UAV HALE at 60,000 Ft (20,000 M) – Flying in the Stratosphere

Basic mission

The basic mission of our HALE (High Altitude, Long Duration) UAV (Unmanned Aerial Vehicle) is to fly over a point on earth for extended periods of time. Exactly why is not important at this point in the discussion. Most likely the goal is to replace a communications satellite.

Speed ​​is not important, since we are only flying over one place. Energy efficiency is. In fact, the less power you need to maintain altitude, the more payload you can carry. Considering what I just said, I hope you have come up with a word: glider. HALE UAVs look like gliders. Low wing loading, slow flight, with long thin wings.

If the UAV is going to stay active for years, it needs a pretty good power source. If a nuclear fusion reactor is the first thought that comes to mind, you would be wrong. Instead, think of something simple and well-tested. Think of solar cells. Many of them. The good news is that our glider already has large wings, so they will be covered in solar cells.

Harsh conditions

At 60,000 feet, 93% of the atmosphere is below you. Air pressure is only 7% of that at sea level. The density of air, or the weight of air, is only 9.5% of what it weighs at sea level. The average temperature is a chill of -75 F (-60 C). Why would anyone want to fly at that altitude?

Why 60,000 feet

As I explained in the first part of this article series, commercial aircraft flights are below 40,000 feet (12,000 m) most of the time. So that’s the absolute floor that we can fly from.

There are a couple of other reasons that push us to fly even higher than that. First, at 40,000 feet the winds can really pick up. Average wind speed at that altitude is 50 MPH (80 KPH). Again, that’s the average, it could easily be double. Not a big problem with an airliner flying over 500 MPH, but a real problem with our slow-flying, fragile UAV.

Average wind speed at 60,000 feet drops to just 15 MPH (25 KPH). This is excellent news. That means we don’t have to fly faster than we would anyway. Slower is usually more efficient.

There is another very important reason why we want to fly higher than absolutely necessary. Flying over a place, we will be in the dark about half the time. Solar cells don’t work in the dark. Maintaining the same altitude for 12 hours would require a large number of rechargeable batteries. To save battery weight, we will slowly lose height at night. We still have to avoid airplanes and weather, but now our UAV glider will be more efficient.

Endurance speed

How fast do we have to fly? Well, an airplane flies at different speeds depending on what the target is at the time. A speed known to glider pilots is called the best glide speed or best lift-to-drag (L / D) ratio. This is the speed at which the wing is most efficient.

The speed we care about is called laziness or maximum endurance speed. Another name for this is the minimum power required speed, for obvious reasons. It is the speed at which we need the least amount of energy to maintain altitude. For a typical aircraft, this is approximately 76% of the value of the best L / D speed.

There is a big catch. At this altitude, the air is so thin that we need to fly 3.3 times faster just to generate the same amount of lift. This means that if the maximum endurance speed is 20 MPH (32 KPH) at sea level, we would have to fly at 66 MPH (106 KPH) at our cruising altitude. That, unfortunately, also requires 3.3 times the amount of energy to maintain. Suddenly designing our UAV glider doesn’t seem so easy!