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Hi I am trying to do a rough estimation of forces acting on an aircraft wing and would like to know what kind of lift to drag ratio is normal. The exact profile hasn't been chosen yet, but the aircraft weighs less than 1800 kg and has a wingspan of around 13 m. The cruise speed is roughly 70 m/s.

As I understand, the maximum load factor to consider is 3.8, so the maximum lift is about 67,000 N; but how would I go about getting an estimate for the drag?

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The maximum L/D values for existing aircraft span a wide range, from 7.5 for the Zodiac Ch701 with floats to 21.5 for the Berkut 360. Source: Light Sport and General Aviation Airplane Comparison and Harloff Performance Factor. This comparison comprises 93 types, and that is normally where a drag calculation starts: You find a similar type and use that as a baseline.

For the wing drag you calculate the lift distribution for several angles of attack (try XFLR5 for this) and get the induced drag for the weights and speeds that belong to those angles. Now you need to know your airfoils and add their drag. Next, add increments for the interference with the fuselage, for fairings, gaps and probes using data from this book. Yes, it is more than half a century old but physics hasn't changed since then.

Drag is normally not the biggest in-plane load on a wing but lift at high angle of attack. If you need to calculate loads for a structural design, make sure you don't omit that particular load case. Don't be surprised: That lift has a strong forward-facing component.

If you are new to this, try to calculate several of the aircraft for which data exists. Only by doing this you can be sure that what you do gives good results. Drag calculation needs some experience and is, when done well, a lot of work.

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  • $\begingroup$ "That lift has a strong forward-facing component." I thought it was rearward facing? $\endgroup$ – DKNguyen May 7 at 0:59
  • $\begingroup$ @DKNguyen: Not in the body-fixed coordinate system. At high angle of attack lift is still about orthogonal to the flow direction which now has a noticeable angle with the body-fixed system. This creates the most important load case for wing in-plane loads. $\endgroup$ – Peter Kämpf May 7 at 3:54
  • $\begingroup$ Oh wait, you're talking about the lift component. I was thinking about the aerodynamic force. $\endgroup$ – DKNguyen May 7 at 4:28
  • $\begingroup$ @DKNguyen Relative to the aerodynamic system, the aerodynamic force is always rearward-facing. That is the contribution of both friction and induced drag. But for wing in-plane loads this needs to be converted into the body-fixed system. $\endgroup$ – Peter Kämpf May 7 at 6:34

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