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OpenWindTunnelAnalysis.html v0.9.36 - 2026-09-22

Open Wind Tunnel Analysis

Analyze and interpret open-wind-tunnel test data.

Choose or drop a JET11 fileJET11 .xlsm or .xlsx workbook, or JET JSON export

Aircraft preview

Load a JET11 file to display the aircraft.
Drag to rotate · wheel to zoom
Objectives
  1. Demonstrate Longitudinal Stability
  2. Assess "Pitch Up" Tendency
  3. Verify Neutral Point Position
  4. Demonstrate Directional Stability
  5. Determine Pitch Trim Position / Control Authority
  6. Determine Yaw Control Authority
Getting Started

Click here for more detailed instructions on how to build your Wind Tunnel Model.


You will need to produce 3 total Wind Tunnel models

  1. Basic: No control surface deflections
  2. Pitch Control: Pitch control surfaces deflected nominally 10 degrees Trailing Edge Up (TEU).
    NOTE: The convention for elevator deflection is + for TED, but we don't want you to be confused... your elevator should be UP!
  3. Directional Control: Rudder deflected 10 degrees Trailing Edge Left (TEL).

Making the Models

  1. Open your *.ntop model.
  2. Position the controls as necessary.
  3. Export the OML to the CAD folder: ./CAD/FILENAME.implicit
  4. Open MakeWindTunnelModel.ntop (which you can download HERE).
  5. Position the Sting/ForceBalance Center at your desired CG.
  6. Export your model in a format readable by the PRUSA 3D-printing software: ./3D_Printing/FILENAME.stl
  7. Here are some templates for your Prusa printing project. Pick the one that corresponds to the printer you want to use:
  8. Get it on the printer!

NOTE:

Enter the denominator to display model stations in inches. If you can't remember your model scale (shame on you), then measure the wingspan in inches, divide that by 12 to get feet, then divide the full-scale wingspan above by that number to get the scale... then hang your head in shame.
Predict \(C_{M_\alpha}\) slope

For a conventional aircraft, the pitching-moment slope relates to static margin by:

\[C_{M_\alpha}=-SM\,C_{L_\alpha}\]

Where Static Margin (SM) is defined:

\[\mathrm{SM}=\frac{x_{NP}-x_{CG}}{MAC}\]

Therefore:

\[C_{M_\alpha}=-\left(\frac{x_{NP}-x_{CG}}{MAC}\right)C_{L_\alpha}\]

Enter your values from JET11 below, then run the code:

Static Margin (SMest)

—

CMα,est

—
Rule of Thumb (RoT) for \(C_{M_{AC}}\)
Rule-of-thumb aerodynamic-center pitching-moment equations for NACA 4-digit, NACA 6-series, and modern aft-loaded high-lift airfoilsMean camber line, chord line, maximum camber, and aerodynamic-center pitching-moment rule-of-thumb diagram

Download MakeWindTunnelModel.ntop

Download File
Estimate \(\alpha_{\mathrm{trim}}\)

NASA’s Static Longitudinal Stability, Chapter 6, particularly equations 6.1–6.2:

\[C_{M_\alpha}=\frac{dC_M}{dC_L}C_{L_\alpha}=-SM\,C_{L_\alpha}.\]

For the linearized moment model,

\[C_M=C_{M_{ac}}+C_{M_\alpha}\alpha,\]

trim requires \(C_M=0\). Therefore,

\[\alpha_{\mathrm{trim,est}}=-\frac{C_{M_{ac}}}{C_{M_\alpha}}=\boxed{\frac{C_{M_{ac}}}{SM\,C_{L_\alpha}}}.\]
Derived from the linear pitching-moment relation and the static-margin relation in Static Longitudinal Stability, Chapter 6, equations 6.1–6.2. NASA technical reference
NACA4-digit analytic profile

Maximum camber

—

\(C_{M_{AC},\mathrm{est}}\)

—

\(\alpha_{\mathrm{trim,est}}\)

—

NOW FOR THE FUN PART -- GATHER WIND TUNNEL DATA

1. Demonstrate Longitudinal Stability
  • Place your Basic Model (no control surface deflections) on the OWT sting
  • Make sure the planned CG is placed at the pivot point
  • Balance it using the nuts and weights as appropriate
  • Turn it on and observe the angle of attack it settles out at

Predicted trim response

Predicted trim responseThe seeded aircraft pivots about its center of gravity against a protractor marked from minus 90 through zero level to plus 90 degrees, settles at the predicted trim angle, and returns after periodic or user-triggered pitch disturbances.CG
Trim target: −5.00°Leveling to trim · ζ = 0.50Drag the aircraft nose up or down, then release it

Enter the observed AoA (this is your actual \(\alpha_{\mathrm{trim}}\)):

DATA PLOT

Estimated pitching moment versus angle of attack

Estimated pitching-moment coefficient line and the estimated trim angle where the line crosses C M equals zero.
Questions to consider:
  • Is your aircraft stable at the planned CG?
  • How do your predictions differ from the measured result?
  • What will you have to do to fly level (~0 deg AoA)?
2. Assess Pitch Up Tendencies
Nose-up perturbation:
0.40

Stable

Choose a nose-up perturbationDamping ratio, ζ = 0.40

Pitch Up Tendency

Choose a nose-up perturbationResponse depends on perturbation
Questions to consider:
  • Is there an AoA when the model wants to pitch up uncontrollably?
  • OR, does the model return to level at any AoA?
3. Verify Neutral Point Position

CG location: the aircraft station positioned directly over the balance pivot (axle).

  1. Set the assessed CG by sliding the model until the desired station is over the pivot.
  2. Rebalance the complete apparatus so its actual CG is at the pivot.
  3. Apply a small nose-up disturbance: returning indicates stability, holding the new attitude indicates neutral stability, and diverging indicates instability.
  4. Move the assessed CG progressively aft, rebalancing each time, to locate the neutral point and demonstrate the unstable region aft of it.
Expected behavior
  • CG forward of NP: statically stable; the model returns toward trim.
  • CG at NP: neutrally stable; the model remains near the disturbed pitch attitude.
  • CG aft of NP: statically unstable; the pitch disturbance grows.
ForwardAft
Assessed CG / axle—
JET11 neutral point—
Static margin — optional override
Calculated: —
PredictionLoad aircraft data
Estimated \(\alpha_{\mathrm{trim}}\)αtrim—

Neutral-point stability response

Awaiting aircraft dataDrag horizontally for CG; drag the nose vertically to disturb
4. Demonstrate Directional Stability
  • Rotate the model 90° about its longitudinal x-axis so the balance rotation represents yaw.
  • Place the planned CG at the pivot and rebalance the complete apparatus.
  • Apply a small left or right yaw disturbance to the nose, release it, and observe the response.
  • A directionally stable model should return toward the airflow centerline.

Predicted directional response

Predicted directional-stability responseThe aircraft is depicted after a 90-degree roll about its longitudinal axis so the balance pivot represents yaw. Drag the nose laterally and release it to observe directional stability.FREESTREAMCG / axle
Aligned with the airflowDrag the nose laterally, then release itThe top view represents the model rolled 90° about its x-axis
5. Determine Pitch Trim Position / Control Authority
  1. Place your next model (Pitch Control) on the open wind tunnel balance. It must have the elevator deflected UP.
  2. Place the planned CG at the balance pivot and rebalance the complete apparatus.
  3. Run the tunnel, allow the model to settle, and record the new trim angle of attack below.
  4. Compare the Basic Model response at 0° TED with the Pitch Control model response to assess elevator control authority.

Basic Model · 0° TED

CG
Enter α_trim aboveAwaiting measured trim angleDrag the nose up or down, then release it

Pitch Control Model · −10° TED

CG
Enter α_trim,deflectedTEUAwaiting measured trim angleDrag the nose up or down, then release it
DATA PLOT

Trim Angle of Attack vs Elevator TED Deflection

Measured trim angle at zero elevator deflection and at the entered TED-positive elevator deflection, connected by a solid line with its equation in the plot legend.
6. Determine Yaw Control Authority
  1. Place your Directional Control model on the open wind tunnel balance. The rudder must be deflected 10° trailing edge left (TEL).
  2. Rotate the model 90° about its x-axis so the balance pivots in yaw, place the CG at the pivot, and rebalance the complete apparatus.
  3. Run the tunnel, allow the model to settle, and record the resulting trim sideslip angle below.
  4. Compare the Basic Model response at 0° TEL with the Directional Control model response to assess rudder control authority.

Basic Model · 0° TEL

CG / axle
Trim sideslip: 0.00°Awaiting disturbanceDrag the nose laterally, then release it

Directional Control Model · 10° TEL

CG / axle
Enter β trim,deflectedTELAwaiting measured trim sideslipDrag the nose laterally, then release it
DATA PLOT

Trim Sideslip Angle vs. Rudder TEL Deflection

Basic-model trim sideslip at zero rudder deflection and measured trim sideslip at the entered trailing-edge-left rudder deflection, connected by a solid line with its equation in the plot legend.
JSON Viewer
Load a JET11 file to populate the JSON structure.