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I've been trying (unsuccessfully) to figure out the relationship between airspeed of a fixed wing UAV at different flight stages. What I mean is, I would like to know the order of airspeeds from largest to smallest that would be expected for an aircraft (in particular an UAV with say 5m wingspan) to have during climb, cruise and descent. For example, the answer I'm looking for is something like: from largest to smallest, airspeed is expected to be ordered like this -> cruise>climb>descent (this would be my guess by the way).

Also I know that lift coefficient varies with angle of attack, airspeed and air density, but is it possible to make such a qualitative analysis to find an expected ordering like this for different flight stages? Looking only at the relationship between lift coefficient and airspeed, according to my previous guess, I would expect the lift coefficient to be smaller during cruise and larger during climb and descent. Is this correct?

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  • $\begingroup$ It is a function of the airfoil design. If you have an airfoil in mind, this is an easily answered question. If you have an airframe defined, it is a much more precisely answerable question. $\endgroup$ – mongo Apr 28 '17 at 0:06
  • $\begingroup$ Your answer relates to the lift coefficient part right? Or is the climb velocity and cruise velocity also dependent on airfoil/airframe configuration? I was wondering if, in general, aircraft tend to travel at higher airspeeds during climb or cruise? $\endgroup$ – UAV student Apr 28 '17 at 20:50
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I reread your question several times, and will attempt to answer. Hopefully I understand you correctly. First, for largest speed to smallest, I would order descent>cruise>climb. The way to think of it is that during a climb, you are building potential energy (altitude). When that energy plateaus at your cruise altitude, you stop investing in potential energy, and maximize your kinetic energy (cruise speed). During descent, there is energy to bleed, because you are lowering your potential energy (altitude) and converting it to kinetic energy (descent speed).

The gross assumption is that your UAV power setting remains about the same during all three phases of flight.

In this example, the lift is greatest during the climb, when you are investing in potential energy, and not using as much kinetic energy.

The descent would have the lowest lift, because you are willing to trade potential energy for kinetic energy, and the altitude decreases and the speed increases.

Forget my original comment, I don't think I understood your question at that time.

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  • $\begingroup$ Just to give you a conceptual speed value set: climb 50, cruise 100, descent 120. $\endgroup$ – mongo Apr 29 '17 at 3:14
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Observation UAVs are flown in a narrow range of lift coefficients. This can be seen from their wings: Like glider wings, they have a very high aspect ratio and a highly cambered airfoil. If you look at the optimum speeds for propeller and turbofan-powered aircraft, a high aspect ratio drives the optimum lift coefficient to high values which translates into a low calibrated air speed. Note that flying high does not necessarily mean that they operate at a low true air speed, however. Details depend on the altitude in which they are operated.

Let's look at the MQ-1 Predator for an example. Its aspect ratio is 19, and with all the protrusions I would guess the zero-lift drag coefficient to be no less than 0.025. This means the optimum lift coefficient for loiter is close to 2, which is probably above the maximum lift coefficient of the wing airfoil. For piston powered aircraft the best climb speed is identical to the optimum loiter speed, so the Predator should be flown close to its stall speed of 54 knots over the whole mission in order to get the most endurance out of this airframe. Note that the maximum g force the airframe can withstand is only 2, which means that top speed needs to be limited to 76.4 knots in order to avoid structural damage from flying into gusts.

Another giveaway is the low ground clearance of the tail surfaces: The predator lands at its regular flight attitude; there is no need to slow the aircraft down when it comes in for landing.

Short answer: The lift coefficient in all flight stages is high (0.8 to 1.4) while the true airspeed depends on the altitude. Turbofans tend to fly faster than piston-powered aircraft, but the difference is marginal.

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  • $\begingroup$ Definitely a great answer, based on engineering judgment. I'd just add that most UAVs fly almost constant aoa (angle of attack) throughout the flight for maximum endurance. unless there's some urgency to speed up (high speed descent, escape weather, leave a scene, etc). During a mission their fuel mass changes considerably which in turn means slowing down gradually to fly the same aoa. touchdown technics may differ from vehicle to vehicle, but climb and approach both are dictated by the STANAG 4671 as 1.3 VS, unless another value is validated to be safe enough. $\endgroup$ – Gürkan Çetin Sep 16 '17 at 22:06

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