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I'm a student pilot at a flight school that primarily flies RV-12s as a trainer. We had one day that through a booking error meant that all the RV-12s were booked so we had to use their Ercoupe 415-C instead.

I was surprised at how easy the plane was to fly and asked my CFI about the "characteristically incapable of spinning" placard on the front panel. He explained that the rudder pedals weren't necessary because the plane maintains coordinated flight by connecting the ailerons to the rear rudder.

I think this is an amazing innovation for the Ercoupe and I'm surprised it's not a design that I can find on any modern small aircraft designs.

Maintaining coordinated flight with rudder pedals isn't a complicated task for a trained pilot but it is another task that the pilot needs to perform. It seems like removing flight tasks from the pilot would make for a safer aircraft so why do manufacturers not include designs similar to Ercoupe's models?

Is it a weight/cost/drag issue, or are spins not as huge of a killer as they were in the 40s-60s?

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    $\begingroup$ Because taking away the rudder pedals harms pilots' fragile egos. And as long as you have rudder pedals, you might as well have a spin-prone aircraft. That's good for pilots' egos as well. $\endgroup$ Feb 13 '20 at 20:40
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    $\begingroup$ Depending on speed, you need different ratios of aileron and rudder for coordinated flight. If you always fly at the same speed, coupling both is fine. However, you will not be able to sideslip into a tight field. And slipping is fun! $\endgroup$ Feb 15 '20 at 21:11
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    $\begingroup$ With regard to the portion of your question about modern small airplanes, Cirrus aircraft feature a rudder-aileron half-interconnect. Applying rudder will move the ailerons, but applying aileron will not move the rudder. And yes, the interconnect can be easily overridden with muscle for crosswind landings. $\endgroup$
    – Steve V.
    Feb 17 '20 at 4:33
  • $\begingroup$ Because the amount of rudder necessary to maintain "coordinated" flight cannot be determined by any simple mechanical system that relies solely on aileron input. It is dependent on AOA, (NOT SPEED, @Peter), pitch rate, flight path angle rate, yaw angle, etc. etc. Even in very sophisticated modern aircraft, the computers that are used to do this can screw it up in ways that a pilot would not. Also, there are times when you want (NEED) to be able to fly uncoordinated. $\endgroup$ Dec 26 '21 at 1:39
  • $\begingroup$ Another interesting approach (which also had it's issues), was used in early F-15s. The effect of pushing the ailerons to one side or the other was different based on the fore/aft position of the stick. When the stick was fully forward, (implying low AOA), lateral movement deflected the ailerons on the wings. When it was far aft, (implying high AOA), lateral movement caused little or no aileron deflection and almost all differential stabilator. Median fore/aft stick positions would cause a mixture of both. $\endgroup$ Dec 26 '21 at 1:58
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The coupling between the ailerons and rudder are designed such that they avoid uncoordinated turns as you described. However, this occurred at the expense of control in other flight conditions.

In fact, there are situations where you need to deflect the rudder without roll input, you need to deflect the rudder more than the preset coupling, or in a manner that is opposite to the preset configuration.

For example, if you turn right, the system will add right rudder to keep the airplane coordinated. But if you want to do a forward slip, you may perform a left hand roll, but with rudder all the way to the right. The system would be incapable of performing this maneuver, meaning you can't fly in high winds with the system installed. This is fine if you live in an imaginary place where the winds are always favourable.

As a result, a common modification on the Ercoupe is to disconnect the coupling between the rudder and ailerons...

Edit: I also wanted to add that spins are recoverable, within reason, by a trained pilot. For some planes, you are simply forbidden to intentionally enter a spin as a remedy to its inability to recover from a spin!

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    $\begingroup$ Very interesting! It makes sense that you couldn't do a slip landing in an Ercoupe. That is certainly a downside. $\endgroup$
    – Burke9077
    Feb 13 '20 at 22:34
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    $\begingroup$ Also, one of the things most student pilots learn is how to avoid a stall or spin, unless you're practicing them. $\endgroup$
    – jamesqf
    Feb 14 '20 at 3:01
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    $\begingroup$ It also adds more things to go wrong. The coupling jams and suddenly you've got problems with both the rudder and ailerons. $\endgroup$
    – TLW
    Feb 15 '20 at 19:52
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    $\begingroup$ "Inability to recover from a spin"? I was thinking that the FAA regulations require at least all civilian airplanes to be able to recover from a spin using the "standard" method? $\endgroup$ Feb 16 '20 at 9:18
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    $\begingroup$ Hey @Burke9077, having a twin tail can have bring benefits when it comes to aerodynamics and redundancy for a given aircraft, and is usually done to get a reduced footprint (height) for storage. I am not sure what the design priority was for the Ercoupe, perhaps that is another question to ask! Take a look at this thread: aviation.stackexchange.com/questions/35871/… $\endgroup$
    – Gerry
    Feb 18 '20 at 20:33
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Rudder pedals are used for more than keeping turns coordinated. Forward slips for crosswind landings, slips to increase drag during descents, and reducing the tendency to weather-vane during cross-wind taxiing are examples.

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The ercoupe was an advanced design for its time, but it represented a bunch of compromises and the inability to do a forward slip was one of them. The plane's designer made up for this by choosing the airfoil so the plane would descend steeply enough with power off so the pilot wouldn't have to bleed off altitude if he was high on final, and by making the landing gear stout enough to manage landing while severely crabbed.

Another compromise was the omission of flaps; this was to simplify both flight operation and maintenance but required an airfoil that yielded a flap-like descent with power off (as mentioned above) which is why the ercoupe glides like a brick.

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    $\begingroup$ It doesn't glide any worse than its contemporaries. The procedure for losing altitude on final was to S turn. $\endgroup$
    – John K
    Feb 14 '20 at 4:23
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    $\begingroup$ handbook value for 65HP Piper J-3 is 10:1, 6:1 for the ercoupe... $\endgroup$ Feb 14 '20 at 6:17
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    $\begingroup$ Yeah but lots of contemporaries, biplanes, the short wing Pipers, also glide like bricks and you still have to side slip them from time to time, and if you had interconnected controls, you'd be left with S turns. 6:1 or 10:1 L/D is neither here nor there. $\endgroup$
    – John K
    Feb 14 '20 at 14:17
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The usual way to make a plane 'spin proof' is to ensure the centre of the wing (near the fuselage) stalls before the tips, and to move the Centre of Gravity far enough forward that the elevator doesn't have the power to keep the nose up below stall speed.

The first is commonly done by twisting the wing tips up at the trailing edge, reducing their angle of attack relative to the rest of the wing. It could also be achieved with a change of aerofoil section, or turbulators.

Both of these cause drag and reduce efficiency, and the second limits your ability to flare for landing. I imagine those were sufficient reasons for designers not to 'stall proof' their designs.

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  • $\begingroup$ Bert Rutan's canard aircraft (Varieze, Long EZ, etc.) were designed to be spin proof. The elevator is on the canard, in the front, and lifts the aircraft up, creating nose up moment, which balances the nose down moment from the lift on the wing. These aircraft are designed so that the canard, even with full aft stick, cannot create sufficient nose up moment to increase the AOA of the main wing to the point where the main wing would stall. The canard stalls first, loses lift, drops, begins flying again, and the cycle repeats. But the main wing never stalls or loses aileron effectiveness $\endgroup$ Dec 26 '21 at 2:01
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    $\begingroup$ @CharlesBretana And in what way is this different from a conventional design? The wing stalls first ... yada, yada, yada. Spin proofing is done exactly as described here, at least for moderate to high aspect ratios. $\endgroup$ Dec 26 '21 at 8:27
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    $\begingroup$ @CharlesBretana - that's basically the same as the elevator not having enough power to keep the nose up when flying at stall speed. You may want to ask a new question about why canard designs are not more common. $\endgroup$ Dec 26 '21 at 11:32
  • $\begingroup$ @Peter, the difference is that when a conventional wing stalls, you lose lateral roll control, and the tail (still producing nose up pitch moment) exacerbates the AOA and stall (even though the nose pitches down, the AOA is still increasing). In a canard, exactly the opposite happens. $\endgroup$ Dec 26 '21 at 15:13
  • $\begingroup$ In a canard, when the canard stalls, the wing is still generating lift, which produces a nose DOWN pitching moment, which reduces AOA on the canard, which breaks the stall on the canard. $\endgroup$ Dec 26 '21 at 15:20
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From actual owners, landings are done crabbed into the wind and the aircraft corrects itself once both mains hit the runway. It’s a lot like the airliners you see landing in a stiff crosswind.
Not really a dangerous design, but you have to adapt to its idiosyncrasies. Lots of dihedral makes it very stable and not being able to use the rudder uncoordinated will keep you from entering a spin. Also, the elevator back-travel is limited to prevent the aircraft from stalling. All of these built-in features can be overcome if you try hard enough, but flown within its parameters, it’s a safe ship.

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  • $\begingroup$ You probably could spin it. A spin is just stall and yaw. If you pitch up, roll into 90 degree bank and pull hard into a stall, inertial coupling (as the flight path turns down away from the nose) will induce a yaw. the yaw will be upwards away from the earth, and so the lower wing will stall first, or more severely, causing a overbank that would induce the pilot to roll away. Unless the rudder is large enough to keep the yaw angle below the spin threshold, it just might spin... $\endgroup$ Dec 26 '21 at 1:52
  • $\begingroup$ @CharlesBretana It's enough to have asymmetric flow from turning. Spin will follow if stalled in a turn AND enough elevator authority AND a sufficiently backwards cg position. $\endgroup$ Dec 26 '21 at 8:30
  • $\begingroup$ @Peter, yes, Anything that creates yaw and high enough AOA to allow one or both wings to stall will by definition create spin conditions. $\endgroup$ Dec 26 '21 at 15:17

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