Building Tips
This page is a place to compile tips, hints and tricks to make building
R/C Aircraft easier and more fun.
If you have any additional items to add, please e-mail the Website Amdinistrator
Center of Gravity (CG) Calculator
|
This will give the wings Center of Gravity (CG) as measured from the leading edge of the Root Cord.
The Equations
|
For S<>0 Forward or Rearward Swept
|
For S=0 Straight Leading Edge
|
| C |
= (S(A+2B)) / (3(A+B)) |
C |
= Zero |
| MAC |
= A-(AC/S)+(BC/S) |
MAC |
= A-(2(A-B)(0.5A+B) / (3(A+B))) |
| d |
= YC / S |
d |
= (2Y(0.5A+B)) / (3(A+B)) |
|
For all Values of S
|
|
CG
|
= %MAC B.P.*(MAC) + C
|
|
|
This will calculate the wings Mean Aerodynamic Cord (%MAC) for a given position of the CG.
The Equations
|
For S<>0 Forward or Rearward Swept
|
For S=0 Straight Leading Edge
|
| C |
= (S(A+2B)) / (3(A+B)) |
C |
= Zero |
| MAC |
= A-(AC/S)+(BC/S) |
MAC |
= A-(2(A-B)(0.5A+B) / (3(A+B))) |
| d |
= YC / S |
d |
= (2Y(0.5A+B)) / (3(A+B)) |
|
For all Values of S
|
|
%MAC B.P.
|
= ((CG-C) / MAC)*100
|
|
|
Adding, Removing or Shifting Weight (Effect on CG)
The following formulas are used to determine CG location after adding, removing or shifting weight.
The best results occur if you use weight in ounces and distance in inches.
For reference:
Moment = weight (in ounces) x distance from CG (in inches)
Original Moment = weight of model (in ounces) x the CG location (in inches)
|
|
Adding Weight (Effect on C.G.)
Original moment + added moment
------------------------------------------------------- = new CG
Original weight + added weight
|
Removing Weight (Effect on C.G.)
Original moment - moment removed
------------------------------------------------------------ = new CG
Original weight - weight removed
|
|
Shifting Weight (Effect on C.G.)
Original moment ± moment change
----------------------------------------------------------- = new
CG
Original weight
|
|
Aircraft Trimming Chart
Notes:
1) Trim in calm conditions.
2) Make multiple tests before making adjustments.
3) If changes are made, go over previous steps and verify or readjust as necessary.
| To Test For |
Test Procedure |
Observations |
Adjustment |
| 1. Controls neutral |
Fly model straight and level |
Adjust the transmitter trims for hands off straight and level flight |
Adjust clevises to center transmitter trims |
| 2. Control throws |
Fly model and apply full deflection of each control in turn |
Check the response of each control |
*Aileron Hi-rate: 3 rolls in 4 seconds.
*Lo-rate: 3 rolls in 6 seconds.
*Elevator Hi-rate: to give smooth square corner.
*Lo-rate: to give a loop of approx. 130' dia..
*Rudder Hi-rate: approx. 30-35 degrees for stall turns.
*Lo-rate to maintain knife edge flight. |
| 3. Decalage |
Power off vertical dive. Release controls when model is vertical (elevator must be neutral). |
A. Does the model continue straight down?
B. Does the model start to pull out (nose up) ?
C. Does the model start to tuck in (nose down)? |
A. No adjustment
B. Reduce incidince
C. Increase incidince |
| 4. Center of gravity |
Method 1: Roll model into near vertically banked turn.
Method 2: Roll model inverted. |
A. Nose drops
B. Tail drops
C. Lots of down elevator required to maintain level flight
D. No down elevator required to maintain level flight, or model climbs |
A. Add weight to tail
B. Add weight to nose
C. Add weight to tail
D. Add weight to nose |
| 5. Tip Weight (course adjustment) |
Fly
model straight and level upright. Check that aileron trim maintains
wings level. Roll model inverted, wings level. Release aileron stick |
A. Model does not drop a wing
B. Left wing drops
C. Right wing drops |
A. No adjustment required
B. Add weight to right tip
C. Add weight to left tip |
| 6. Side Thrust |
Fly model away from you into any wind. Pull it into a vertical climb (watch for deviations as it slows down). |
A. Model continues straight up
B. Model veers left
C. Model veers right |
A. No adjustment needed
B. Add right thrust
C. Reduce right thrust (move thrust line left) |
| 7. Up/Down Thrust |
Fly
model on a normal path into any wind. Parallel to strip, at a distance
of around 100m from you (elevator trim should be neutral as per test
No.3). Pull into a vertical climb & neutralize elevator. |
A. Model continues straight up
B. Model pitches up (goes towards top of model)
C. Model pitches down (goes towards bottom of model) |
A. No adjustment needed
B. Add down thrust
C. Reduce down thrust |
| 8. Tip Weight (fine adjustment) |
Method 1: Fly model as per test No.6 and pull it into a reasonably small dia. inside loop (1 loop only).
Method 2: Fly the model as per test No.6 and push it down into an outside loop (1 loop only & fairly tight). |
A. Model comes out with wings level
B. Model comes out right wing low
C. Model comes out left wing low |
A. No adjustment needed
B. Add weight to left tip
C. Add weight to right tip |
| 9.(a) Aileron Differential Method 1: |
Fly
the model towards you, before it reaches you, pull it up into a
vertical climb. Neutralize controls, then half roll the model |
A. No heading changes
B. Heading change
opposite to direction of roll commands (ie. heading veers to models
& your left after right roll).
C. Heading changes in direction of roll command |
A. Differential OK
B. Increase differential
C. Reduce differential |
| 9.(b) Aileron Differential Method 2: |
Fly the model on a normal pass and do 3 or more rolls |
A. Roll axis on model center line
B. Roll axis off to same side as roll command (ie. right roll, roll axis off right wing tip)
C. Roll axis off to opposite side of model as roll command |
A. Differential OK
B. Increase differential
C. Reduce differential |
| 10. Dihedral |
Fly
model on normal pass and roll into knife-edge flight, maintain altitude
with top rudder (do this test in both left & right knife-edge
flight) |
A. Model has no tendency to roll out of knife-edge flight
B. Model rolls in direction of applied rudder
C. Model rolls in opposite direction in both tests |
A. Dihedral OK
B. Reduce dihedral
C. Increase dihedral |
| 11. Elevator alignment (for models with independent elevator halves) |
Fly
model as in test #6 and pull it up into an inside loop. Roll inverted
and repeat the above by pushing it up into an outside loop |
A. No rolling tendency when elevator applied
B. Model rolls in same direction in both tests
C. Model rolls in opposite direction in both tests |
A. Elevators are in correct alignment
B. Elevator halves misaligned. Either raise one half or lower the other half
C. One elevator half
has more throw than the other (model rolls to the side with the most
throw). Reduce throw on one side or increase throw on the other |
| 12. Pitching in knife-edge flight |
Fly model as per test no. 10 |
A. There is no pitching up or down
B. The nose pitches up (the model climbs laterally)
C. Nose pitches down (model dives laterally) |
A. No adjustment needed
B. Alternate cures:
1. Move the CG aft
2. Increase wing incidence
3. Add down trim to ailerons
C. Reverse the above |
|
Installing control horns
When installing
control horns onto control surfaces the screwdriver invariably slips.
The result is a hole poked into the covering material or a gouge in the
balsa. There is a simple tool you can make that will eliminate this
damage. Take a small piece of thin plywood and cut a rectangular opening
in it just slightly larger than the base of the control horn. Place
this opening around the control horn base before tightening the mounting
screws. Now when the screwdriver slips there will be no damage to your
new aircraft! (Courtesy Fred H., Derby Radio Control Club, Derby Kansas)
Mark Hinges
When using CA
hinges use a marker to draw a black line across the middle of the hinge.
This way you can tell if the hinge is being pushed into
the wing when you put on the aileron. I have had some hinges do this and
end up with a sixteenth of an inch in the aileron and the rest in the
wing, not very strong. If you can't keep the hinge from being pushed
into the wing stick a pin through the middle of the hinge it will not
weaken the hinge at all.
Fiberglass Hint
How do you get
the creases or lumps out of the fiberglass cloth we use to reinforce the
center section of the wing? This method will probably eliminate them.
Prior to applying the fiberglass cloth to the center section of the
wing, take the time to iron it flat with your clothes iron. This will
make it soooooo much easier to achieve a FLAT surface. Next, place the
glass cloth on the center of the wing and tack it down to the surface
with one drop of CyA (each corner, top and bottom).
This may require
that you have to pull the cloth taught, but don't overdo it! Now you
should have the cloth resting smoothly on the top of the wing. Now apply
the resins (or CyA) over the cloth. You may find that after 3/4 of the
cloth is attached that you now have puckers along one edge or the other,
but this is easy to fix. Lift the material where you tacked it to the
wing (remember I said to tack it down, not permanently attach it), pull
taught, and tack it down again. there you have it!
COWL AND CANOPY MOUNTING
Referring to the photo below, glue balsa blocks in
the proper locations desired for mounting your cowl or canopy. Drill an
appropriately sized hole through the cowl and into the balsa block.
Drill out the balsa block to accept Nyrod. Press a short length of Nyrod
in the hole and wick thin CA around the Nyrod. You now have a very
vibration-proof mounting method that will undoubtedly outlast your
model.
Vertical Fin Alignment
To get a fin in
correct alignment with a fuselage, try using thread. Make sure you have
an accurate center mark near the top-front of the fuselage, and
tack-glue a long piece of thread to the top near the nose, a distance
from the centerline equal to half the thickness of the fin. Run the
thread back to the tail, and hold it against the side of the fin. The
thread should touch the side of the fin evenly overall. If it doesn't,
then rotate the fin until it does, then tack glue the fin into place,
reinforcing it later. Last, remove the thread you tack-glued.
Balancing Planes
Here's a good
way to balance airplanes. While building your plane, insert a half-inch
square piece of plywood where the balance point should be. For a low
wing, this should be on the bottom of the wing, and for a high wing this
would be on top of the wing (Note: sometimes something will be in the
way, like a canopy, and you can't use this technique). When the plane is
finished, put a small hook into the plywood and suspend the plane with
wire or string. This way you can check the fore-aft balance AND the
lateral balance at the same time (Note: a low wing will be suspended
inverted).
Fiberglassing Wing Centers
Whenever I
fiberglass a wing center section, I've found it's difficult to get the
fiberglass cloth to lay flat after it's been folded in a bag. Here's two
ways to make this easier: (1)Use thin CA to tack it down. You may
saturate the whole cloth with thin CA, or apply epoxy. On foam wings,
make sure you use CA safe for foam. (2)Give the cloth a light spraying
of 3M Spray Adhesive, then apply it to the wing. I've found this method
to work extremely well, and it's safe for foam. Then apply the epoxy as
usual.
Cutting Dowels Straight
When cutting a
dowel, it's easy to make the cut crooked. To help ensure a nice
90-degree end, especially on larger diameters, try rolling the dowel
into the bandsaw or scrollsaw blade.
Firewall Fuelproofing
Firewalls of
planes are normally coated with epoxy to help prevent fuel and oil
damage to the wood. On planes with no cowling, apply a coat of epoxy on
the firewall after you cover the plane with film covering. Make sure the
film overlaps a little onto the firewall. This way the epoxy seals the
edges of the film covering. Besides, most film adheres better to wood
than epoxy, so that's another plus.
Installing Triangle Stock
For me, triangle
reinforcements have always been difficult to handle due to their shape,
especially if they're coated with epoxy.
Try sticking
your Xacto knife loosely into one end of the triangle. Then lay it on
the bench so that the wide part of the triangle (the hypotenuse) is
against the benchtop. Now apply the epoxy or other adhesive to the sides
that will contact the airframe.
Next, by using
the knife handle, insert the triangle into position in the airframe.
Press down with your finger onto the wide side that has no glue, and
carefully slide the knife out of the piece.
This way you can cleanly install triangle stock, and not get any glue on your fingers.
Rib Maker
Cut two ribs
from 1/16-inch steel. Drill two holes along the center line, one near
the leading edge, one near the trailing edge, for 1/4-inch bolts to pass
through. Make sure both steel ribs are identical.
Use a steel rib
as a template to draw ribs onto balsa sheet. Leave room around each rib.
Cut each rib "block" out of the sheeting, and drill the holes in each.
Assemble all
ribs on the correct length bolts, and sandwich all between the steel
ribs. Using nuts, tighten the assembly down, making sure it's straight.
Now, using a
belt sander (a disk sander will work too), remove the extra wood around
the ribs down to when the steel begins touching the sander. Cut out the
spar notches with a hand saw, and clean them out with a file.
This will make
all the ribs for a wing at once, and they'll all be identical, resulting
in a straight, uniform wing. It can also be used for a tapered wing
(with all the ribs of different size), and bulkheads and formers can be
made using this method too.
Curving Balsa
Get some
ammonia, found in the household section of the supermarket. Put some in a
spray bottle, and spray both sides of balsa sheet liberally. Carefully
bend the sheet to the right shape. You can even tape it to a form, such
as aluminum soda cans, and let it dry. Once dry, it may be used as
turtle-decks, etc.
Addendum:
To soak wood, get a piece of PVC pipe the wood sheet will fit in. Cap
one end, and stand upright. Fill with water (You can't buy pure ammonia,
the stuff on the shelf is low grade, water works fine) and drop in the
sheet and cap the top if it floats out.
Wing-Tail Alignment
Get an old (but
straight) telescopic antenna, the same type as on transmitters. Use it
as an adjustable-length measuring rod to compare critical measurements
on planes during construction. I use this idea to compare the distance
from one wingtip to the stabilizer, and to make sure this distance is
equal on both sides of the plane. This ensures that the stabilizer is
parallel to the wing.
Engine Mount / Nosegear
If you have a
small plane with a very tight engine installation (usually resulting
from a very streamlined cowl), often there's no room for a nose gear
assembly. Try drilling holes through the engine mount to accept the nose
gear wire, and hold it in place with wheel collars. The steering arm
can be placed below the engine, even on the outside of the plane. This
will work with most engine mounts, even the two-piece ones as long as
the engine is rotated 90 degrees.
Servo Blanks
Here's an easy
way to make sure your servos will fit in your plane properly, especially
helpful with scratch-built designs: Take the measurements of your
servos, and make a few from wood, identical to the real ones. This may
be easy if the manufacturer supplies full-size drawings of the servos. I
made my servo blanks from pine blocks, a little plywood for the
mounting hole piece, and a dowel for the motor shaft. These servo blanks
will not only help in drilling the holes to mount servos, but will
assure adequate clearance on all sides. In addition, the dowel is the
correct size to press on an actual servo arm, which will help in
aligning pushrods or cables. Using this method will help keep your real
servos safe and clean during the building process.
Poke a hole in your covering
Gee why would
anyone want to make a hole in your nice new covering job. Well holes for
wing bolts, switches, hatch screw holes, pushrod openings, etc come to
mind. Sure you could cut the hole/opening with a Xacto, or razor blade,
but then you have to adhere the fresh cut covering to the surrounding
wood. The solution; get an old soldering iron tip (pointed preferably)
and cut the hole/opening with it. I use a 25Watt Weller, and it cuts
through the covering with ease, and makes a perfect seal. Once you try
this you won't want to do it any other way. One word of caution, clean
the tip after each cut. I use a wet sponge like that used for soldering,
but use a different sponge as to not foul the clean tip used for
soldering. If you don't clean the tip regularly the burned covering will
cake on, and not only smell really bad, but will inhibit the cut, as
you will not have maximum heat. I thoroughly clean the tip with a wire
wheel after each use. After it completely cools of course.
Hardening Mounting Holes In Balsa
To harden small
holes in balsa in order to better retain wood screws or threads, use
thin CA. Sometimes the CA will get on a finished surface doing this
step. To prevent getting CA on the outside surface, use MicroBrushes. If
you do not have MicroBrushes handy in the shop, you can also use one of
the plastic coffee stirrers cut at a sharp diagonal to form a thin
point. A drop of CA in the cavity at the sharp tip can be used to place a
small amount almost anywhere.
Alignment Of Wings And Tail Surfaces
A very important
task in building an RC model is alignment of the flying surfaces with
respect to the fuselage. Most of the time there is no absolute reference
which will allow you to measure the mounting angle and be certain that
it is square. One method is to mark a point at the nose or tail
(depending on whether you need a reference for the horizontal stabilizer
or the wing) at the center of the fuse and measure to the tips of the
respective flying surface until it is properly centered. If your
aircraft model is in the "bare-bones" stage, using a large heavy
straight-edge can easily create hangar rash even before you have done
the finishing. Here are a couple suggestions to avoid the large metal
straightedge problem.
One is to use a
length of ordinary lamp cord. If you get a piece of it fresh off a roll
and hang it in your shop, with a small weight at the bottom end, for
several days it will be straight enough to ensure accurate measurements.
Put a piece of heavy tape around one end (about the last 3/4" or so)
and use a pin immediately in front of the tape and through the center of
the cord to locate the reference point on the aircraft fuselage. Then
hold the other end at a reference point on each end of the stab or wing
to compare the two measurements. A small piece of duct or masking tape
will mark your measurement on the lamp cord while you make adjustments
to the mounting location of the airframe component.
Another trick is
to use what home builders refer to as a "story stick". This is merely a
small piece of wood (carpenters use a 2 x 4 but that could be as
dangerous as the metal straight edge) possibly a long, 1/2" dowel or
similar sized piece. Drill a hole about 1/2" from one end which accepts a
heavy building pin to use as the locater. Use the other end to mark
wing and stab locations. This can be a good reference tool to ensure
your wing mounting is secure. Mark the dowel with the name of the plane
next to the final dimension to the wing tip and check your model after
several flights.
Locating Engine Mounting Holes
Engine thrust
angle can be affected by small errors in the location of the mounting
holes. To ensure the mounting holes are properly located on the engine
mount, the most important task is marking the holes to match the engine
case lugs. One good way to mark these holes is with a machine screw of
just the right size to pass through the hole in the mounting lug. Cut
the screw so it is just a few thousandths of and inch longer than the
thickness of the lug, and, cut it on an angle so there is a sharp edge
which extends just through the lug. Now, use two small pieces of double
sided tape to hold the engine temporatily in place while you rotate the
cutoff screw in each of the four mounting lug holes. This will leave a
mark which is the exact diameter of, and in the proper location for, the
mounting screws.
The next part is
to get a true center mark for this mounting hole. If you have an
automatic center punch, you may be aware that some of them come with
replaceable tips. Buy an extra tip or two and grind them so they just
fit through the mounting lugs and use them to center punch the holes.
Dust collector
Next time your
wife yells about the balsa dust from sanding steal one of her large
fluffy bath towels and use it to sand on. It not only protects the
airplane from dings, but it will trap a huge amount of dust. When done
sanding fold it carefully then take it outside and shake it out.
Note: see disclaimer above, I am not responsible for what happens when your wife catches you doing this!
Cutting covering
When cutting
sheets of plastic shrink covering nothing beats glass for a surface to
cut on, it will not dull the knife or slow it down when cutting. The
covering material will kind of stick to the glass if the backing is
removed all by itself for easy cutting. You can also use low heat to
make it stick even better for critical cutting. You can use solvent to
put together large panels of covering without it sticking to the work
surface. The best place to find a suitable piece of glass is at the flea
market. Look for an old glass top coffee table. The rectangle ones work
super if you have the room. Stay away from non-safety glass it breaks
too easy…Saw this one somewhere….
Holding canopies
Use electrical
tape to secure the canopy on airplanes. (Striping tape that will hold a
canopy on surprisingly well however the oil always gets under it and
loosens it in time.)
Rib holder
Get a piece of
aluminum 1 or 2 inch right angle that can be found at most hardware
stores. Make sure it is really square (90%) then cut off 1/2 inch wide
pieces.
Next drill small holes in each end about the size of a push pin or T pin.
Use the angles
to hold ribs perpendicular to the building board by putting one on each
side of the rib and then pinning it to the board.
Rotary cutter
A good way to
cut thin balsa sheets is with a rotary cutter made for cutting fabric.
This also works well to cut plastic covering .
Better holes
IF you have to
drill a hole in thin balsa do not try to use a common wood drill it will
usually leave a ragged hole that is the wrong size. Go to the hobby
store and get a piece of thin wall brass tubing the same outside
diameter as the hole you want to drill. Mount the tubing in an electric
drill (preferably a drill press) then hold a piece of sand paper or a
file against the end while turning to sharpen it. Use this to drill your
holes it will cut perfect holes. After cutting several holes you may
have to push out the wood that collects inside the tube with a stiff
wire. If you can find a drill the same size as the inside of the tube
stick in in the top part so the drill chuck does not crush the tube.
Also watch that the tube does not flare out at the bottom and change the
size of the hole. From the instructions for the Great Planes Wing Jig.
Better plans cover
Waxed paper is
the standard covering for plans when building an airplane on top of them
I find that waxed paper does not come in large enough sheets and tends
to stick to the airplane. Last time I built I used some left over
plastic that is used to cover windows. This stuff is made by 3M Scotch
and comes in large 3X5 foot sheets this is just the size of the plans I
was using. the covering will shrink if heated but this is not necessary
as it lays very flat. CA and other glues will not stick to this stuff at
all and it is crystal clear unlike waxed paper. There was even a roll
of double stick tape in the box. The price on the box (Which was several
years old) was $5 this is cheaper then the Great planes plan protector
stuff but I don't think you get as much.
Better screw holes
When you have to
use screws that will be removed often drill the holes for the screws
large enough to allow you to insert a small piece of Sullivan Gold-N-Rod
glue the rod in the hole. The screws can then be removed and reinserted
with out weakening the wood. John does not recommend this for high
stress areas.
Mixing epoxy
When mixing epoxy use an old coffee can lid, after the epoxy hardens just flex the lid and the epoxy will pop off.
Easier plastic bolting
After you cut a
plastic wing bolt stick it in one of those cheep plastic pencil
sharpeners and give it a beveled tip this makes it much easier to insert
the bolt and cleans up the start threads .
Bending wood
Use 409 spray cleaner to bend balsa wood. It also makes a good airplane cleaner for you and the airplane.
T-bar sanders
If you use T
bars for sanding, gluing sand paper on them can be a mess and the stick
on paper at the hobby stores is expensive. I find that stick on
sandpaper made for electric sanders at hardware stores works just as
well, or even better.The only problem is you need to cut it to size.
If you use the
old style T bars the kind that have a thin vertical handle, you will
find putting a piece of masking tape on where you hold it makes it much
easier to hold on to.
Take a file and round the corners of your T bars you will be much less likely to gouge what you are sanding.
Mark Hinges
When using CA
hinges use a marker to draw a black line across the middle of the hinge.
This way you can tell if the hinge is being pushed into the wing when
you put on the aileron. I have had some hinges do this and end up with a
sixteenth of an inch in the aileron and the rest in the wing, not very
strong. If you can't keep the hinge from being pushed into the wing
stick a pin through the middle of the hinge it will not weaken the hinge
at all.
Use for an old sheet
A handy thing to
have around when covering a airplane is a old folded king size bed
sheet. It is soft and heavy enough to hold down a wing when stretching
the covering over the ends or use it to prop up a fuse upside down when
working on it.
Fiberglassing Wing Centers
Whenever I
fiberglass a wing center section, I've found it's difficult to get the
fiberglass cloth to lay flat after it's been folded in a bag. Here's two
ways to make this easier: (1)Use thin CA to tack it down. You may
saturate the whole cloth with thin CA, or apply epoxy. On foam wings,
make sure you use CA safe for foam. (2)Give the cloth a light spraying
of 3M Spray Adhesive, then apply it to the wing. I've found this method
to work extremely well, and it's safe for foam. Then apply the epoxy as
usual.