Proper Use of Scale Models for Aircraft Development

There are many obstacles engineers must overcome to create scale-model aircraft that are effective development and testing tools. To design and calculate a viable aircraft is one thing. But to take it from the mind’s eye to actually building and testing it can prove to be challenging. At Morgan Aircraft, we have designed, built, flown, and tested ten distinct small-scale airframe models of our vertical-take-off-and-landing (VTOL) aircraft technology. This modeling work has greatly accelerated and enhanced our program.

When developing scale-model aircraft, one of the most basic but very important factors to keep in proper balance is the overall Horsepower-to-Weight Ratio. It would be easy to install an engine in a model aircraft that is too powerful. If such an overpowered model were to be scaled up to a full-size aircraft, it would be impractical to achieve the same power-to-weight ratio that the scale model achieved. The full-size aircraft would also not be able to match the extraordinary flight performance that scale-model would exhibit.

On the other side of that equation is the Weight. It is possible to build model airframes that are unrealistically light compared to the overall wing area and engine power necessary to fly that particular scale model. This can result in an unusually low wing-loading that would, again, yield over-performing behavior, while requiring less power. It would not accurately represent the actual in-flight characteristics of the final full-size airplane.

Many other aircraft design features and specifications can be misrepresented as well. Parasitic Drag from erroneously low wing loading is much lower than it would be if the aircraft were weighted properly. Often these scale-model aircraft act nothing like their full-scale counterparts. Control Responses and Sensitivity Rates are very light to the touch and actually make the plane harder to fly in many cases. Take-off Distances are artificially short and Landings can be abrupt and hard to manage. The same standard atmosphere that pilots fly real aircraft in remains constant as these models fly with relatively less effective wings and aero shapes. In other words, they simply do not closely represent or predict the flight characteristics of the full-size version of the same plane.

This is not to say that all smaller scale models are so difficult to fly that no one could possibly attain reasonable control of them. On the contrary, there are many highly skilled modelers and radio-controlled (RC) aircraft pilots that, in fact, master and even purposely manipulate small model designs to perform incredible stunts at air shows. Some purposefully design toward the unstable but radical-performance end of the continuum, while others design toward the super-stable end, making certain models extremely easy to fly. For example, there are several model kits that employ extra-stable modifiers in order to help new RC pilots gain confidence and spatial skills before they move on to faster, more complex (more real) model aircraft.

At Morgan, we are very careful to maintain proper ratios with all these factors in order to use scale modeling as a real and effective design evaluation tool. The scale models we build have correctly sized wingspans and wing areas, properly weighted airframes, and appropriately proportioned engine installations. Careful modeling has enabled us to greatly accelerate our overall efforts and minimize development expenditures.

Another major challenge that we face, particularly because we are working on a VTOL aircraft concept, is Aircraft Density. There are a lot of systems and pieces of equipment that must be packed into the airframe to properly test and feel the various control aspects and flight characteristics of our aircraft. In addition to all the normal aircraft flight controls, we need to fit in lift-rotor systems, redundant gyro-stabilization systems, and extra power to run these, while minimizing excess radio frequency interference.

This required us to engineer miniature versions of rotor hardware, mountings, and airframe sections that are not only true to scale, but also are structurally sound and sufficiently light weight to yield a model airframe that has the correct overall weight, and one that is actually usable. This describes, in part, one of the biggest challenges for any VTOL program: How to integrate an effective and safe VTOL apparatus that does not add too much dead weight to the overall airframe, so that it can achieve its targeted forward-flight parameters.

As we developed and tested our advanced rotor systems and strived to fully model and program our stability augmentation system (SAS), we found ourselves no longer able to use off-the-shelf (OTS) RC model components. We had to create custom components that would accurately represent the form and function of the corresponding full-size components, yet that would fit into, be light enough, and interface well with the OTS components we were able to use. Furthermore, we were required to integrate many OTS components with each other in ways that were not originally intended by the OEMs. This was both challenging and rewarding -- we achieved great results.

Careful modeling of our designs has helped us to fine tune the control methodology of the aircraft. We have achieved stable vertical and forward flight, even in the smaller scale models. The larger airframes benefited from the control rates and sensor ratios that were developed on the smaller airframes, proving the scalability of the technology. We are now taking the strong results from our modeling work and are translating them into computer analytical models that will be verified and calibrated to larger scale and full-scale aircraft.

Scale aircraft modeling can be a very effective tool. Care must be taken to properly ratio the vital factors. Sport and hobby aircraft modeling can be loads of fun and very rewarding, but it can also be an effective engineering tool that saves money and helps to accelerate the aircraft design process, especially at the early stages of a new program.

copyright Morgan Aircraft 2011

copyright Morgan Aircraft 2010