# Low density air on a planet - effects on civilian and military aircraft (not spaceworthy)

Would aircraft have problems creating lift in a low density atmosphere? How plausible would it be to have jet biplanes, triplanes, possibly even quadruplanes under these circumstances?

EDIT: As a clarification, the planet is Mars.

• So, how low is "low"? – Sec SE - clear Monica's name Aug 29 '17 at 13:26
• Related xkcd – Sec SE - clear Monica's name Aug 29 '17 at 13:28
• This question might be more appropriate on Aviation.se. At the very least there is some crossover. – MozerShmozer Aug 29 '17 at 19:12
• @MozerShmozer It's asking about aircraft designed to operate on non earthlike planets. To the best of my knowledge Aviation only considers questions set on Earth as on topic. Asking Could planes fly in the low density atmosphere of another planet, and what would they look like? is definitely a question about worldbuilding. – sphennings Aug 29 '17 at 19:37

Lift is dependent on air density. This is why planes have a maximum altitude. They physically cannot fly any higher.

Other circumstances that reduce air density, such as high temperatures have been known to ground planes.

There are many ways of increasing lift. The two simplest are improving the geometry of the wing and traveling at a faster speed.

Your basic premise about increasing the area of the wing (bi, tri, quad-wings) would in fact increase the lift to the plane. Unfortunately it also increases the weight and drag of the plane.

To effectively fly you need to balance the lift vs. weight and drag vs. thrust. Generally adding more wings increases the weight of the aircraft and has other effects on the planes aerodynamics, (bi-planes are amazingly maneuverable at low speed and can takeoff in very short distances, but they have lower top speeds because of the increased drag from the wings) so it would be a balancing act to make this achieve whatever aircraft design criteria you wanted in a lower density atmosphere.

Another concern would be that pressure drops with altitude, so if you could fly in a lower pressure atmosphere you may have increased altitude restrictions compared to Earth.

Further complicating your problem would be the engines, if you want an air breathing jet engine, lower pressure is going to make them less efficient requiring a greater level of compression to function or even the injection of an oxidizer making it more of a rocket plane than a jet engine.

An area you could research for the extremes is various studies done on proposed powered flight in the very thin Martian atmosphere, they are all single wing, very very light planes, more comparable to ultralight aircraft than a jetliner.

• Also, the biplane was primarily a structural design choice. Early airfoils were quite thin. With the wood and fabric materials of the day, it was hard to make a monoplane wing strong enough to bear the necessary loads. A biplane can be made into a strong box structure with diagonal bracing wires, thus solving the strength problem at the cost of increased drag. Thicker airfoils and metal construction made monoplanes practical, so biplanes are only found in e.g. acrobatics or bush flying, where the increased roll ability & shorter takeoff runs are desireable. – jamesqf Aug 29 '17 at 17:36

As seen here, the equation for lift involves the lift coefficient Cl, the density, $\rho$, the velocity, $V$, and the wing area $A$ in this equation: $L =C_l\frac{\rho V^2}{2}A$.

The key thing for your consideration is that there are a few design variables we could play with to try to maintain lift. If density ($\rho$) is decreased, other variables must increase proportionally to keep lift a constant. The $C_l$ term can be increased to an extent to provide more lift but there are drawbacks. Velocity($V$) is nicer to increase, since it has he squared relation. It only needs to be increased by a square root factor. (If the density is halved, the velocity needs to be increased $\sqrt{2}$ times). Lastly, we can increase the wing area. Super simple, right?

Wrong. Unfortunately, these design variables have other effects that aren't immediately noticed. Increasing the area often means more weight and/or drag. Velocity increases often require larger powerplants, and Cls can be picky in their performance across your flight envelope.

That all said, its totally possible to design for and fly in low density, there are just a few things to really look into.

As to the specifics of Jet Bis, Tris, and Quadplanes, I don't know. A lot of high altitude (thats low density on earth) aircraft have very high aspect ratio wings (quick google search). From what I understand much of the reasoning behind bis, tris and anything above, was that the materials available didn't support the necessary structure, at least not easily. So, to get around long, broken wings, they added multiple shorter wings (still getting the same wing area) that the materials (wood, wire, canvas) and the structure (wire box frames) could support. They did have plenty of drawbacks. Those structures with all those wires added a ton of drag. Stronger materials like aluminum came into phase because they could support the wing, without the exposed "draggy" wireframe structure. In, my mind, If you have the tech for jet engines, likely you have tech for long single wing structures.

Check out this Aviation SE post about biplanes.

If you decrease air density, yes of course you make it harder to generate lift

because lift is created from the difference in air pressure/density above and below the wing.

If it's too little you can't fly at all. If it's less than what's found on Earth you are gonna need bigger wings, if it's greater, you can use smaller wings.

This applies to all AIRcraft, civilian or military makes no difference.

Another aspect is all aircraft with exception to gliders and blimps require oxygen to fuel their energy producing combustion reaction.

This is why the spaceshuttle uses a rocket and isnt used on Mars. (the wings dont create enough drag)