Since a few months, I enjoy flying a Troll. As François is used to, this glider uses SB96V and SB96Vs airfoils.
These airfoils has been quite surprising for me, because I was used to fly with TP42/TP29 which are symetrical airfoils.
Quickly, I felt many differences in flight. These were so clear that I decided to investigate. May I be able with PredimRC (a tool designed by Franck Aguerre for aerodynamic analysis of plane models) to quantify these differences with some simple values?
Now I can answer « yes! ». This article will deal with my steps to this conclusion.
TP42 and SB96V: any difference in flight?
I was used to fly my Snowflak with TP42/TP29 airfoils. With the Troll, the first good point for SB96V I felt was a better ability to take altitude and to pass through. First I though this was du to plastic wings (much better than my wood ones) and to higher wingloading.
But something else indicates that airfoil has an important role. Flying inverted, it is more difficult to gain altitude with thermals. More of that, resources and recovery with small radius (in and out of stall turns for example) induce big loss of speed. This loss is significantly smaller while using flaps amply.
It is these behaviors that I sought to find in PredimRC. The idea is to know what to expect by using one or another airfoil for a aerobatic glider, but also to adjust the use of flaps to keep energy.
Airfoil performances, with or without flap, normal and inverted
So I began to analyse with Xfoil (in PredimRC) the SB96V in all its aspects: in normal flight without flap, then with different flap angles. Then I reversed the profile and I also blew it in smooth, and with different flap angles, favorable to inverted flight.
I thus obtain 9 airfoils and their characteristics that will interest me in the continuation of my study:
-SB96V inv 0°
-SB96V inv 5°
-SB96V inv 8°
-SB96V inv 11°
-SB96V inv 14°
At this point, it is important to dissociate the performance of the single airfoil from the performance of the complete glider. That is why the next step is to modelize the glider.
Then I put the geometry of the glider in the sheet « 3-Geometrie » of PredimRC. The graphical visualization helps much to verify that everything is ok. You may notice that I customized a little PredimRC, by changing some colors and other stuff.
At this step, various results are provided by PredimRC. Most of them with graphs. For my part, I want to be able to compare quickly some outstanding and significant values, that could be related to flying sensations I described. That’s why I added some formulas just next to the graphs.
3 performance indicators
We started this article talking about 3 feelings in flight : gaining altitude, passing through, resource with small radius.
In PredimRC, these feelings may be related to : rate of sink, glide ratio, Cz/Cx in short turn.
Thus, I added in PredimRC some formulas to present as a table the maximum of rate of sink, maximum of glide ratio, and Cz/Cz in short turn for Cz = 0.85 (arbitrary value for short turn). As PredimRC allows us to analyse 3 airfoils together, and 2 masses for the model, it leads to 3 indicators x 3 airfoils x 2 masses = 18 values in my table. You can find them below black cells on following picture.
By taking only the optimal values instead of the curves, it makes comparison easier, but with a loss of informations. The comparison between curves gives informations for a large speed range, while the optimal values are only singular points of flying possibilities.
Smooth airfoil, normal flight
Then with airfoil SB96V 0° I obtain following values:
– min sink rate = -0.61 m/s
– max glider ratio = 17.8
– Cz/Cx in short turn = 15.8
Admit that is good. How does that change with inverted flight?
Smooth airfoil, inverted flight
With the same glider geometry but with inverted airfoil SB96V with no flap, we obtain:
– min sink rate = -0.98 m/s
– max glider ratio = 12.6
– Cz/Cx in short turn = 4.1
At this step we have something instructive, because the values match with my feelings in flight. Sink rate and glider ratio a little less good, but Cz/Cx in short turn terribly less good (from 15.8 to 4.1).
Now we may wonder if the performances could be increased by using flaps. Answer is yes, without any doubt. Here are two tables obtained with airfoils with flaps, in normal and inverted flight. Values of flaps are those surrounding the optimum.
So about the best:
– min sink rate = -0.53 m/s
– max glider ratio = 18.4
– Cz/Cx in short turn = 19.0
– min sink rate inverted = -0.69 m/s
– max glider ratio inverted = 16.1
– Cz/Cx in short turn inverted = 14.5
This is much better than without flap. Mostly for inverted short turn, growing from 4.1 to 14.5 with 11° flaps.
Let’s imagine a Troll with TP42
Making analysis again with TP42, with 0°, 5°, 8°, 10° and 13° of flaps, we can imagine what would be the performances of a Troll with TP42 airfoil and compare it to SB96V performances:
As we could guess, it is less good in normal flight and better in inverted flight. All depends on the time you fly inverted then… We still notice that TP42 is closer to SB96V « normal » than to SB96V « inverted ».
Influence on settings and flying
The practical conclusion consists of adjusting the values of flaps. I will therefore go up to 11° down and 14° up for flaps (piloted on the right stick for my part).
And from now on, I will amply use flaps for all the passages from horizontal flight to vertical or vice versa! So I keep a lot more energy.
If flight phases are used, they can be adjusted with 8° for the min sink rate and 5° for the maximum glider ratio, in normal flight. The fans of inverted flight could go up to make a phase of inverted flight with 11° up for inverted min sink rate and with 8° up for inverted max glide ratio.
In conclusion, PredimRC allowed me to find « in numbers » my sensations in flight. The simple fact that the figures evolve well in the same way that the sensations testifies that PredimRC is a very powerful tool to analyze a glider geometry, going as far as to know what rate of flap is the best for the min sink rate, max glider ratio, the short turns.
Of course, as it is, it does not tell me to what extent the calculated values are accurate on my glider in the air. The use of telemetry may give other interesting conclusions, and I believe that François is on the point …