimating sink rate?

[davidleitch]

If one has a glider with a known airfoil, let's say mh32 and a wing loading expressed in say oz/sq ft and assuming one is flying at the minimum sink rate in still air, is there a way of estimating the sink rate in say metres or feet per second? what other variables does one need to know?

Any pointers appreciated. I'd like to compare what I actually achieve with perfection.

tia
dave

[Ollie]

At minimum sinking speed, induced drag is more than half the total drag of the sailplane. This means that aspect ratio has a very important effect, in addition to wing loading, on minimum sinking speed. Induced drag is proportional to coefficient of lift squared and inversely proportional to aspect ratio. The performance polar diagram of the plane can be calculated and displayed in a fraction of a second by a program like PC SOAR. It is rather tedious to calculate manually. Once the polar is calculated and graphed the minimum sink can be read directly. The main errors in calculation are in estimating the various components of lift and drag. For example the airfoil drag polars assumed by calcualtion may not match those of the nominally similar model. The lift diatribution assumed may not perfectly match the lift distribution of the model. The parasitic drag estimates used in calculations are usually crude at best. Calculations are best for comparisons between competing designs.

An alternative to calculation is experiment. Launch to a measured altitude at dawn when the air is stable, fly with absolute minimum control movement and measure the time and air speed. This must be repeated over a range of elevator trim settings that involve flights at various constant airspeeds, both above and below the air speed associated with the minimum sinking speed. The sinking speed is just the altitude divided by the time. The performance polar is a graph of sinking speed versus air speed The main sources of error in this experiment are altitude measurement, vertical movement of the air mass, airspeed measurement, and ground effect. Experimental determination of minimum sinking speed is usually the best approach for measuring sinking speed of a particular model rather than for comparisons between competing designs.

When comparing calculated polars with experimental polars it is important to take into account the affects of difference between the air density assumed by the calculations and the air density associated with your flight measurements.

BTW, all measurements and estimates have errors associated with them. Trying to campare your model with "perfection" will be an elusive goal.

[Dick Huang]

dave,
Every thing Ollie said is good advice; however I have some quick and dirty methods of estimating sink speed. All are based on SL air density.
Vsink(ft/sec)=7.25*(w/s)^0.5*(Cd)/(CL)^1.5; (w/s) is wing loading in oz/ft^2.
Vsink(ft/sec)= 11*(Wo oz)^0.5/(Sw in^2)^0.5

Both formulas work for me.
Dick Huang:)

[davidleitch]

Dick

I've finally got around to needing to use these formulae. Could you remind me what Wo and Sw are in version B of your quick and dirty formula are please.

I am trying to measure whether the increased climb rate of a heavier power system will compensate for the resulting faster sink rate. In Australia our 7 cell gliding competitions require 5 minute duration with points deducted for every second of motor run.

I can add 8 oz to my current 54 oz AUW (and a lot of $ to the credit card) by switching to a power system that will increase my theoretical climb rate from 1800 ft per minute to 2500 ft per minute. This could cut my motor on time from 15 seconds (height of 450 ft) to 11 seconds. Or alternatively I could climb another 150 ft.

Unfortunately for my credit card I am almost sure that the height gains will easily exceed the sink penalty.

[Ollie]

David,
Your comparison of altitude gained due to better climb versus altitude lost due to higher sink rate does not take account of possible thermal activity. It is directlly applicable only to air that isn't rising or sinking. Altitude gained in the climb exposes the plane to the probability of bigger and stronger thermals. Altitude gained in the climb increases range and the probability of encountering thermals. The better you are at predicting where the lift is before you launch, the less important sinking speed considerations are.

Winning is much more about pilot skill than about equipment. Should you prioritize your resources for skill development or equipment improvement? Only you can deside.

The main disadvantages of increased wing loading are that it not only increases minimum sink rate but further increases sink rate in a thermal turn by increasing the angle of bank for a given size circle. This seriously decreases the ability to work small, low altitude, weak thermals.

[davidleitch]

Ollie
I appreciate that. I actually managed to win sportsman class at a small comp on Sunday last, and 5th overall. I have been getting up every morning at 5:30 to practice landing, however there is little or no lift at that time so don't get much of a chance to read the air.

Neverthless the competitions in Australia are been won by people who are emphasizing power. They are also fantastic pilots etc etc.

The extra oz only increases the wing loading by 1 oz sq ft (from 11-12).

It seems to me that the higher the wingloading the greater the skill required to recognise and stay in thermals, however the greater the power, the higher you will be and the greater the chance of finding strong lift, plus you have further to sink. Landings will be harder with a heavier plane, but speed between thermals will be higher.

Its a huge nuisance to have to buy a new motor and batteries and will alter the character of the plane, but I suspect its what has to be done in this comp.

regards
Dave

[Ollie]

The sinking speed and airspeed increase as the square root of the wing loading. An increase in wing loading from 11 to 12 ounces per square foot will increase air speed and sinking speed about 4.3 % at every coefficient of lift or angle of attack short of a stall. It will increase the diameter of a circle at a given angle of bank and coefficient of lift by about 9.1% while also increasing the sinking speed by 4.3% compared to a circle at the lighter wing loading.

[Dick Huang]

Dave
Wo is the all up weight in oz and Sw is the wing area in squar inches. If you have Moto-cal it lists the sink speed as a minus number on the last line.
Dick Huang:)
Here is the formula that Ollie was talking about:
Vsink2=[Vsink1*(W/S)2^0.5]/(W/S)1^0.5