Ultralights with slight dihedral will roll and yaw on rudder input (e.g the Bloop 3, Maxair Hummer) so some don't have ailerons for the sake of simplicity. Is this kind of control input advisable, or are there disadvantages?

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    $\begingroup$ Related $\endgroup$ – Pondlife Jun 18 '18 at 14:34
  • $\begingroup$ differential elevator is much better choice if you design the airplane with simplicity in mind from scratch. $\endgroup$ – user3528438 Jun 18 '18 at 16:03
  • $\begingroup$ Not sure I'd say it's a "better" choice, but it's at least as valid, and the mechanical mixer required is pretty simple to build. The roll authority is pretty small, but as an adjunct to rudder/dihedral, it should improve the coordination of turns. $\endgroup$ – Zeiss Ikon Jun 18 '18 at 18:49
  • $\begingroup$ One thing to add about the Maxair Hummer is the fact that both center of gravity and Vtail center of lift are far away under the wing's center of lift, which configuration provides a much quicker transformation from skid to roll. $\endgroup$ – qq jkztd Jun 18 '18 at 21:41

It works in a half-assed way but the key word is half-assed. You'll always be skidding around the sky since sideslip is required to obtain and maintain any rolling moment. Control response can be somewhat laggy, since you have to induce a skid and wait for the roll, depending on how much dihedral you have. So to have something approaching snappy control response, you need way more dihedral than is necessary for normal lateral stability. The inability to cross control limits the ability to handle crosswinds and prevents you from using sideslip to increase descent rate.

So it works after a fashion, but will never be as good as full 3 axis. The thing with ultralights is that because of the really low mass/large surface area, they are really strongly affected by turbulence, and when close to the ground, half-assed control authority is the LAST thing you want.

Control authority is not a place I would cut corners in the interest of simplicity. I think that the attraction of 2 axis control for ultralight designers was in the idea that non-pilots getting into ULs would think "Hey that must be only 2/3rds as difficult to learn as 3 axis. Must be better."

  • $\begingroup$ At some point, question could be: If the only way to fly this plane is 2 axis, would you get rid of the ailerons, or the rudder? Ailerons + elevator is also one lighter solution than ailerons + elevator + rudder. $\endgroup$ – qq jkztd Jun 18 '18 at 21:48
  • $\begingroup$ Also consider the Predator drone: inverted Vtail transforms rudder input into same direction roll. Otherwise standard Vtail transforms rudder input into adverse direction roll, which is contradictory regarding 2axis authority. At some point, large span inverted Vtail becomes one deadly effective yaw-roll device. $\endgroup$ – qq jkztd Jun 18 '18 at 21:55
  • $\begingroup$ Yes that is one of the benefits of the inverted V tail (Molt Taylor, inventor of the an FAA certificated flying car in the early 50s, was a big advocate). But the rolling moment you get from an inverted V tail is pretty mild unless you make it really big and then you are building way to much tail volume (area x arm), so you are still dependent on dihedral. Plus the other negative from V tails, a tendency to yawing moments in gusts (tail wagging in bumpy air) you have to live with. $\endgroup$ – John K Jun 19 '18 at 0:38

Rudder only limits banking as described very well by John K, and might be a distorted view of increased safety by helping the pilot to not become disorientated. It may also be an attempt to not over stress the structure or prevent a stall in a high G turn.

After considering John K's comment it appears to me this is more a marketing or cost savings stunt, then a safety consideration.

A stall in a skid or slip is all that is necessary to enter a spin! So, essentially you are trading a less hazardous situation (disorientation) for the biggest pilot killer on the planet! (spin)

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I have some tens of hours flight experience with radio control aircraft that fly this way -- throttle, elevator, and rudder. How well it works depends very much on how much rudder authority you have, and how much dihedral.

I have seen and flown aerobatics with these no-aileron aircraft, including prolonged, controlled inverted flight (tricky, because yaw-roll coupling reverses when inverted), outside snap rolls (again, roll is reversed relative to rudder and the aircraft may tumble if the wing unstalls before rudder is neutralized), as well as more conventional slow rolls, chandelles, Immelman turns, etc.

I wouldn't expect an ultralight to be flown through any of these maneuvers (at the least, you'd need full aerobatic stress limits), but I would expect one to fly well in normal operation if the rudder authority and dihedral are correctly balanced. If you're interested in building one and don't want to follow an existing design, I'd very strongly recommend building a radio control test model to verify the setup (tail moment, surface deflection, CG position, thrust axis, etc.) before cutting any tubing or fabric.

  • $\begingroup$ The configuration works perfectly fine within its limitations, just as hang gliders work fine within their limitations. The issue is what is optimal on a machine with wings and tail and control surfaces, and the acceptance of the limitations of skid-turning for the sake of some simplicity gained by leaving out the ailerons will be considered a poor choice by pilots experienced on normal aircraft. I mean, there's a reason in the end as to why almost nobody does it. $\endgroup$ – John K Jun 18 '18 at 17:30
  • $\begingroup$ It need not look and feel any different to the pilot from an original no-pedals Ercoupe (ailerons and coupled rudder) -- in my experience (with R/C) the "skidding turn" isn't noticeable (it might be more so from the pilot's seat, of course). There have been other factory-built GA aircraft that flew with rudder and no ailerons, mostly before WWII, as well as the whole Pou de Ciel family (of which only one model is known as a pilot killer). $\endgroup$ – Zeiss Ikon Jun 18 '18 at 17:50
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    $\begingroup$ Yeah and they pretty much all suck. The Ercoupe was an interesting one. To land it in crosswinds they designed a crab tolerant gear because that's how you had to land it. I would say most Ercoupes today have had separate rudder controls added by STC because most pilots hate not being able to cross control the airplane. $\endgroup$ – John K Jun 18 '18 at 20:05
  • $\begingroup$ At the time the Ercoupe and Pou de Ciel designs appeared, crossed controls were considered as much a pilot killer as they were necessary (most aircraft also lacked flaps at that time). Cross up between a slip and a skid at low altitude, you can wind up in a splintered, burning wreck instead of shortening your approach. These aircraft were attempts to prevent this, and some were "spin proof" as well. They may not fly the way you're used to after learning three-axis control, but they fly and some fly very well. $\endgroup$ – Zeiss Ikon Jun 19 '18 at 11:10
  • $\begingroup$ The feature that made the Ercoupe spin proof was a limited elevator range that prevented stalling deeply enough to enter a spin, not so much the interconnect (modern airplanes barely need any rudder to coordinate turns anyway). That also limits things you can do however, like have flaps. And its accident record is, surprisingly, no better than its contemporaries. In any case, forget about RC... believe me if you fly full size aircraft and are presented with a 2 axis airplane you will not like it once you experience its limitations. $\endgroup$ – John K Jun 19 '18 at 13:20

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