Tandemwings

In the tandemwing both arrows go up. So...both wings generate lift. And that sounds nice when you want to stay up in the air, doesn't it. Both moments are countered by a delicate balance between moments, placement of the wings and center of gravity of the entire airplane. I guess that this delicate balance scared many designers to use tandemwings. But ...these days ...with the aid of computers ...why would they not be tried? They have serious advantages.

Why a tandemwing?

Why would anybody make a tandemwing? Well, i have already found a few advantages that make it worth looking at the concept. The most remarkable one came from Scaled Composites (the firm that Burt Rutan started) about their Proteus project.

Why? Reducing dimensions!

When you have a airplane. You need to place it under a roof. But hangar space costs. And the access to a hangar is through a door. Sounds logical, isn't it.

Well, the hangar door was one of the issues to use a tandemwing design for the Proteus. It had to get through the hangar door of Scaled Composites! (source of info: Kaye LeFebvre of Scaled Composites)

If you are hoping to fly at low costs, you want to avoid the cost of hangar space. Placing your airplane in your own garage is a good idea in such a case. Since the 40's Henri Mignet used hinges on his Pou du Ciel designs. This way the wing could fold and the complete airplane could fit into a garage. A big advantage for a lot of flyers.

But ... make sure that you first check the dimensions of your garage door. I came to the surprise that I missed a few centimeter (inches). So I had to dismount the front wing and hang it on the ceiling of my garage. Anyway... I don't have to pay hangar space costs.

Why? Stability!

A airplane without the deathly stall is a must-have for a beginner pilot. Stall is a situation where your wings don't generate enough lift to keep you in the air. Many pilots were killed due to this loss of lift when making a take-off or landing.
Well, the tandems are stall-free like the canards are. Let me explain.

In the page "design tips for tandems" you will read that two identical wings are hard to control. The airplane does not know which is the main one. For that reason they shift the center of gravity a bit more forward. This result in the airplane pushing harder on the front wing that one the rear wing. In technical terms this is called "a higher wing loading on the front wing".
Now, if you knew that if two wings fly at the same speed the one with the highest wing loading will first stall, you will understand that the front wing will stall. But ...hey, didn't I tell that stall was deathly. Yes, but here there is a strange but good thing into this stall. Lets look step by step what happens if a conventional wing stall and when a front wing of a tandem stalls.

Conventional airplane:

1. The speed of the airplane is reduced (you are landing or taking off).
2. The wing with the highest wing loading gets nearer and nearer its point of stall. One can sometimes feel it. The airplane starts to vibrate. If you don't change the speed (by giving more power or pointing the nose more downwards), the wing will enter a stall.
3. The wing enters a stall. There is no more lift from that wing. Here, in a conventional wing, the wing that stalls is the largest wing. The surface of the remaining wing surface, the tail, is not able to lift the airplane (It cannot do that even if it was a bit larger, because the tail is generating a downwards force).
4. Oeps, the wing has no more lift, the weight of the airplane cannot longer be held in the sky. The airplane falls like a brick. The nose will drop fast and you will dive. You can correct from this situation by gaining speed and pulling up once the speed is high enough to get lift again from the wing that did stall. If you are close to the ground (when making a approach for a landing), you might not have the time to correct this situation. Sorry, you are dead.

Tandem winged airplanes:

1. The speed of the airplane is reduced (nothing new here)
2. The wing with the highest wing loading gets nearer and nearer its point of stall. Here you have also the possibility to react. If you don't the wing with the highest wing loading will enter a stall. Nothing new again.
3. The wing enters a stall. But here, in tandem wing configuration, the front wing will stall. What happens now? Well, the nose drops a bit. The rear wing is still holding the airplane up a bit. But it cannot hold it completely, so the airplane will sink while its nose drops a bit. But, the nose that did drop makes the angle of the airplane (and its wings) less, so ...the wing lifts again once the angle of the airplane is good enough. The nose rises again. You get another change to react to the stall.
4. The result is that you enter a wave like movement, you sink while you have your nose going down and up again. You can keep on to this situation and even use it as a kind of hard controlled descent. Or you can get more time to react to this situation and give more power or lower your nose. Anyway, you are given a chance to live and tell.

A even deathlier thing is a spin. What is a spin? Well, it starts in a stall.  Your nose drops and you start falling like a brick AND your left (or right) wing starts to drop too. Now you are in a spin ... loosing maybe 100 m or about 300 feet per second. When flying low there is not much time to react. Yes, one can get out of a spin. But you need the height to do this!

Many conventional airplanes have a design which makes it possible to steer to avoid a spin. But you still need to steer. In a tandem ... a stall gets avoided automatically.

When a canard and when a tandemwing

Some of the designs are very close to a canard. I started asked myself "why is the edge between canard and tandemwing?" I found a possible answer in the homebuiltairplanes forum. In short it says that it is hard to tell when it is a canard and when a tandemwing. But if you need to place a figure on it, below 2:1 or 1:2 wing area ratio is a canard, higher is a tandemwing. 

Special section of Flying Flea

I have a entire section about the Flying Flea. Sure is a good place to look if interested in tandemwings.