Glossary of Aeromodelling terms
I had an email suggesting that a glossary of terms used in aeromodelling would be a useful addition to the site, so I thought I'd give it a go.
It's neither exhaustive nor cast in stone - suggestions for new additions and edits are welcome.
This is a water-based adhesive usually with a creamy yellow colour that is similar to PVA, but easier to sand, stronger
and more heat resistant. This is my glue of choice for building balsa airframes as you get plenty of working time to adjust
joins before it sets and it doesn't tend to pull or distort delicate structures. When glueing end grain, such as on cross-pieces,
it is best to pre-glue the ends so that the adhesive is drawn into the grain before applying the final glue layer. My favourite brand
of aliphatic is Titebond I with the red label. Titebond II has better water resistance but is more rubbery to sand.
This is the negative angle of the wings relative to a horizontal line when a model is viewed from the front (i.e. the wings droop).
It is usually defined on a model plan as the distance the wingtip is below the root rib, though an angle in degrees can also be specified.
Anhedral is usually only seen on models with swept wings where the full size aircraft also had anhedral. Anhedral usually adds to the lateral
instability of a free flight model, but you can get away with it for some jets because a swept wing has a positive dihedral effect offsetting the anhedral.
My Martin XB-51 is a case in point.
A cellulose based adhesive containing solvents which evaporate fairly quickly, so giving a quicker grab than typical water based adhesives.
Pre-gluing of joints gives greater strength - especially with end grain. Joins can be released later by applying cellulose thinners, which is
This is an alternative to dope for sealing tissue. It does not shrink the tissue, just seals the surface.
It doesn't need thinning before application and I find two brushed coats are needed to get the best surface finish.
According to the internet it is a solution of cellulose nitrate in pentyl acetate or a similar solvent.
Centre of Gravity (balance point)
For a free flight model to fly successfully the balance point has to be in the correct position. If you are building from a kit or plan, the centre of gravity
should be marked, so make sure the model balances close to this point before attempting test glides. With rubber powered scale models you
will almost always need to add some nose weight. If the centre of gravity position is not shown on the plan, around 1/3 of the wing chord,
measured from the leading edge, is a good starting point. Non-scale rubber duration models often have a centre of gravity further
back than this as their airframes tend to be inherently more longitudinally stable.
Cracked rib wing
See "Stringered Wing"
Cyano or cyanoacrylate adhesive
Also known as superglue (UK) or crazy glue (US) this adhesive is basically an acrylic monomer which forms a bond by polymerising
in the presence of moisture. Bonding is very rapid and it sticks to skin better than pretty much anything else! There are many different types,
including thin (wicking), gel, foam safe and toughened versions. Kicker sprays can be bought to give instant bonding - useful for field repairs.
Many modellers use CA to build their models as it sets so quickly, but I prefer to keep it for specific applications and repairs.
For example, CA gel is very useful for anchoring rigging wires.
This is the positive angle of the wings relative to a horizontal line when a model is viewed from the front. It is usually defined on
a model plan as the distance the wingtip is above the root rib, though an angle in degrees can also be specified. Most free flight model
aircraft require some dihedral for lateral stability and low winged designs generally require more dihedral than high wingers.
The term "domestic tissue" can be applied to several types of tissue paper, many of which are not specifically created to
be used on model aircraft. Some work better than others, so if you see some likely looking sheets in a craft store, best to try them out on
an old model or test frame before using it on your latest creation. Generally these papers are heavier than Esaki tissue and often have a
more open weave. Wet strength is also variable, as is the water-fastness of the colour. Some suppliers of modelling products have their
own range of domestic tissue which are proven to work well on free flight models - for example Easy Built Models "Lite" tissue available
in 31 colours. SAMS Models also did their "Superlite" tissue in a large range of colours.
Dope is a traditional cellulose finishing material for tissue or fabric. It is still used to seal fabric on full size aircraft,
as it was in world war 1.
Various types are still available from model shops:
Nitrate dope. The traditional stuff I used back in the 1960's. This has a shrinking action to tauten tissue as well as sealing it
This is what I use to apply tissue with now, applied to the edges of the balsa frame and reactivated by flooding thinners through the tissue.
Butyrate dope. This shrinks rather less than nitrate dope but apparently keeps on shrinking for longer. It is less flammable than Nitrate dope.
Non shrinking dope. This is nitrate dope which has had a plasticiser added to it to stop the shrinkage as it dries. I do all my tissue
shrinking with steam and/or water before applying the final sealing coat.
For modelling purposes it is required to thin dope 50/50 with cellulose thinners (lacquer thinners in the US) before applying to tissue.
For tissue with a more open weave, like Modelspan, several coats may be needed. For Esaki tissue I only use one coat and simply brush it on.
This refers to the downwards tilt of the propeller shaft at the front of the model. Most plans show
two or three degrees built in as a starting point, but how much you need depends to a large extent on the model design.
When trimming a model if you find your model does not stall at a low number of winds, but then stalls when you add more power, then
that is an indication that you need to increase downthrust. The simplest way to do this is to shim the nose block with a piece
of thin ply and then see if the stalling disappears.
A specific brand of Japanese tissue noted for its light weight, excellent wet strength and smooth surface.
The tissue shrinks quite strongly and has a definite grain. A single coat of 50/50 thinner dope
gives an excellent surface for a painted finish. Various colours were produced, including white, red, yellow, red, green,
black and blue. Sadly, this tissue is no longer produced.
Also known as a Gurney Flap, this is named after the U.S Racing driver Dan Gurney. On a racing car it is a small vertical strip
mounted at the rear of a wing, protruding upwards to give increased downforce without the drag associated with a more steeply angled wing.
On a model aircraft, the strips are usually balsa sticks glued to the outer trailing edges of a wing. Glued under a wing
trailing edge the strips give increased lift (similar to gluing on a trim tab bent down). Glued above a trailing edge, the strip
will push the wing down. They can also be used on the fin trailing edge instead of angling the rudder. Gluing a strip on the left
side of the fin trailing edge will give the same effect as left rudder. Gurney strips are helpful in that fine tuning of the trim is
relatively simple - to reduce the effect of a strip, it can simply be shortened. To increase the effect, it can be made longer or deeper.
My most frequent use of Gurney strips is to place them under the left wing trailing edge to hold up that wing when a model is circling left
and hence prevent it spiralling in.
Incidence (of wing)
This refers to the angle of attack of the wing relative to a horizontal reference line.
Putting a number on this is complicated by the shape of the wing section. The simplest way is to measure the angle
of the flat bottom of the wing rib, however if the wing section has a raised leading edge, the effective incidence
could be considered to be a line drawn from the trailing edge to the middle of the leading edge. My rule of thumb for a
free flight monoplane would be to have 3 degrees positive for the wing and zero for the tailplane. However, you could
equally well have zero on the wing and minus 3 degrees on the tailplane - it's the relative incidence between the wing
and tailplane that is important (see "longitudinal dihedral")
This is a method of producing neat curved outlines for items such as wings and tail surfaces. A series of thin strip balsa (usually between three and four)
are soaked in hot water, then laminated with a suitable adhesive around a curved form and allowed to dry. The process is detailed
This is the incidence of the wing in relation to the tailplane when the model is viewed from the side. So, for example if the wing had an
incidence angle of plus two degrees and the tailplane minus one degree, the longitudinal dihedral would be plus three degrees.
This was the standard model shop tissue when I was growing up - it was available in lightweight and heavyweight versions in a variety of colours.
It is quite a soft tissue with a more open weave than Esaki and is forgiving when covering double-curved surfaces. To properly seal the surface of the tissue
you need several coats of dope. Mike Woodhouse still sells this tissue, but only in white.
Delicate structures on balsa models, such as tailplanes and fins, can be distorted out of shape when the tissue is shrunk, especially with Esaki
tissue which shrinks quite strongly when sprayed with water or exposed to steam. To counteract this, tissue can be pre-shrunk before applying to the model.
There are various ways of doing this - my preferred method is to stick a piece of tissue onto a wooden frame using a glue stick, then waft this
over the spout of a boiling kettle. This removes some of the shrink from the tissue and leaves it drum tight on the frame. Then cut the tissue
from the frame and apply as normal.
A water based wood glue - polyvinyl acetate. Colour is white but dries clear. Bonds well to balsa wood but doesn't sand well because it dries
"rubbery". Often used in schools for craft activities.
A lightweight but torsionally stiff type of wing design developed by Dave Rees, one of the USA's outstanding free flight scale
modellers, sadly no longer with us.
Once a model has been covered with tissue it will need shrinking, unless the covering has been applied wet, when it will shrink as it dries out.
Shrinking can be achieved by spraying the tisue with a water mist from a spray bottle, or by holding the covered component over a the spout of a boiling kettle.
For wings and tail surfaces, they should be held flat while the tissue dries to stop them warping. To get reduced shrinkage for delicate parts,
rubbing alcohol can be used as shrinking medium instead of water. Alternatively, pre-shrink the tissue before application.
This refers to the angling of the propeller shaft to the right or left relative to the centre line of the airframe.
For most models the prop rotates clockwise when viewed from the rear of the model. The model reacts to the torque of the motor by trying to
rotate the model anticlockwise when viewed from the rear, that is, into a left bank. The addition of right thrust to the prop, for example by
shimming the nose block, counteracts this tendency and reduces the left bank. The more powerful the motor, the more side thrust may be needed.
From my own experience, electric powered models tend to need more side thrust than rubber models - probably because of their higher revs with a smaller prop.
Stringered wing (also known as cracked rib wing)
A type of lightweight wing design that does away with conventional ribs, instead using at least one full depth spar and balsa sticks.
This results in a facetted wing section where the ribs do not show through the tissue, you only see the wing spars.
I tend to use this construction for models of prototypes with metal skinned flying surfaces. For fabric covered types
I would use conventional ribs. For an example of a cracked rib wing, see the PZL P.24 plan here.
The process of adjusting elements of a free flight model in order to achieve a stable and reproducible flight pattern. Typical steps
could be adding nose weight, adjusting the angle of hinged control surfaces, changing the thrust angle by shimming the nose block and adding trim tabs to the flying surfaces (see below).
This is a piece of acetate or cardboard attached to the trailing edge of a flying surface of a model which can be bent one way of another to
change the trim. Examples are a rudder trim tab attached to the back of the fin (useful if the model does not have a hinged rudder) and a
tab attached to a wing trailing edge out towards the wingtip. This latter can be helpful in preventing a wing dropping during a turn.
For example, if a model is turning left and you want to keep the left wing up, then you can either add a tab to the right wing trailing edge
bent upwards (also called a drag tab) or a tab to the left wing training edge bent down, to increase the lift on that side. Gurney tabs can
be used instead of trim tabs to give the same effect.
The opposite of Wash Out (see below) - the wing has more incidence at the tip than it does at the root.
When a model wing has wash-out built in, it means the incidence at the wing tip is less than the rest of the wing.
A slight twist is given to the structure as it nears the tip so that the trailing edge is raised. The twist
can start from anywhere between half way and 2/3 of the way from the root to the tip. I tend to put in wash-out at the
tissue shrinking stage by weighting down the wing on a flat surface with a shim at the trailing edge near the wingtip while
the tissue is drying. Wash-out helps to prevent tip stalling and is particularly useful on aircraft designs with tapered wings.
Differential amounts of wash-out on left and right wings can be used as a trimming aid, for instance on a model turning left,
less wash-out on the left wing will help to keep the wing up in the turn.