X21 /UMS Carbon
X21@at Ranapanb
Still today I feel sun, fresh air and spirit of this famous F3F side in Bohemia, Ranapan. Crazy weather here. If two weeks ago there was -10C (14F) now on sun is temperature raising to +20C (68F). Not so good for skiing, but for flying F3B, excellent. (By Endlich 23, December, 2001) A F3B glider arrived before Christmas from Mr. Jiri Tuma.
Wing Length
Cord Length
Wing type
Wing Area
Wing Load
See the comments
2 pieces
Tail plane
Tail type V-Tail,2 point screw
Total Weight 2,180g(Ballast:800g)
Radio gear
6 Servos, 7 Channels
Fuselage Length
X21, a F3B model development has been just finalized by Mr. Jiri Tuma who is famous as the designer and manufacturer of Xantipa one of F3J models. It took more than three years since he started to hatch up his new model. We may have finally new X21 model. . A prototype model of X21 was unfortunately jackknifed while launching in Japan. The reason of this accident did not come from its design, but entry level mistake during manufacturing process (Spar did not fully contact with the inside of skin.) Through lots of test flight of X-21 in Japan, many comments (see below) have been issued by Japanese fliers the X-21 and many improvements have been added into X-21. Here we may have an exquisite new F3B model, X-21 using UMS carbon (light and stiff) at last. We must thank for the contribution by the Japanese fliers in Kamisato flying club, Mr. Iwata, Mr. Hamada and Mr. Hashimoto.
In Japan, Mr. Hamada recorded 18 seconds constantly in the all Japan F3B Championship, March 2002. Mr. Iwata and Hashimoto have sometimes recorded 15 seconds, informal flight. Those records as well as the record by Mr. Tuma himself, proves that X-21 is one of F3B competition model and that X-21 is an ultimate F3B fighter so far. New X-21 is available since August, 2002 in Japan.

In European countries(Norway, etc.), X-21 is famous as a slope fighter, too. The beautiful picture on the top was taken at Ranapan in Bohemia and uploaded in the site of Roman Vojteck. Most of F3B models made in Czech Republic are designed as F3J model originally, and are reinforced such as Pike/F3B and Eraser2000/F3B. And there are few F3B models that was designed as F3B model originally in Czech Republic. It is X21 that is designed as a F3B model originally in Czech. Mr. Tuma himself is a excellent pilot as well as a manufacturer who is a flier who flies his prototype model until he understand it comfortable enough. This has been already proved by Xantipa, a excellent F3J model. Since I heard his plan to develop new model at the end of 20th century, the name of new model was presumed as "21st model type X" tentatively. However a name, X21 became its formal name. X21 was stared to be rolled out since August 2001. X21 looks like Estrella, because of its skinny fuselage and its narrow airfoil. The airfoil is divided into two wing pieces. Though its flying performance is unknown, it seems to be valuable for us to expect his bigotry. Flight performance is available in the comments bellow.
Jiri Tuma/JITOM (Czech Republic) Plan
Dealer RC-Sailplane.com (Please contact with RC-Sailplane.com) Mail
Price Full Carbon airfoil (excluding tax and shipment cost) TBD
D-Carbon airfoil (excluding tax and shipment cost) TBD
Glass airfoil (excluding tax and shipment cost) TBD
Ranapan in Bohemia, famous slope side.
On Dec. 23, 2001. We got F3B glider from Jiri Tuma before Christmas.
Radio Gear. Slim body seems to refuse to install some electronic stuff... ,An cantina line can be seen out of fuselage.
.. Hook ...
ballast inlet. (Original brass ballast's are available.. )
..for tuning flying phases
Waiting for wind..
...but in the air its the best ...
Yes, I 'm ready...
.Go Go!!
.. In the high place,...
.Through into airborne
...always under control..
. ... .crowing quite well...
will you hit me or not ?...
Final approach..
. .. last flight and lets go to the pub.
X21 Whole view
Radio@Gear Servo@Cover
Nose cone
X-21 with beautiful aspect
Radio Gear Flat Servo Cover

Nose cone with

V-tail Carbon Joinner
V-tail (original)
V-tail (Current model) Fat carbon joiner with Kevler core
Hook and electric cable connector.
(There is water proof type)
F3B Flight of X21 in Japan of Mr. Hamada. Trial of X-21 experiments.
Mr. Hamada and X-21 in landing approach
X-21 turn with background of Mt. Akagi, Japan
...Good flight today, too.

Comments during improvement experiments of X-21
There were many issues in the prototype of X-21. The first one is that there was not any clear pass for the Aluminum control rods for V-tail in the fuselage if Futaba servos were used, because servo horn height was too short. Therefore the rods must be bent like S-shaped. When servo height is too short, because the control rod contacts with ballast tube end, the control rod must be bent. So Mr. Hamada recommended to use long carbon rods and vinyl pipe with 2mm diameter, which fixed this issue. Improved X-21 can equip either vinyl tube or aluminum rods as an option.
As the priority was put on the light weight in the stage of prototype, the strength of the fuselage was very light but, not strong enough. The inner surface of fuselage was tried to be reinforced by such various type as carbon roving, or spiral carbon cross tape or carbon cross, because of experiments. But Kevler and Carbon cross was adopted finally for the inner surface reinforcement of fuselage.
The servo mount in the fuselage of X-21 is a very conventional veneer plate. As those who know legacy sailplane must know, the plate adhered by normal Epoxy can be unstick by small force or shock. Mr. Hamada recommends to use silicone adhesive or bath cork adhesive.
There is a hole of head of fuselage where nose ballast is equipped. This hole help you to place ballast at very end of fuselage and therefore reduce the total amount of ballast in the fuselage for catching balance.
Flat geometry: The airfoil of X-21 strongly tapers at the end of the wing tip, This flat geometry for the airfoil tends theoretically to cause stalling at wing tips within the low speed range such as a final stage of Task B. This argument does not only to the case of X-21. However X-21 has an excellent performance in cycling of Task A (thermal duration) as carrying our mind back to Xantipa.
Though COBRA or ERASER has MH32 profile which is the same of that of X-21, this tapering airfoil of X-21 shows completely different flying performance from them because of the high aspect ratio of airfoil In Task C, this airfoil with high aspect ratio also achieves remarkable speed up to 15 sec (informal record). Some X-21 prototypes load ballast at both before and after spar in the airfoil to reinforce airfoil stiffness. However it resulted in a heavy airfoil In addition, the stiffness can be achieved by contacting spar and wing skin properly without reinforcement by ballast tube before an after spar. So we recommended that the ballast tube in the airfoil should be optional. Since there were partially unstiff portion at the end of wing, it was reinforced. (This portion need not be reinforced, but there are someone who lands at the end of wing and another checks stiffness of the airfoil with holding one side of the wing tip end and shaking it!!.)
Stiffness of the airfoil:
We asked to make such airfoil as carbon-balsa-carbon sandwich to reinforce airfoil But total weight became 60g heavier. Mr. Tuma decided to use UMS carbon to reinforce airfoil and reduce its weight. UMS carbon is well known as the stiffer and lighter material than normal material.

V-tail mount: V-tail is mounted by two screw bolts such as following figure. Screws are made tight from under V-tail. In the prototype, flat veneer was used as mount. But we had accident three times that this flat veneer plate pushed the V-shaped mount in the hard landing and broke both V-shaped mount and V-tail itself. So Mr. Hashimoto and Mr. Hamada recommended that V-shaped carbon plate should be used instead of flat veneer which is able to reinforce V-tail mount and reduce weight.
The spar in the V-tail of prototype scanned up to the middle of V-tail. There remained some un-sharpness of elevator control in the steep turn of Task C because of small deflection of V-tail. There is another reason for this un-sharpness as a matter of course, such as control rod, pilot skill and so on. Therefore this factor is minor one.
The levers for moving wing of V-tail positioned at blue line originally. This position tends to loose control. Mr. Hamada recommended to change location of lever to fat solid line in the right figure. It will improve the strength of adhesion between lever and moving wing. This idea is adopted by Ellipse, Tragi and so on.
It was September, 2001 that the maiden flight of the prototype of X-21 in Japan was executed. Mr. Hamada belong to Kamisato Sailplane club flew X-21 in Japan. Mr. Hamada is a man of power who always challenge anything. He is the best flier of X-21 which attempts to the limit of speed. Mr. Iwata, leader of this club and Mr. Hashimoto, a member of this club also flew X-21 during the same time frame. After only two weeks, Mr. Iwata met such an accident that his X-21 jackknifed while launching. The reason was the spar in the airfoil did not contact with inner skin. This is an error during assembling models. The above improvements relates to only design not assembling. There was no need to change any fluid-dynamics design of X-21. The flight impression of X-21 with original design are as follows;

angle just after releasing X-21 is very steep and good tension can be obtained. However enough tension of launching string is needed, otherwise X-21 can not have enough buoyancy just after release. The response of elevator is sharp enough when carbon rod is used. After launching, X-21 did not lose speed and obtained enough height which is the same as Estrella. The airfoil did not deflect while launching. Therefore the stiffness of airfoil seems to be enough.

Thermal duration task: Since the wing area of X-21 is smaller than F3B fighter with normal aspect ratio, it was thought that X-21 was not good at thermal duration. Contrary to expectations, the flight performance is very good in Task A. The response seems to be excellent. In case of the profile MH32, sailplane tends to have its head down in the high speed range. As for X-21, such tendency is very mild. It may be because of washout of airfoil of X-21. Task B by X-21 can be expected. Gliding ratio seems to be very high, that is, long foot.

Speed task:
A remarkable characteristics of X-21 is that its attained height is very high even without ballast. When X-21 dives from high altitude, acceleration is very comfortable, the speed decrease very little even at final lap of Task C. X-21 is algate, very rapid. Though the elevator response was a little mild, when aluminum rods are used. In other words, flight curve at turn of Task C might become bigger circle. But after replacing rods by carbon rods, its response was extremely improved. Negative displacement at Task C maybe 1-2mm enough from neutral.

Anyway, X-21 is cheap but excellent F3B fighter.

Standard trim and How to Make X21

Basic trim are shown here.

Detail infromation for assembling X-21 is available in the homepage of Mr. Kanoh. (Sorry. It is writen only in Japanese. But beautiful pictures will give you some idea for you.)


Since X21 was veiled out and its production began August 2001, there are very new number of X21 in the world. The pictures of the first X21 in Japan is available, if you click here.

Updated on 17 Aug. 2002.