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With two engines, one above and one below the wing, will the lift be the sum of two lift increments (produced by upper surface blowing and external blowing)? I am trying to make a jet suit and I am interested in incorporating this phenomenon in it.

When a jet of air blows over or under an airfoil with a deflected flap, it gives an immense amount of lift due to the change in direction of the jet via the Coanda effect. The engine can be placed either above and below and can achieve lift coefficients up to 7 (when any one of the methods - upper surface blowing or externally blown flap - is used) What if we use two of them at the same time? To which value can the lift coefficient increase?

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  • $\begingroup$ Is the wing going the same speed? Then lift would be the same, yes? $\endgroup$ – CrossRoads Apr 4 '18 at 15:10
  • $\begingroup$ didn't understand your statement @CrossRoads $\endgroup$ – Aman Thangellapally Apr 4 '18 at 19:22
  • $\begingroup$ Is there really space to have two engines top and bottom of wing? Review Boeing YC-14 or MD YC-15. $\endgroup$ – INGSOC Apr 5 '18 at 0:55
  • $\begingroup$ What creates the lift - the wing, or the engines? If the wing, then lift is determined by speed. If it's the engines, then you have more of a rocket than a flying suit. $\endgroup$ – CrossRoads Apr 5 '18 at 1:40
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Two Engines Would Probably Be a Waste

First the basics about lift. Lift can be seen as occurring from one of two effects, a pressure difference across surfaces, or in reaction to the acceleration/deflection of the air (or whatever fluid you are moving through). Any case can be calculated from either method and you will come to the same result. You can't exploit a pressure differential without accelerating air, and you can't accelerate air without creating a pressure differential. Since force in these cases is governed by $F=\dot m\Delta v$ and required power is $P = \frac{1}{2} \dot m (v_{out}^2 - v_{in}^2)$. This means that to maximize efficiency you should try to accelerate the largest quantity of air by the smallest amount.

Now let's talk about flow control. Blown flow control systems increase lift in two ways, first is that they help keep the flow attached, and second, especially in propulsion driven systems like you are describing, is by deflecting part of the thrust downwards. Attaching the flow helps to allow greater angles of attack without stalling, once you have have generated this flow attachment, further actuation of flow control will not help generate any more lift by this method, though it may allow further angles of attack. If you are trying to improve low speed characteristics by allowing extreme angles of attack remember that the increase in lift will approximately follow a sin curve as it results from increasing the angle of deflection for flow across the wing, the first little increase in angle will give lots of lift with little drag, the last will give almost no lift for tons of drag if you want to take it as far as 90 degrees. Also lift is related to speed to a power of two, that means you need a lot more deflection as your speed drops.The second method that blown systems generate lift is by deflection, basically you gain some lift by sacrificing thrust. In the end this is no better than just pointing the engines slightly downwards.

If you want to perform flow control you also have to remember how much area on the wing your exhaust is going to effect. Your jet will likely be small in comparison to the total size of the wing.

At the end of the day, it is probably better to make a system that provides some flow control to the entire wing rather than a massive amount of flow control to a small portion of the wing so spread out those engines rather than trying to stack them; and remember, flow control will give no improvement over simple thrust vectoring at a standstill.

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Let's first consider a simpler example. Have two engines on the same side of the wing, close together. Both experience the same effect, so you can still get that lift coefficient of about 7.

That's no surprise; that's how coefficients work. They're multipliers. And by the basic mathematical rule c*(x+y) = c*x + c*y multipliers can be distributed over terms.

Now moving one of the two engines to the other side of the wing doesn't improve the other engine, so the multiplier doesn't improve either.

As for your second question, that's not simple mathematics anymore. Both effects are ultimately due to the deflection of the airflow. As air is forced downwards by the wing, Newton tells us the wing is forced up. That up force is what we call lift. And it's true that you can deflect a single airstream twice. But after the first time, you've already traded some horizontal speed for vertical speed. And it's not the goal to have just vertical speed in the airflow - that's what we call a helicopter. So the more you deflected the air flow the first time, the less you can deflect further down the streamline.

I can't give you theoretical results here, this will be down to engineering. And of course, a more complex mechanism definitely adds weight and cost.

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  • $\begingroup$ yeah my doubt is not if the other engine improves it's performance.. I meant to ask, since we are using both of the methods at the same implying twice the amount of flow being directed down, wouldn't lift double(approximately) $\endgroup$ – Aman Thangellapally Apr 5 '18 at 20:51
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I'm not sure this is viable on a wingsuit; it may be too complex or heavy. However, I do see the concept perfectly viable on a larger type aircraft that may need short take-off/vertical takeoff capability. Consider just splitting the airflow into two, above and below the wing using a single engine. Initial engine airflow may have to be different on the wing chord above/below the wings as well. Study how such a design would compare in efficiency over just pointing the engine airflow downward.

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