Why should I not use a "lifting tail" on my ultralight design?

Question

In my misspent youth I designed and built many rubber, gas, and RC models. When I wanted good climb and glide I used a aft CG [60 to 80%] and a lifting tail. I had no stability or control problems, though CG was critical. I'm a 75 yr old retired air traffic controller, pilot, flight instructor, A&P mechanic who would like to get his feet off the ground again with a flying wing or tandem wing ultralight.

I don't need speed, but I would like to climb in thermals and ridge lift. So far a flying wing like the Monarch with 15 to 20hp and a wing loading under 4.5 lbs/ft. and a span of 30/40ft sounds good. With two lifting surfaces I might keep span under 25ft with a good aspect ratio. Please try to keep answers in laymen's terms.

Answer 1

You can use a lifting "tail" for any plane design but it will depend on the design. The only requirement for a naturally stable flying aircraft is that it must have a reasonably good positive static margin. And for that to happen the rear flying surface must have a lower wing loading than the forward flying surface - this does not necessarily mean downforce though that's an easy way to comply with the rule.

In one extreme case you must use a lifting "tail" or the aircraft would not have enough lift to fly. These are canard planes such such as the Rutan Long-EZ. Now, you may make the argument that the thing at the back is not a tail but a wing. Yes, if you were to insist on classifying planes by rigid categories then the rear flying surface of canard planes are normally called "wings" but this mindset will make one blind to what really happens to aircraft stability as you change the planform design.

CG placement, forces on the tail and planform

The correct CG placement for stability depends on several things including the distance between the wings and stabilizers and the airfoil of the wings but the dominant factor is the relationship between the area of the wings and the area of the stabilizer.

Most full-scale aircraft including ultralights tend to have very small stabilizers:

In this configuration the stable range of CG is usually between the leading edge of the wings and the midpoint of the chord. Typically this manifests itself in the rule of thumb of between 25% to 33% of the chord but for actual values you need to calculate the CG based on your desired static margin.

You will notice that most model planes tend to have larger stabilizers. This is especially true for non-scale models and even more so for free-flight models:

In this configuration the CG for the same static margin (stability) appears to be quite far back. In most cases it is 50% of the chord - right near the middle of the wings. It only appears that way compared to full-scale planes or scale models. Aerodynamically the CG placement gives exactly the same level of stability.

This is when some planes can start to be trimmed with a lifting tail. Some real-world planes with large tails such as the Avro Lancaster may in some conditions fly as if the tail is lifting.

As you increase the size of the stabilizer further the CG for stable flight shifts further back:

This is commonly seen in free-flight duration models. Almost all planes with such planform have a lifting "tail". It is rare to see a real plane with this planform however there is one very famous design where you can obviously see the lifting tail - the Pou-de-Ciel:

(Note that the original design of the Pou-de-Ciel has a catastrophic fault where the rear wing interacts with the downwash of the forward wing - later revisions moved the wing back several inches - DO NOT BUILD THIS unless you know all the issues)

If you change the relationship between the forward and rear lifting surfaces further you will end up with a tandem wing. If you've been following the trend at this point I think you can see where this is going: yes, the CG moves even further back:

The typical CG for tandem wings is completely outside the forward wings and sit between the forward and rear wings. Remember, all the planforms so far including this was calculated to have exactly the same static margin - they are all similarly stable.

All tandem planes have both forward and rear flying surfaces configured to be lifting.

Tandems are less rare in manned airplanes than the almost-tandem configuration above. The most well known is probably the beautiful Rutan Quickie:

As we progress with changing the relationship between the forward and rear flying surfaces we will eventually end up with the canard configuration:

Canards obviously have a lifting rear flying surface. So much so that people instinctively refer to them as "wings". And the CG moves further back still (in relation to the forward flying surface, obviously it is in front of the "wings").

TL;DR

As you can see, weather or not your tail is lifting or generates downforce is not simply a choice you make. In some instances you will be forced to make your tail lifting. In some instances it will depend on your current maneuver. It all depends on the design of your plane.

Side note: Why most planes have (relatively) tiny stabilizers?

There are two main reasons why most designers chose to make the tail small:

First is to reduce weight in the tail. The larger the tail the harder it is to get the weight distribution right for the correct CG. Note however that this is mostly due to most designers using conventional wisdom because as we see above if you make the tail large enough you can shift your CG further back for exactly the same stability.

But this leads us to the second reason: to reduce drag. The larger the tail the more drag it generates. Tandems are the worst-case scenario where both the wings and "tail" generate the same amount of induced drag. However several designers such as Burt Rutan have taken advantage of the characteristics of tandem wing planes to produce very efficient designs.

Answer 2

When I wanted a good climb and glide, I used aft CG, and a lifting tail...

That all goes great, until you stall and have to recover. Models generally have a much higher thrust to weight ratio than full scale recreational aircraft, which are generally 1:4.

With an aft CG, the tail will tend to "sag" down as velocity decreases, which will result in a higher Angle of Attack, producing a straight velvet-like glide right up to the point of stall. You definitely do not want this full scale.

Tail heavy models are require less trimming with change of speed, but lack of "static stabilty" is dangerous in slow flight. Excess power covers a "multitude of sins" with models because adding throttle can increase airspeed almost instantaneously, whereas at full scale precious seconds can pass having to accelerate more weight with proportionally less thrust.

It is power that enables the climb, not the "lifting tail". You just do not want one with excessive down force, which can not only be the result of excessively forward CG, but also excessive downthrust angle of a nose mounted prop.

If you want to "get your feet off the ground again", why not go flying with an instructor in a safe and proven design like a Cessna 172? They've got all the theory figured out all ready, leaving you with a safe and fun aircraft to fly.

Answer 3

With a conventional (downforce) tail, a CG forward of the center of pressure would cause the nose to drop as airspeeds decrease. The less airspeed, the less downforce. This nose-down pitch would increase airspeed until a stasis is reached. The system would be statically stable, decreasing pilot workload.

If you moved the CG further back for a lifting tail, as speed decreased, this would cause the nose to pitch up. The less airspeed, the less upward lift. This nose-up pitch would cause the airspeed to decrease further which would cause more nose-up pitch. The system would be statically unstable, increasing pilot workload.

You could put the CG at roughly the same place as the center of pressure. You could even balance the system to provide equal amounts of lifting force and torque with the CG between the two lifting surfaces. Unfortunately, having two lifting surfaces of different sizes and design would create different proportions of the total force of lift as airspeed varies.

Furthermore, a CG closer to the rudder and elevator (specifically the rudder) would make recovery from a stall and/or spin more difficult. The moment arm of the corrective force would be less.

Answer 4

If you want a low sink rate, make sure that induced drag is low. This can best be achieved by a wide wing with a small tail surface. Distributing the wing area between two wings equally will result in one large wing flying in the downwash of another large wing and higher induced drag in total.

With only 25 ft wing span and a total mass of between 120 and 150 kg, your span loading will be 18 kg/m (12 lbs/ft). This is clearly too high for weak thermals, so more wing span is needed for soaring. Aspect ratio is not all - your induced drag is dominated by span loading, so keep that low.

Going with a flying wing as your first design is very ambitious – a decent tail surface makes things a lot easier. Keep the loading of this tail surface low so you have enough margin for maneuvering. Yes, a more forward c.g. location ( = more tail downforce) gives better spin resistance, but by selecting a wing tip airfoil with good post-stall behavior you can gain the same benefit with better performance overall.