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Most aircraft feature the tailplane configuration, which requires the main wing to provide higher lift than the weight of the plane itself, what seems counterproductive.

Few aircraft have featured tandem-wings like the Rutan Quickie, Proteus or the Maurice Delanne 10 with limited success. I usually read that the main disadvantage of the tandem-wing is that the rear wing is subjected to the downwash from the front wing, what greatly reduces its efficiency Q&A link.

My point it that standard tails suffer from the same downwash effect, and on the other hand this effect can be reduced by:

  • a T-tail
  • Making the front wing much larger than the aft-wing.

enter image description here

  1. Thus, which are the flaws of this design (top)? Given that it could still have the CoG in front of the Neutral Point.
  2. Why are lifting tailwings refered as unstable? Q&A link
  3. Is this layout suitable for VTOL aircrafts, in which horizontal take-off and landings are not an issue?

enter image description here

Thank you for your attention. I would appreciate remarks of any wrong assumptions.

Extra

  1. Why the Proteus features a tandem-wing, and why this layout unusual?

Edit: The reason for considering a lifting tail is that to make VTOL possible for this layout, the CG must be moved between the rotors, thus far aft.

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  • $\begingroup$ That planform diagram looks like a conventional wing and tail design to me. What is different about it? In fact both designs pictured look very conventional... $\endgroup$
    – Michael Hall
    Commented Apr 15, 2020 at 17:31
  • $\begingroup$ the section about pitch wise stability of how it flies is a good reading tp better understand your question. $\endgroup$
    – Manu H
    Commented Apr 15, 2020 at 17:54
  • $\begingroup$ I think birds get lift from their tails. Fuel is more expensive but the "avian-onics" are better, so stability is less important. I think. $\endgroup$
    – Kevin Kostlan
    Commented Oct 12, 2022 at 6:34

3 Answers 3

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A lifting tail just means lower static stability.

You are correct, flying in the downwash of the wing makes the tail less efficient. But that does not mean it is all bad: Since downwash increases with lift coefficient, the tail effectively sees a smaller angle of attack variation than the wing, so it maintains a healthy margin from stalling even while the wing is stalled. This helps to keep the aircraft controllable well into a stall (within limits, of course).

The reduction in downwash as the center wing stalls will even add a positive AoA gradient during the incipient stall which adds some more stability, making it harder for the pilot to command a further AoA increase.

And performance-wise, the download on the tail in conjunction with downwash actually might create a bit of thrust on the tail.

Concerning Rutan Designs: Their different looks are not for better performance but merely the expression of an oversized ego and his way of self-promotion.

EDIT:

The many downvotes I now collect for speaking my mind freely shows how well the self-promotion of Burt Rutan works. But it cannot bend the laws of physics which guide my opinion. Which I will continue to express freely.

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  • $\begingroup$ I've been following Rutan since the 70s and I've always had a similar take. His brother, a former F-4 pilot who once did a record endurance flight in a Long Eze where he had fatigue induced hallucinations during the overnight part, is pretty much the opposite. $\endgroup$
    – John K
    Commented Apr 16, 2020 at 0:12
  • $\begingroup$ @JohnK -- "where he had fatigue induced hallucinations during the overnight part" -- just like Lindbergh on his 33 1/2 hour flight... $\endgroup$
    – quiet flyer
    Commented Apr 16, 2020 at 0:38
  • $\begingroup$ Yep I read Lindbergh's book. He crossed the Irish coast only a couple miles off track in spite of having dozed off, and wandered this way and that before waking up, numerous times. I guess his wanderings all evened out. He also had plenty of fuel left when reaching Paris and mulled over continuing to Rome, but the thought of crossing the Alps at night was enough to squelch that idea as maybe pushing his luck just a little too much. $\endgroup$
    – John K
    Commented Apr 16, 2020 at 1:02
  • $\begingroup$ @JohnK: What I remember most of Burt's brother is his foul language. He called every aircraft a "flying piece of excrement". That does not endear me to him. But to his credit I have to admit that he flew Voyager for a week. Jeana did not do much flying but was there to keep Dick awake. $\endgroup$
    – Peter Kämpf
    Commented Apr 16, 2020 at 14:24
  • $\begingroup$ One cannot write off the record-breaking achievements of a plane like the Voyager as down to "the expression of an oversized ego and ... self-promotion." It was a brilliant piece of aircraft design. Burt Rutan cannot be written off just because you personally object to his style. $\endgroup$
    – Guy Inchbald
    Commented Apr 16, 2020 at 17:01
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Focusing on the aircraft in the picture, lifting tail aircraft are essentially bi-planes. The single lifting wing has proved to be more efficient long ago. Simple adjustment of the CG removes the need for a lifting tail. This plane could be flown that way.

It is the job of the tail to set the wing at a given AOA, using aerodynamic force in flight to hold that AOA. The thought of tail "downforce" creating a huge lift penalty is not true. In flight, it is the angled, lift producing wing that creates the "induced drag", while the tail rides along at its lowest drag. Torque around the Center of Gravity keeps it that way, unless some disturbance changes the wing AOA. Many aircraft, like the Piper Cub, use a low aspect flat plate as a horizontal stabilizer, with an elevator to create a crude, but adequate "tail wing" to change pitch or trim as needed.

Only when CG is placed behind the center of lift does a lifting tail become necessary, but directional stability is affected because now more wing and fuselage area is ahead of the CG.

Many planes on the ground show the "decalage" of the tail appearing to provide a lot of downforce, but in the air it will "weathervane" into the wind, torquing the wing to the desired AOA.

Moving the weight forward on that model needs only adjusting the fore and aft engine thrust percentages for VTOL. In the air, treat it like an airplane, with one lifting wing and CG slightly forward. And yes, keep the T-tail.

An add on for those who wish to further understand lifting tails. The whole point of a tail is to increase directional stability. Taking a wing without a tail, and balancing the center of lift with the CG still will have some directional stability because the center of lift is usually at least 2/3s of the way forward and the back portion of the wing acts as a "stabilizer". Add a tail to this and you are far more directionally stable, and there for able to move CG behind wing center of lift and still have directional stability. This is by no means fatal in all cases, but will make the plane less stable and easier to turn. Good for fighters, not for cruisers.

For the modeler building the VTOL above, no need to get stupid when simpler solutions are at hand. Make it an airplane first, and use differential thrust for VTOL. At least 2 of the motors could be idled in cruising flight.

Regarding the Proteus, Rutan wanted a high flying heavy lifter. A very high aspect wing is most efficient, but presents structural challenges. So he made a bi-plane. Colonel Pezzi flew one to 51,000 feet in 1937. With Rutan, one strives to understand function as well as form.

PS. A lifting tail is entirely safe as long as the CG is within design limits. The XB-70 certainly is a "lifting tail". People get into trouble moving CG behind design limits because they use up elevator throw to create lift and also make the plane less stable than it is designed to be.

But take a model of an XB-70 to the hill and fly it off with an average sailplane model of equal weight and wing area. No contest for gliding efficiency, but the XB will have a much wider speed envelope under power.

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  • $\begingroup$ it's Center of Gravity applying torque about the Neutral Point (Center of Lift adjusted to account for external moments increasing or decreasing the torque). Gravity acting at the C of G is the force APPLYING the torque. Move the CG aft to the NP and the torque goes to zero. $\endgroup$
    – John K
    Commented Apr 15, 2020 at 19:11
  • $\begingroup$ @JohnK you can take moments about any point of a solid as long as the sum of forces on that solid is zero. Taking them about the center of mass is advantageous because it removes one of the moments. $\endgroup$
    – AEhere supports Monica
    Commented Apr 15, 2020 at 19:18
  • $\begingroup$ @John K interesting that you mention that (twice). Theoretically, both explainations work, and yours was actually favored because it provides an easier explanation for static stability. However, the effects on moving the CG back on the plane are the same no matter how we explain it theoretically. That plane in the picture looks like a winner to me, maybe with a hair more dihedral. $\endgroup$
    – Robert DiGiovanni
    Commented Apr 15, 2020 at 19:20
  • $\begingroup$ My point is moving the GG fwd increases the torque, increasing tail downforce reqmt, and moving it aft decreases it. When CG is at NP, torque is 0 and tail downforce required is 0, a rather unstable craft. Keep going aft, to a negative value, forcing tail to lift up to keep the nose down, you have a fatal condition normally. Lifting tail airplane is really a tandem wing with a small aft wing; problem becomes how to get a desirable pitching tendency w speed; slow down pitch down, speed up pitch up. Canards do this by having a steeper lift slope for the front wing by using a different airfoil. $\endgroup$
    – John K
    Commented Apr 15, 2020 at 20:17
  • $\begingroup$ @John K yes, if CG is forward some down force is required, but not necessarily 100s of pounds. The plane pictured, especially as a model, with mixing of forward and rear VTOL thrust, would fly well without the need for a lifting tail. Sort of reminds me of another way to make an Avro Lancaster, which flew well enough to gain admiration from the fighters. $\endgroup$
    – Robert DiGiovanni
    Commented Apr 15, 2020 at 20:27
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It is not actually true that

... the tailplane configuration ... requires the main wing to provide higher lift than the weight of the plane itself

What matters for stability is that the change in lift moment of the tail should be greater than that of the main wing; the absolute value of its lift is not relevant.

The lifting tail has been well understood since around 1918.

In a tandem wing, both planes provide lift. Now consider the rear plane being progressively shrunk to the size of a tail plane. It is quite false to imagine that it must necessarily pass through a zero-lift condition at some point and end up with downforce. Practically, it might do, but that is another issue.

The tail creates least drag when it has zero angle of attack and for maximum economy the aircraft is trimmed to approach that condition (although, as pointed out in another answer, operation in the wing downwash can make a small downforce the ideal condition). However trim frequently changes during flight and the ideal condition is seldom met in practice, and then only transiently as the trim adjusts from one side of the balance to the other.

What often confuses students is that the wing must be designed for maximum lift at the moment of rotation for takeoff, when it is fully laden at its slowest flight speed and the tail is pushing down strongly to rotate the nose up. But that condition is only transient, it soon passes and the cruise condition is quite different.

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  • $\begingroup$ A conventional aircraft with the CG aft of the neutral point, required if you want to make the tail make positive lift, has a negative static margin and is unstable in pitch.Trim Drag is the energy lost in creating downforce due to the positive static margin and you can minimize it but can't eliminate it because you have to maintain a positive static margin unless you have artificial stabiltiy. This idea of cruising with a lifting tail, on an airplane not designed as a tandem wing aircraft from the start, is crazy. boeing.com/commercial/aeromagazine/aero_02/textonly/… $\endgroup$
    – John K
    Commented Apr 17, 2020 at 1:48
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    $\begingroup$ To someone who does not appreciate the difference between lift moments which resolve around the centre of lift, vs changes in lift moments which resolve around the aerodynamic centre, it might seem crazy. To others, including experienced pilots I have discussed it with, trimming the tail for positive lift is a familiar and safe phenomenon. en.wikipedia.org/wiki/Aerodynamic_center $\endgroup$
    – Guy Inchbald
    Commented Apr 17, 2020 at 7:04
  • $\begingroup$ To trim the tail for positive lift the CG must be aft of the NP. If the CG is forward of the NP, there is a ND pitching moment and making the tail lift just adds to it. Static margin requires CG to be a minimum distance ahead of the most forward NP. I don't know where you are getting this from and cannot find any source anywhere that says a conventional tailed aircraft can operate with the CG aft of the NP, except aircraft with artificial stability designed to operate in an unstable state for maneuver purposes. If you have a source, NOT wiki or ASE, that covers this, I'm all ears..well eyes. $\endgroup$
    – John K
    Commented Apr 17, 2020 at 22:42
  • $\begingroup$ Sorry, your conclusion is just plain wrong. There is not space for the analysis here but I urge you to consider the marginal case between a tandem with small real wing and a conventional tail; what changes and why? More importantly, what does not change? What effect does a lifting rear surface have on the NP? Understand these and you may yet get there. $\endgroup$
    – Guy Inchbald
    Commented Apr 18, 2020 at 10:38
  • $\begingroup$ Well for starters, with a tandem/canard the stabilizing surface with the steeper lift slope and variable camber element, the "elevator", has to be on the forward lifting surface, with the NP between them and the CG, still, forward of the NP. You don't seem to understand the basic difference between a tandem/canard and conventional tail, and won't address my question directly. Fine. Provide a link to an authoritative source, NOT an anonymous wiki or another ASE post, somewhere like NASA or a university source, that describes what you are saying. It should be easy. $\endgroup$
    – John K
    Commented Apr 18, 2020 at 13:10

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