HALE UAV at 60,000 ft (20,000 m) – Flying in the stratosphere
basic mission
The basic mission of our HALE (high altitude, long endurance) UAV (unmanned aerial vehicle) is to fly over a point on earth for long periods of time. Exactly why is not important at this point in the discussion. Most likely, the goal is to replace a satellite communication.
Speed is not important, since we are only flying over one point. Energy efficiency is. In fact, the less power you need to maintain altitude, the more payload you can carry. Given what I just said, I hope a word has come to mind: glider. HALE’s UAVs look like gliders. Low wing loading, slow flight, with long, slender wings.
If the UAV is going to stay in operation for years, it needs a good power source. If a nuclear fusion reactor is the first thought that just popped into your head, you would be wrong. Instead, think of something simple and well proven. Think about 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 terms
At 60,000 feet, 93% of the atmosphere is below you. The air pressure is only 7% of what it is 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 chilling -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, most of the time commercial jet flights are below 40,000 feet (12,000 m). So that’s the absolute floor 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. The average wind speed at that altitude is 50 MPH (80 KPH). Again, that’s the average, it could easily be double. Not so much a problem with a passenger jet flying over 500 MPH, but a real problem with our fragile and slow flying UAV.
The average wind speed at 60,000 feet drops to just 15 MPH (25 KPH). It’s excellent news. That means we don’t have to fly any 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 half the time. Solar cells do not work in the dark. Maintaining the same altitude for 12 hours would require a large number of rechargeable batteries. To save on battery weight, we will slowly lose altitude at night. We still have to avoid airplanes and weather, but now our UAV glider will be more efficient.
maximum endurance speed
How fast do we have to fly? Well, a plane flies at different speeds depending on what the target is at the time. A speed well known to glider pilots is called best glide speed or best lift-drag (L/D) ratio. This is the speed at which the wing is most efficient.
The speed we care about is called loitering or maximum endurance speed. Another name for this is the required minimum power 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 best L/D speed value.
There is a big trap. At this altitude, the air is so thin that we need to fly 3.3 times as fast 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 need to fly 66 MPH (106 KPH) at our cruising altitude. That, unfortunately, also requires 3.3 times the amount of power to maintain. Suddenly, designing our UAV glider doesn’t seem so easy!
