# Does an instrument exist to measure the different types of drag?

Does a 'drag meter' exist that separately shows total, induced and parasite drag? If it doesn't, and in theory, would it be possible to build one?

Yes, partially. A wind tunnel's wake rake (an array of pitot tubes) can be adapted to measure in-flight profile drag (form drag plus skin friction).
Measuring Wing Profile Drag using an Integrating Wake Rake, by Pifer and Bramesfeld, in Technical Soaring 36(3):74-82, 2012.
The article doesn't have any in-flight photos, so it may not have actually been flown at time of publication.

@quietflyer mentioned Richard Johnson, who indeed has flown an airplane with a drag rake.

The wing drag rake was installed on N3176T’s left wing trailing edge, 1.0 meters outboard from the wing root joint. ...

R. Johnson, "A flight test evaluation of the SZD-55-1," Soaring, March 1992.

The drag rake itself is described and shown in another article:
R. Johnson, "At last: an instrument that reads drag!," Soaring, October 1983.

• JZYL's answer explains how a wake rake works, and suggests that it would help to measure not just pressure but also cross-flow. But it would be hard to tow an array of cross-flow sensors (hot film, maybe?) without affecting the airplane's drag and center of mass. – Camille Goudeseune Feb 6 '20 at 21:02
• @xxavier and others-- Drag rakes have certainly been used in flight to measure the drag of sailplanes-- for examples, see some of Richard Johnson's work. I am fairly sure the instrument is rigidly mounted to the aircraft, not towed. This answer could be improved by incorporating references to this work. In fact I'm fairly sure I saw a glider with a drag rake on it at the soaring museum in Moriarty, New Mexico. If I can locate photos I may compose an actual answer along these lines. – quiet flyer Mar 24 '20 at 20:28
• Drag rakes have been flown behind wings for at least half a century already. However, photographic evidence is missing from the web. – Peter Kämpf Sep 7 '20 at 15:52
• Can rake measure pressure drag separatetly from skin friciton? Is this rake motorsport.tech/wp-content/uploads/2020/02/…? – user52248 Sep 7 '20 at 16:18
• 1, no, a rake can't know what caused the change in pressure. 2, great photo, maybe it is a drag rake. What context is there for this photo? (This is worth asking as a new question, instead of hidden in these comments.) – Camille Goudeseune Sep 8 '20 at 2:00

In theory, it's possible to distinguish induced drag from parasite drag, at least in subsonic flow. The drag force can be quantified through conservation of momentum in a control volume (note that the coordinate system is in the stability axis):

$$D=-\oint_{S_\infty}p\hat{i}\cdot d\vec{S} - \oint_{S_\infty}\rho (\hat{i}\cdot\vec{u})(\vec{u}\cdot d\vec{S})$$

where $$p$$ is static pressure and $$\rho$$ is density.

If the sides of the control volume are far away from the wing/airplane, then the flow field tends to the free-stream values and their contributions go to zero. The only side that matters is the aft boundary, called the Trefftz plane.

After some algebra (Ref. Drela, Flight Vehicle Aerodynamics), we can separate the induced drag and parasite drag (subscript $$_\infty$$ denotes free-stream values):

• Induced drag ($$v$$ and $$w$$ are cross-flow components): $$D_i=\int_{S_T}{\rho_\infty \frac{v^2+w^2}{2}dS}$$

• Parasite drag ($$u$$ is axial flow speed, $$V_\infty$$ is the free-stream speed): $$D_p=\int_{S_T}{\rho u(V_\infty-u)dS}$$

To summarize, you can measure induced and parasite drag contributions by casting a large net aft and far of the wing/airplane (Trefftz plane). To measure induced drag, you would need an array of probes that can measure the cross-flow at individual locations on the Trefftz plane. To measure parasite drag, you would need an array of pitot tubes to measure the total pressure and the resulting momentum defect.

• cross-flow components v and w in induced drag formula ,have dowanward direction,prepedicular to freestream? – user52248 Sep 7 '20 at 16:07
• @Сократ v is lateral component, w is vertical – JZYL Sep 8 '20 at 2:02