Saqr ConsultingDesign sheets
Saqr Consulting

Parasite Drag Build-Up

Wetted area · skin friction · form factor · interference · add-ons
Sheet 07
A file you keep, not an account. Nothing is stored on this site.
Reference
dm²
km/h
Drag coefficients are referred to wing area, so both sheets that consume this figure must use the same one. Speed matters because skin friction falls as Reynolds number rises — this is not a coefficient that holds still.
Wing
mm
% t/c
%
Only the wing in the airflow has wetted area. The part buried in the fuselage does not, though the junction it makes there is charged separately below.
Sets how much of each surface keeps a laminar boundary layer. Laminar skin friction is roughly half the turbulent value, so this is worth more than it looks — and it is also the figure a scruffy build loses first.
Interference. Two clean surfaces meeting badly cost more than either does alone.
Tail
% of wing
mm
% t/c
Tailplane and fin together. The short chord puts the tail at a lower Reynolds number than the wing, so its skin friction coefficient is higher.
Fuselage
mm
mm w
mm h
Maximum width and height. A long thin fuselage carries more skin but less pressure drag; a stubby one the reverse.
Sets the fuselage form factor, the markup a body pays for not being a flat plate. From Raymer's published method, applied here well below the Reynolds numbers it was validated at.
Add-ons
off
mm ⌀
mm w
Drag coefficients on frontal area: a bare wheel 0.45, faired 0.30, fully panted 0.15. Compiled figures, not measurements — a wheel sitting in the wing's wake does worse than any of them.
off
mm
mm thick
A round wire has roughly five times the drag of a streamlined section of the same thickness. Legs are usually worse than the wheels they carry.
cm² drag area
Antennas, exposed horns and linkages, camera pods, cooling air, hinge gaps. Drag area is the item's frontal area times its drag coefficient. A 5 mm round antenna 100 mm long is about 0.55 cm²; a plain flat plate is roughly 1.2 times its own area.
Markup & air
What you would have guessed on Sheet 04 without this sheet. The comparison below is against this, not against whichever category happens to sit nearest the answer — that would agree by construction and tell you nothing.
%
Wrinkles, tape seams, screw heads, misaligned hinges, a covering iron's mistakes. Full-size practice adds 2–5%; a model is not built to full-size tolerances and 10–25% is more honest. Read the note under the comparison before settling on a figure.
m ASL
°C
% RH
Only through the Reynolds number, and weakly. Hot thin air raises viscosity and lowers density, so Reynolds falls and the friction coefficient creeps up.
Parasite drag coefficient
—
Enter a wing area and a cruise speed.

What this sheet works out

Where the drag comes from

Stacked bar of parasite drag by component A single bar divided by component in proportion to what each contributes to the parasite drag, widest segment first. The component table below gives every segment as a number and a percentage.

Against the categories the other sheets offer

Where this build-up lands against the airframe cleanliness categories used on Sheets 04 and 06 A horizontal scale of parasite drag coefficient with the named cleanliness categories laid along it, from a clean moulded airframe at the low end to a draggy one at the high end. A marker shows where this build-up falls, so you can see which category to pick on Sheets 04 and 06 rather than guessing.

How these are worked out — the equations behind this sheet, their sources, and what each one assumes.

Component by component

Where people go wrong

The errors this method actually invites, and what it does about each. Every one of them is already a warning somewhere — in a note that appears when your own figures trip it, in the scope statement, or in the equations behind the page. Collected here so you can read them before you need them.

Taking the total as the drag. A component build-up reliably lands below measured drag, and more so at model scale — 24% under on the default trainer. The sheet asks which category you would otherwise have guessed and reports the gap against that, rather than against whichever category happens to sit nearest the answer, which would agree by construction and tell you nothing.

Forgetting the legs. The landing gear is typically a fifth of the total on a trainer, and the legs usually cost more than the wheels they carry. Round wire is 1.10 against a streamlined leg's 0.20 for the same job.

Using it below a Reynolds number of 100,000. A laminar separation bubble forms on most sections down there and adds drag no flat-plate correlation knows about. The sheet warns when the wing falls below it; the warning is not decoration.

Expecting the form factors to be about models. The friction correlations are exact for a flat plate and approximate for everything else, and the form and interference factors were fitted to full-size aircraft. They are the best available, which is not the same as being right here.