Search Dryden



Text Size

Past Projects - APEX - Frequently Asked Questions

The following is a list of questions, and corresponding answers that have come up repeatedly during the project. This is not a complete list, however, the project will update this list whenever possible.

Q: How many different airfoils will be tested on the Apex aircraft, and will the test section airfoil be the same as the rest of the wing?

A: The entire wing (including the test section) will use the Apex 16 airfoil designed by Mark Drela for initial flights from the balloon. This airfoil has been optimized for the flight conditions of the transition maneuver from vertical to horizontal flight (M = 0.65, Re = 300k). It also has robust, yet benign characteristics over the entire flight envelope. The inevitable upper surface separation at high subsonic Mach numbers affects the lift and drag gradually, not suddenly.

After the performance of the Apex 16 has been measured and documented, other candidate airfoils could be evaluated in a similar manner. No candidate airfoils have been identified at this time.

Q:What is the maximum Mach number that the vehicle will be constrained to?

A: Based on the predictions of the performance of the Apex 16 airfoil, the maximum Mach number that the vehicle will be constrained to during the transition maneuver (after aircraft release) will be Mach 0.65. During the data-gathering portion of the flight, the flight-test plan may call for the aircraft to exceed that Mach number (most likely at reduced lift coefficients.) The result of exceeding 0.65 Mach could be a large loss of lift and a corresponding loss of altitude. One of the goals of the experiment is to measure airfoil characteristics at Mach and Reynolds number combinations that cause airfoil performance to degrade.

Q:Will the Apex aircraft use a glove over the existing wing or a new wing for the high-altitude flight?

A:A new wing will be designed for the Apex aircraft for flights at high altitude. The existing American Spirit wing may be used for any low-altitude system checkout, or airdata calibration flights (details will be described in another document). The new wing will be designed to provide a large, "clean" test section for airfoil measurements while minimizing 3D effects. The wing design will be structurally compatible with the existing fuselage and will be sized to provide the desired Reynolds number and wing loading. The wing will include internal volume and mounting provisions for embedded instrumentation and actuators. A structural model of the wing design will be created and analyzed for static and dynamic loading.

The following is a list of reasons why a new wing will be built rather than applying a wing glove to the entire span of an existing wing:

  1. Vehicle weight will directly affect the final altitude that can be reached using the balloon-assisted launch technique. The project would like to maximize that altitude in order to reach very low Reynolds numbers. A new wing can be built significantly lighter and stronger than a gloved wing.
  2. It is required to measure strain on the wing for load-testing and in-flight gust response determination. A gloved wing introduces unpredictable, non-linear load paths into the structure that would make the accurate measurement of wing loads very difficult.
  3. Flutter analysis of a gloved wing is much more complicated than for a wing alone. If the gloved wing does not meet flutter criteria, modifications to the underlying structure might be required.
  4. The new wing offers a great deal of experiment and aircraft design flexibility (actuator mounting, sensor mounting, control-surface size and location) that solves problems across all disciplines.
  5. Existing wings have significant taper, which can introduce significant 3D flow effects on the test section.

Q:What surface quality is necessary on the test-section airfoil and the rest of the wing to provide both good experimental data and to ensure airfoil performance?

A: Low Reynolds number laminar flows are less sensitive to surface waviness and roughness than high Reynolds number laminar flows. However, low Reynolds number, high Mach number flows are quite sensitive to pressure distribution changes. Therefore, the Apex airfoil will be more sensitive to the accuracy of the airfoil contour than its surface finish. The recommended approach is to specify to what tolerance the finished wing must match the requested airfoil shape. By also specifying that the manufacturer use production sailplane construction techniques, the surface roughness and waviness should be quite adequate. If there is a significant cost/quality trade-off for this construction technique, the manufacturer could pay more attention to the first 60 percent of the upper surface and leave the rest of the airfoil somewhat rougher.

Q: What role will wind tunnels play in accomplishing the Apex objective of validating the CFD code?

A: Several organizations may have the capability to perform appropriate wind tunnel experiments to match the Mach and Reynolds numbers expected in flight. However, a basic problem exists with using wind tunnels to predict low Reynolds number, high Mach number airfoil performance. The problem stems from the level of freestream turbulence inherent in wind tunnels and the strong effect of freestream turbulence on boundary-layer dominated flows. Determining the relationship between freestream turbulence and airfoil performance would require the measurement of freestream turbulence in flight. Quantifying this relationship is not an objective of the Apex project. Since this measurement is extremely difficult and not required to satisfy the Apex objectives, no such measurement is currently planned. Therefore, there is no plan to use wind tunnel data to validate the CFD code as a part of the Apex project.

Q: What attitude will the Apex vehicle be in at the time of aircraft release from the balloon?

A: The optimum aircraft attitude at release is the one that will allow the wing to start generating lift as soon as possible. Releasing the aircraft pointed straight down (in its lowest drag attitude) will allow it to accelerate to substantial dynamic pressure within 10 or 20 seconds. Given the aircraft's large static margin, the nose will tend to point into the relative wind (straight down) naturally. If the aircraft is released in any other attitude, the angle of attack would have to be reduced below alpha for Clmax (< 8 degrees) before the wing could begin to generate a significant amount of lift. For these reasons, the aircraft will be released with the nose pointed straight down. (NOTE: Roll attitude at release will only effect cross-range location after pull-out.)