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I have a project for university to design and build a model RC aircraft. Tt's main design principal is to carry a high payload, so I was thinking of going for a high camber, thick aerofoil. Hopefully this will allow it to carry a heavy weight at low speeds.

However, one of the other requirements is that it must be able to fly inverted for 4s without load.

How should this impact my choice of aerofoil, is at all? And what are some suggested types of aerofoil I should look at for these requirements?

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  • $\begingroup$ I’m voting to close this question because questions about RC aircraft belong on Drones.SE. $\endgroup$
    – Ralph J ♦
    Commented Sep 28, 2022 at 14:35
  • $\begingroup$ Even though the scenario is unlikely for full-size aircraft, the reasoning would be exactly the same, so I don't see why it should be closed. $\endgroup$
    – Raketenolli
    Commented Sep 29, 2022 at 13:27
  • $\begingroup$ First a couple of suggestions. When you are talking about "high weight payload" this is completely arbitrary. Instead try to think about it in terms of weight fraction of design gross weight or similar parameter. This puts the problem into perspective. a 10 lbs payload for a 15 lb aircraft is a big deal. Also low airspeeds equates to what? 30 kts? 15 kts? Giving more specifics on design criteria will help you get a better answer. $\endgroup$
    – spacegirl1923
    Commented Sep 29, 2022 at 20:22
  • $\begingroup$ How big in meters (or cm) should it be? MTOW? Configuration (conventional, canard, payload in cargo bay or underwing, swept wing, delta wing, flying wing)? Max speed, stall speed, ... $\endgroup$
    – sophit
    Commented Sep 29, 2022 at 21:14
  • $\begingroup$ @Raketenolli The Reynolds number would be fairly different for an RC vs a full-size aircraft, and that drives some differences. Picture the wings of a simple paper airplane... that shape works fine for fold-it-and-throw-it, but not so well for larger, heavier, and faster. Since the assignment here is literally a drone, this would fit better at Drones.SE. $\endgroup$
    – Ralph J ♦
    Commented Sep 30, 2022 at 3:55

2 Answers 2

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In general to start a preliminary aircraft design problem you need to start with basic parameters such as configuration and geometric parameters which will contribute to GTOW, available power, target stall speed etc. Intro to Flight by John D Anderson is a great starting point for basic design parameters. In particular look at chapters 5,6 & 7.

If you know your target stall speed and guess at your max weight you can determine the lift coefficient you will need. This will be a starting point for evaluating airfoils. Airfoil Tools is a great resource for searching sorting and gathering data. You'll also want to look at performance at a target cruise/loiter speed. You can approach it from different directions, also depending on control surfaces. Yet another factor is flight performance. How long does it need to fly for? 10 minutes carrying a heavy payload is pretty easy compared to 3 hours.

These are all things you need to consider. Saying a thick high camber airfoil is a generic starting point and maybe totally unnecessary. Think about the Piper J-3 that uses the tried and true Clark Y but achieves a really low stall speed. You really need to factor in the whole vehicle to get a "good" design.

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However, one of the other requirements is that it must be able to fly inverted for 4s without load.

How should this impact my choice of aerofoil, is at all?

I'm going to assume that "without load" means "without payload", not "in a zero-G condition".

I'm going to also assume that there is no requirement that the altitude be constant during the inverted segment-- i.e. I'm going to assume that descending flight is allowed.

If so, the 4-second inverted requirement should have no effect on your airfoil choice.

As a strategy to intentionally lose altitude, I've held a "E-flite Radian" radio-controlled motor glider in unpowered (descending) inverted flight for literally minutes on end. The Radian has an undercambered airfoil that is far from optimal for efficient inverted flight. Yet the resulting best glide angle in unpowered, inverted flight is still relatively flat (perhaps 10 to 20 degrees below horizontal?)

Your primary issues in a brief segment of inverted flight will revolve around structural integrity and controllability (especially if the wing has dihedral, which degrades controllability in inverted flight), not airfoil efficiency.

(Hint: don't assume that the requirement to demonstrate inverted flight rules-out a no-aileron design, with ample dihedral, and with rudder and elevator as the only control surfaces. Such designs can in fact be flown inverted for prolonged periods by a proficient pilot, so long as the rudder is sufficiently large and has ample throw. The "Radian" glider mentioned above, when set up optimally, is an example of such a design.)

In fact my recommendation would be the same even if the requirement were that the aircraft must maintain altitude while inverted, with zero payload. Back to the "Radian" motorglider as example-- despite the undercambered airfoil, it is perfectly capable of a modest sustained climb rate in inverted, powered flight. So design the airfoil for efficient sustained upright flight with the required payload, not for the brief no-payload inverted segment.

Another note-- depending on the exact rules at play, and the discretion of the judges, you might consider doing the inverted section as part of loop. Would the judges count all portions of the loop where the aircraft was even slightly inverted, as contributing to the required 4 seconds? If so, depending on entry speed (the higher the better), it seems quite possible that you could meet the 4-second-inverted requirement with the G-loading (and therefore the lift force) never even going negative at all, or only going slightly negative for a brief interval.

Another way to show how much you can "get away with", for brief intervals of time, in terms of non-optimal airfoil choices, is to take a radio-controlled model airplane with flaps and experiment with sustained inverted flight with the flaps down. You might be surprised at what is possible! Design your airfoil for the main mission, not for the 4 seconds of inverted flight.

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  • $\begingroup$ So thorough! The only point you might add is, while flying inverted, to put the flaps "up" to restore the desired camber. $\endgroup$
    – Camille Goudeseune
    Commented Oct 1, 2022 at 19:15
  • $\begingroup$ @CamilleGoudeseune - actually I was thinking of flaps "down" from airplane's point of view, just as a demonstration, to make the airfoil really non-optimal! $\endgroup$
    – quiet flyer
    Commented Oct 1, 2022 at 20:59
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    $\begingroup$ Yes, you were clear: if a shape that terrible still works, there's zero justification for compromising the main task's performance. I only noted that, hey, while we're talking flaps, reversing them is a cheap way to improve performance while inverted. $\endgroup$
    – Camille Goudeseune
    Commented Oct 2, 2022 at 19:25
  • $\begingroup$ I read the initial statement "without load" as specifically not in a loop where there will be G loading, but rather as steady-state inverted flight. Are you trying to suggest that it also has to fly with no payload for a separate flight or that it somehow has to dump its payload to fly inverted? $\endgroup$
    – spacegirl1923
    Commented Oct 4, 2022 at 20:44

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