Topless Vs Kingpost Gliders In Region #9: Which One's For You?

by Larry Huffman

With the advent of topless gliders in the last couple of years there has been much talk about the merits of these new wings. Many of us are swaying back and forth trying to decide if we should buy a new glider and which one it should be. Most conversations on the subject are based on hearsay, the perceptions of our flying buddies, and remote competition results. The conclusions range from the topless being very good to a belief that they don't make any difference. The truth lies somewhere in between and really depends on the type of flying that we as individual pilots do. In presenting the different factors that effect the type of flying we do here in Region #9, this article will hopefully shed some light on the matter and allow pilots to make more informed judgments as to the usefulness of owning a topless glider in Region Nine.

Polars
First we must set some parameters for our comparison, ones that will reasonably represent the conditions we most commonly encounter in this part of the country. To start we will look at two generic polars comparing a topless and a kingpost glider. The two polars here are close derivations of polars actually used by competition pilots for both types of gliders. In order to represent our performance a bit more accurately the polars are for an average flying altitude of 5000 ft. MSL. It should be clear that the performance of our two gliders are reasonably close below 30 mph but as the speed increases the topless glider shows an increasing performance advantage. The minimum sink rate of the topless glider is only about 20 fpm less than our glider with a kingpost at very close to the same speed. This amount would be very hard to see in the air therefore pilot skill and technique would make much more of a difference. The best L/D speed for the kingpost glider is 29.3 mph while the topless' is only 1.7 mph faster at 31.0 mph. This 5.5 % speed difference at best L/D speeds is part of the story and the actual L/D is another part. With a 13.6 to 1 glide ratio of the topless glider it has a 12.5 % advantage over the 11.9 to 1 of the kingpost glider. So now we know that the topless is slightly faster at best glide speed and has a better overall glide but what does that mean to us in the air?

To illustrate this we will assume that both gliders start gliding at their corresponding best L/D speed from 5000 ft. agl toward a thermal 3.5 miles away. Without encountering any sink the topless glider will arrive first at 3640 ft. agl and will have taken 6.75 minutes. On the other hand the kingpost glider will arrive at 3448 ft. agl in 7.16 minutes or 192 ft lower and .41 minutes behind the topless glider. The difference between the two gliders in this example is not exactly staggering.

Average Speed
To further explore the differences let's look at a chart comparing the average xc speed of our two gliders. For this example we will again assume that there is no wind and the average air mass sink rate is zero fpm. This means that the air we are flying in is not going up or down. Because the scope of this article does not include racing all speed rings settings are zero for the expected climb rate. Remember that in this part of the country the climb rates and altitude gains are usually relatively low and racing isn't a prudent option most of the time. We can see that even at a climb rate of 1000 fpm (a rarity in this area) the average xc speed difference is only about two mph or 7.7 %. At lower climb rates the difference will be slightly less but 2 mph is a general rule of thumb for this comparison.

Recreational XC
Now we ask what does this really mean to us while flying cross country? For our next example we will use common conditions encountered in our area for a 50 mile flight. The following table shows the outcome of a flight with our two gliders. Remember that the times listed here are optimal for the parameters given and are subject to change with pilot skill of finding and centering lift. Also remember that the 300 fpm climb rate used is an average and is representative of the climb rates we will often see on a xc flight. We will certainly see more and quite often we will see less but the average is what determines our average speed.

Additional Factors
Thus far we haven't seen a major difference between a topless and a kingpost glider with the examples used. This could lead us to ask why anyone but a competition pilot would spend the big bucks on a topless glider for such a small return. All of our examples above assumed that we were flying at slower speeds (under 30 mph) where the differences weren't. Now we will compare performance at faster speeds. In order to understand the performance advantage it is necessary to know why there is a need to fly faster at times. Racing, flying through sink, and flying into a head wind are the three basic reasons for flying faster than our best L/D airspeed. Racing is a more complex skill that most recreational pilots never use so we will confine our discussion to the latter two.

Flying Through Sink
We should all know that when we encounter sink we should speed up. The following data is based on the mathematically correct speed to fly for our two gliders when flying through sink. For an example if the air mass we are flying through is going down at 200 fpm then the topless glider should be flying at 37.8 mph with the vario indicating -464.7 fpm which results in a 7.2 to 1 glide ratio. In the same air a properly flown kingpost glider would be indicating 33.8 mph and -464.2 fpm with a glide ratio of 6.4 to 1. By comparing the two at various air mass sink rates we will see that the vario indication and the glide ratio are not that much different. The real performance advantage is the correct airspeed difference between our two gliders. The topless is clearly faster than the kingpost glider. This means that it will be able to get through the sink faster and will not lose as much altitude.

Flying Into a Head Wind
The following table shows the performance differences between our two gliders with different head wind components. The airspeeds are the correct speed to fly into a head wind without any air mass sink. We can see that the topless glider really out performs the kingpost glider with any kind of head wind. The additional glide performance helps but the real difference is the faster speed performance of the topless.

Getting The Performance
All of the above comparisons are based on our two generic polars and reflect optimum results for each glider. The real world outcome may not be the same for all pilots depending on their gliding skill and the rest of their equipment. The type of harness and the angle at which the harness is adjusted in flight can reduce much of the gliders' performance differences. For most of us that don't have natural abilities for speeds to fly a good speed to fly vario may offset most if not all of the performance advantage gained by flying a topless glider, if the topless is being flown at improper speeds particularly on down wind cross countries. If we are to get the most out of any glider we have to look at the rest of our equipment with a critical eye.

Summary
In the previous sections we have compared topless and kingpost gliders in a variety of circumstances that have hopefully shed some light on the differences in performance. It is clear that topless gliders do have some performance advantage in all cases but less in some instances than others. The major disadvantages to topless gliders are their price and increased weight. We as prospective buyers of new gliders have to decide if the performance increase is worth it for the type of flying that we do. If we are to be serious competition pilots then a topless glider is a must. But how many of us fit into this category? For the average pilot that boats around his or her home site and occasionally goes cross country the answer may be different. Each of us should honestly evaluate our skills and needs for the type of flying we do without succumbing to peer pressure in deciding which glider is best for us.