# How can a designer deal with a strict requirement when designing an aircraft?

I'm working on designing an aircraft with some specific requirements (Vstall, Vmax, Ceiling, Payload, Range, RoC, Slanding/takeoff etc).

I'm following Anderson's and Raymer's ways (trying to find the best for me).

Right now I'm stuck at the step of estimating for the first time the Wing Loading (W/S). Doing the calculations about the W/S constraints (Vstall, Landing distance from Anderson) I found that the Stall constraint is very strict. The result that I get is a W/S = 24 lb/ft2 for a twin turboprop with a estimated gross takeoff weight of 16k lbs. The constraint of landing distance gives a W/S of 62.

Looking at historical data (and some similar aircraft) and Raymer's/Anderson's tables the W/S = 24 is way off the ordinary values. I do understand that it is just an estimation.

What would you suggest to do so I can satisfy the requirement and be able to get a higher W/S?

I thought about changing the flaps that I used (Split Flap) but I don't know if it's possible and realistic to have "Double Slotted Flaps and Slats" for a simple twin turboprop that has a simple mission.

edit: After the comment, here are the equations about the Vstall and Landing Distance.

and

• No VtC from me, but this may be far too broad. I'm not an expert, though, and could be totally wrong... – FreeMan Jun 24 '20 at 18:38
• Can you list the equations that derive the "stall constrained W/S" and "T/O constrained W/S"? – JZYL Jun 24 '20 at 19:27
• @JZYL Check the post, I edited it. – ChrisP Jun 24 '20 at 19:40
• This smells a lot like the wrong c$_{L_{max}}$ is used for the stall speed. Are you using the clean c$_{L_{max}}$ when the task asks for the c$_{L_{max}}$ in landing configuration? The formulas are fine and dandy, but what numbers do you use for the parameters? Single slotted flaps with a bit of rearwards extension would be adequate, but the choice really depends on the requirements. Maybe that specific combination can only be fulfilled with double slotted Fowler flaps and slats. A split flap is a bit old-fashioned, don't you think? – Peter Kämpf Jun 24 '20 at 21:08
• @ChrisP: Keeping the low stall speed means you need to carry around a huge wing which creates more drag at cruise speed, so the engines need to get bigger. And the fuel tanks. Which in turn requires a bigger wing. Repeat. The result is an economic disaster for the customer. Of course, with complex flaps and slats you can raise the lift coefficient, but still this will result in a big and heavy wing which creates more drag than necessary. In short, renegotiate that particular requirement because it will lead to absurd results. Could it be your supervisor has even less experience than you? – Peter Kämpf Jun 25 '20 at 18:46