The Airframe
A VTOL aircraft with its larger installed power must be aerodynamically efficient at high cruise speeds if it is to use that installed power efficiently. Also, if the airframe of the volantor is not appropriately aerodynamic, fuel consumption increases and its maximum travel distance (range) becomes unacceptable. The ideal airframe must also be lightweight so the craft can obtain a favorable power to weight ratio. Lastly, it must be strong for stabilization and safety.
The determination of aerodynamic efficiency comes down to the following:
1 |
When the aircraft is moving at high speed, does the propulsive air move efficiently through the propulsion or thrust system? |
|
|
2 |
Does the aircraft have a small frontal area? |
|
|
3 |
Does the aircraft have a small wetted area (surface area in contact with the airstream)? |
|
|
4 |
Is the vehicle sufficiently streamlined to ensure that its aerodynamic surfaces are free from airflow separation and therefore present a clean aerodynamic design? |
|
|
5 |
Does the configuration achieve a good lift/drag ratio at high cruise speeds? |
Some light planes built today, particularly those in the experimental or homebuilt category do an excellent job satisfying the above conditions. A measure of aerodynamic performance is the passenger transport efficiency (PTE) as measured by: PTE = (Passenger Miles / Gallon) A few four-seat aircraft have a PTE near 70 at 250 MPH although they will generally have a fairly high landing speed without STOL (Short Takeoff and Landing) provisions (flaps, slats, etc.).
The key to a successful high-speed design with a high PTE is finding a way to simultaneously satisfy the five stated aerodynamic requirements. For example, a long, very narrow aircraft could certainly be streamlined and have a small frontal area, but might have an excessive wetted area.
An efficient VTOL aircraft requires the propulsive airflow to move almost horizontally through the system during cruise because even a modest bending of the flow can introduce a substantial drag due to momentum losses.
| • |
A frontal area under 25 ft2 is realistic for a VTOL aircraft carrying up to four passengers. |
| • |
A wetted area to frontal area ratio under 15 is achievable with an efficient airframe design. |
| • |
The drag coefficient based on wetted area (CDwet) is a good measure of the aircraft's freedom from airflow separation. A well-designed aircraft should achieve CDwet of .005 at cruise while a state-of-the-art design arrived at after extensive wind tunnel testing could have a CDwet <.004. |
| • |
Lift/Drag Ratio (L/D) will be high with a large wing area and a high aspect ratio. However, this high L/D will only occur at speeds significantly lower than desired cruising velocity. A high aspect ratio is also hard to achieve in a light vehicle where the fuselage becomes a lifting body, due to its size relative to the small wing area required for efficient high-speed flight with a modest payload. For this reason, the maximum lift/drag ratio of a powered lift aircraft is likely to be less than that for a conventional airplane, but could match or exceed its PTE at higher cruise speeds (>250 MPH). |
The Skycar volantor's composite airframe is constructed mostly of FRP (fiber reinforced plastic) which enables it to be both lightweight and strong. We have our own 250 mph wind tunnel in which we have performed over 1000 hours of detailed flight testing using both powered and un-powered models to ensure that we have chosen an optimum design.
