SR X250 Assembly Instructions Manual

X250tm Assembly Instructions ©1999

Introduction

You are in for a real treat. You’re going to find that your new
X250 is one of the fastest building and best flying aircraft you’ve ever
owned. Nothing has been taken for granted. If you follow our
instructions, you’ll be amazed at how fast you’ll have your X250 in
the air.
We’ve photographed just about every step in completing the
X250. The notes that accompany each photograph will give you a
“heads up” about any tricks or possible problems you could run into
while completing that particular step. Please read the instructions!
We used thin, fast setting, CA glue in assembling all of the X250
prototypes. If we recommend a different glue for a particular step,
we’ll mention it in the notes.
Although any small radio system can be used, we’ve specifically
set up the X250 for the very popular Hitec HS-60 servos and the
Hitec “555” or “Super Slim” Series sub micro receivers. These servos
and receivers will work with any brand of transmitter and we strongly
recommend them. If you can’t find them locally, we always keep
them in stock at SR.
Finally, you’ll find that a 5/64” Ball Driver and a Higley
Trimmer will really speed things up. Both are available from SR if
you don’t already have them.
On the following page you’ll find detailed technical
specifications and power system recommendations.

Preparing the parts...

We’ve included a master parts sheet that identifies each part in
the kit and tells you where the part is located. We’ve deliberately left
all of the MicroLaser Cut parts for the X250 in their sheets to protect
them. Use your modeler’s knife and a straight edge to finish the cuts
and release the parts. Just about all of the cuts will be with the grain
to make it easier for you. You can either cut out all of the parts before
you begin or leave them in their sheets until you’re ready for them.
It’s up to you, but you’ll be less likely to lose a part if you leave it in
its sheet until you’re ready for it.
In most cases, the SR MicroLaser Cut process leaves the balsa
parts with a honey colored edge that needs no sanding. Unfortunately,
the glues used in making plywood tend to glaze the edges of the
plywood parts when they are laser cut. We use a special plywood that
is designed for laser cutting, but we recommend that all plywood
parts be lightly sanded along their edges before gluing the part in
place. In addition, like spruce, plywood parts tend to have an oily film
on their surfaces which prevents CA glues from taking a good hold.
For this reason, we recommend that all plywood parts be given a light
sanding on all surfaces before you glue them in place.

X250tm Assembly Instructions, Page 1

Technical Specifications...

In spite of the X250’s small size, it’s probably one of the
most carefully designed model aircraft ever kitted. Literally
hundreds of hours of computer time were spent optimizing the
design. Rather than just designing an aircraft, we designed an
entire system. There’s no point in designing an aircraft that
requires a motor, gear ratio, battery, or prop that doesn’t exist.
Instead we evaluated dozens of combinations of components
to come up with the optimum, complete system.
We know that some of you will be tempted to change
some of the components in the power system. We suggest that
you don’t. Our target was an aerobatic aircraft that would
easily do consecutive loops yet still give you 7 to 9 minutes of
aerobatic flight and 12 to 15 minutes of sport flying. The
X250 fulfills this goal. If you change any of the components,
you may gain a small advantage in one area of performance,
but you’ll definitely be losing performance in another area.
Here is our definition of Optimum Per formance:

Optimized Performance
Aircraft SR X250 (aileron version)
Wing area 266 square inches

Wing Span 36”
Aspect Ratio 4.3:1
CG Limits 2” to 2.75”
Airfoil Computer Optimized by SR
Typical Wt. w/ 10 cell pack 24 oz.
Typical Wing Loading 13 oz. / sq. ft.
Stall Speed 13 MPH
Max. Speed 40 MPH
10 Cell Current Draw 10 Amps
Thrust 18 oz.
Climb Rate 900’/min. @ 23 MPH
Flight Time, Aerobatic 7 to 9 min.
Flight Time, Sport 12 to 15 min.
Minimum Sink Rate 2.761 ft./sec.
Maximum Glide Ratio (L/D) 11.595
Best L/D Speed 25.157 MPH

The following table lists the specific equipment we’ve
evaluated and tested to yield the above performance data. You

can purchase these items locally, or SR has several packages
available which will save you time and money.

Optimized Components
Aircraft SR X250 (aileron version)
Prop Graupner 9x5 Slim, Fixed
Motor Graupner Speed 400, 7.2V
Gear Ratio 2.33:1
Speed Control Jeti 350
Cell SR 500 Max Series
Cell Count 10
Pack Shape Rectangular, 5x2
Connector Sermos
Alternative Prop Graupner CAM 9x5, Folding
Receiver Hitec, 555
Servo Hitec, HS-60
Covering Material Goldberg, UltraCote Lite

Why the Jeti 350 Speed Control?
The Jeti 350 is overkill as far as current draw goes.
However, it has one feature we really like. Its BEC circuit is
designed to handle up to 4 servos rather than the usual 3. As
we’re using 3 servos, we liked the extra safety margin. In
addition, it’s one of the few speed controls that will allow you
to turn off the Brake function.

Why the SR 500 Max Series cell?

With a capacity of almost 600 mah, yet still having a very
low internal impedance, this cell is optimum for Speed 400
applications. At only .7 oz per cell it’s hard to beat.

Which prop, folding or fixed?
Originally, we recommended the CAM 9x5 folding prop.
The advantage of the folding prop was that it would fold back
rather than break if it hit the ground. However, after over
1,000 flights on our various test aircraft, we’ve come to the
conclusion that the fixed Slim prop gives a slight performance
edge for aerobatic flight. You wouldn’t be wrong if you
decided to use the CAM folding prop, but our first choice
would be the Graupner 9x5 Slim prop. If you do decide to use
a folding prop, be sure you turn off your speed control’s brake
so that the prop blades won’t fold back when you shut down
the motor in a stall turn.

X250tm Assembly Instructions, Page 2

Warning, Warning, Warning!!!

This is the single most important step in building your X250. The
entire trueness of the fuselage depends on you properly aligning the
fuselage doublers to the sides of the fuselage before you glue them to
each other. Take your time and don’t rush this step even though it
may seem very simple. DON’T MAKE TWO LEFTS!!!
There is a right and left fuselage side and a right and left doubler.
You can identify the left fuselage side and doubler by the cutout for
the speed control switch. Carefully align the doubler with the fuselage
side. Hold them together with clothes pins and check that they are
absolutely lined up with each other in the nose, wing saddle, and
fuselage top areas. When you’re satisfied with their registration, glue
them together. Glue the doubler around its edge and around all of the
cutouts in the doubler.

Trial fit the left motor mount and firewall...

Insert, but don’t glue, the left motor mount and firewall into the
left doubler. Note that the lower right corner of the firewall (viewed
from the front) has 3 holes for mounting the nose gear!
The forward, bottom edge of the firewall needs to be reshaped.
When the 1/8” bottom sheeting is added to the fuselage, it will rest on
the bottom of the firewall. If you locate and trial fit the bottom
sheeting, you’ll see how much of the bottom, forward edge of the
firewall will have to be sanded away to form the proper angle.

Align

Left doubler and side
have the switch cutouts

Align

Forward bottom edge has to

be sanded at an angle to

match bottom sheeting

Align

Use the alignment jig...

We’ve given you a plywood alignment jig to keep everything
aligned properly. The jig has three slots. The center slot is used with
plywood parts. The largest slot is for aligning the vertical stabilizer
after it’s covered and the narrowest slot is used with 3/32” parts. The
outer corners of the jig are cut at exactly 90° which will come in
handy later.
With the firewall and motor mount firmly seated in their recesses
in the doubler, use the alignment jig to make sure the firewall is
square with the doubler. Tack glue the firewall and motor mount to
the doubler in a corner of each and let the glue set. Remove the jig
and then glue the motor mount and firewall in place along their entire
edges. Don’t forget to glue the motor mount to the firewall too.

Bottom sheeting

will go here

3 Holes!

Alignment

Jig

Tack glue

X250tm Assembly Instructions, Page 3

Front battery pack mounting plate...

Use the alignment jig to make sure the front battery pack
mounting plate is square to the side of the fuselage and that it is
firmly seated in the recess in the doubler. Tack glue the mounting
plate in place. When the glue has set, remove the jig and finish gluing
the mounting plate in place.

Rear battery pack mounting plate...

Use the alignment jig to position the rear battery pack mounting
plate in place. You don’t have full support along the bottom of the jig
so be careful. A double check that the alignment is correct is that the
rear motor mounting plate should line up with the front motor
mounting plate.

Front battery pack mount

Rear battery pack mounting plate

The front and

rear plates should

line up with each

other

Servo mounting tray...

The servo mounting tray is setup for Hitec HS-60 servos. Check
to see if the cutout is properly sized fo r the servos you’re going to
use. We’ve left plenty of extra material in the plate so just open up the
hole until it fits the servos you’re going to use.
Use the alignment jig to hold the servo mount perpendicular to
the side of the fuselage while the glu e sets. Again, tack glue first.
Then remove the jig and complete gluing the servo mount in place.

Be careful! No support

Servo mounting tray

X250tm Assembly Instructions, Page 4

The left side is done...

Here’s what the left fuselage side should look like at this point.
The left motor mount, firewall, battery pack mounting plate and servo
tray are in place.

Attach the right fuselage side...

Following these instructions will assure a straight and true
fuselage without any twists.
Dry!!! fit the right side of the fuselage. Insert the right motor
mounting plate, firewall, battery pack support plate, and servo tray
into their corresponding recesses in the right side doubler. Make sure
that the parts are firmly seated in their recesses and that nothing is
preventing them from completely seating.
Before gluing any parts together, use a weight to hold the parts
in place and use the right angle jig to check how closely the two
fuselage sides line up at the tail. Put the jig flat on the building
surface and see how close each of the fuselage sides comes to the jig.
If you were careful about lining up the doublers with the fuselage
sides and all of the parts were firmly seated in their recesses before
you glued them into the left fuselage side, the fuselage sides should
line up very closely with one another.
If the fuselage sides are close to being square (within an 1/8” or
so), go ahead and glue the right side and doubler in place. If the two
are way off (1/4” or more), try to sand and adjust the ends of the parts
and the recesses in the doubler until you can align the two fuselage
sides with one another. They should be within 1/8” of being square.
However, if you’re within an 1/8”, just pull the fuselage sides into
perfect alignment before you glue the right side in place.

Left fuselage side

Weight

Right fuselage side

Don’t Glue Anything Yet!!!

Right angle jig

With one fuselage side
flush up against the
right angle jig, the other
fuselage side should be
within 1/8” of the jig.

X250tm Assembly Instructions, Page 5

1” Wide ply plates...

There are three, 1” wide plates that you should glue in next.
Before you do, lightly sand them to remove any oil or glazing from
their surfaces.
The upper rear plate is for securing the hatch and the lower two
plates are to reinforce the fuselage and provide mounting points if you
should choose to add floats for flying off of water at a later date. Do

not glue the upper front plate in! It will be glued to the hatch rather
than the fuselage top!

Do Not glue this
plate into place!

Glue rear hatch mounting
plate into place

Glue these two plates
into place.

Glue tail former into place...

Hold the sides of the fuselage together and tape the tail former
into place. Make sure you don’t glue in the tail former upside down
as there’s a right way and a wrong way! If you’ve got it right, the
former will be flush with the top and bottom of the fuselage sides.
With the tape holding the tail together, check the alignment of the
fuselage. Either lay the fuselage over the top view on the plans or
draw three parallel lines on your building board. The tail former
should be centered. If need be, pull it to the right or left to center it
and glue it in place. You’ll be able to correct minor misalignments
when you glue the top and bottom sheeting in place, but that’s no
reason to not make it as right as possible now.

X250tm Assembly Instructions, Page 6

Tail former should be

centered and not

upside down!