## Area for lift coefficient calculations for Grid Fins?

## Area for lift coefficient calculations for Grid Fins?

(OP)

Hi All,

I have some test data from a wind tunnel for several designs of grid fins with minor design tweaks. I'm trying to plot the aero efficiency (C.L / C.D) to compare them, but I feel like my resulting values aren't quite making sense. Compared to some studies I've looked at, my drag coefficients are in the right ballpark, but my lift coefficients are way low, and I'm thinking it's the area values I'm using.

For for the lift coefficient, would the area be Fin Length * Fin Depth? Effectively, the planar area of the side view of the fin? Or would it be the sum of control surface area on one "side", i.e. 2 * Cell Width * Cell Depth * No. of cells? Also, when changing the AoA, would you account for that in the area value used to calculate the lift coefficient? For my drag coefficients, I have been using the planar area of the top view (looking down through the lattice) and do account for the change in projected area with a cos(AoA) added into the mix, and my values are roughly aligning to the studies I mentioned.

Any input greatly appreciated - thanks in advance!

I have some test data from a wind tunnel for several designs of grid fins with minor design tweaks. I'm trying to plot the aero efficiency (C.L / C.D) to compare them, but I feel like my resulting values aren't quite making sense. Compared to some studies I've looked at, my drag coefficients are in the right ballpark, but my lift coefficients are way low, and I'm thinking it's the area values I'm using.

For for the lift coefficient, would the area be Fin Length * Fin Depth? Effectively, the planar area of the side view of the fin? Or would it be the sum of control surface area on one "side", i.e. 2 * Cell Width * Cell Depth * No. of cells? Also, when changing the AoA, would you account for that in the area value used to calculate the lift coefficient? For my drag coefficients, I have been using the planar area of the top view (looking down through the lattice) and do account for the change in projected area with a cos(AoA) added into the mix, and my values are roughly aligning to the studies I mentioned.

Any input greatly appreciated - thanks in advance!

## RE: Area for lift coefficient calculations for Grid Fins?

another day in paradise, or is paradise one day closer ?

## RE: Area for lift coefficient calculations for Grid Fins?

## RE: Area for lift coefficient calculations for Grid Fins?

## RE: Area for lift coefficient calculations for Grid Fins?

as 3DD suggested I'd use the planform of the fins (maybe the planform of one fin *number of fins ?).

another day in paradise, or is paradise one day closer ?

## RE: Area for lift coefficient calculations for Grid Fins?

Seems like an aero engineer would be useful on your project.

## RE: Area for lift coefficient calculations for Grid Fins?

## RE: Area for lift coefficient calculations for Grid Fins?

## RE: Area for lift coefficient calculations for Grid Fins?

## RE: Area for lift coefficient calculations for Grid Fins?

A quick glance show how complex this topic gets...

What airspeed range: Subsonic, transonic, supersonic, hypersonic?

What materials: composites, aluminum, titanium, steel/CRES/HRA.

What size/purpose: small/handheld tactical, large(er)/transporter tactical, ballistic missile, space-launch, emergency escape, etc...?

Probably lots of proprietary stuff that is sensitive/classified/trade-secret.

Devil-is-in-the-details.

Regards, Wil Taylor

o Trust - But Verify!

o We believe to be true what we prefer to be true. [Unknown]

o For those who believe, no proof is required; for those who cannot believe, no proof is possible. [variation,Stuart Chase]

o Unfortunately, in science what You 'believe' is irrelevant. ["Orion", Homebuiltairplanes.com forum]

## RE: Area for lift coefficient calculations for Grid Fins?

-We operate in the subsonic range, ~M0.1 to 0.3.

-Our materials are not set in stone, but at the moment we're utilizing 3D printed Nylon 12; longer term we may looking into glass filled injection molded Nylon or potentially Bulk Molding Compound carbon fiber.

-Fins are in the range of 1 sqr ft of planar area, though that's still changing depending on how developments go. We are in the transport / logistics space of applications.

I hate to backtrack, but with regard to the area used for Cd and Cl calcs, is it typical that the area value is changed with each change of AoA? Or is the 0° AoA plan area used regardless of the AoA value? I had been opting for the former so far. Obviously, the projected area of a thin, flat surface relative to the flow is going to decrease with increases in AoA, so I'd been multiplying in a cos(AoA) to approximate that change (not perfect since there is some thickness to the grid fin but close enough for our purposes).

## RE: Area for lift coefficient calculations for Grid Fins?

## RE: Area for lift coefficient calculations for Grid Fins?

One other thing I forgot to mention; we have also been exploring Cascade Fins, which are an offshoot of Grid Fins where the lattice density is reduced and they align 0/90 rather than ±45. A few studies have shown they have about twice the aero efficiency for subsonic speeds, and can be increased beyond that if the cross members are shaped like airfoils, which is far easier to do with Cascade Fins than Grid Fins.