Choosing Endplate Shapes for a Miata Wing

Introduction

I’m in the middle of designing a wing for my project car, and I’m not sure how I should shape my end plates. I know from the aviation industry that endplates(called winglets in aviation) can have large effects on drag and lift. Thus, I should optimize the endplates on my wing.

However, there are thousands of winglets around, many of which achieve different results. To find one that works for me, I'll look at common winglet shapes for airplane wings, and then common endplate shapes for car wings. I'll describe what different design choices do to drag and lift, and then I'll choose a design for my wing.

Why Endplates? Why Winglets?

Winglets have been used on planes since 1987, and for good reason. A good winglet design reduces drag and increases lift, resulting in a more fuel efficient and higher performing aircraft. But how do winglets work?

The shape of an airfoil creates different paths for air below and above the airfoil. On an airfoil designed to create lift, air flowing over the top of the wing must follow a long path, encouraging the air to speed up to stay attached to the wing's surface. Basically, the shape's wing encourages air to accelerate over the top to maintain smooth, non seperating flow. The airfoil's flatter bottom profile encourages air flowing under the wing to compress and slow down.

By Bernoulli's principle, slower moving air creates higher pressures, while fast moving air creates low pressures. By this concept, pressure under the wing is positive, and pressure above the wing is negative. This creates what we call lift. I've illustrated this concept graphically to the left.





At an airfoil's tips, wihtout a winglet, air flowing under the wing naturally wants to curve around the wing, and enter the wing's low pressure zone. This is called a wingtip vortice. A winglet or endplate creates a barrier between the two pressure zones. This keeps the pressure zones from mixing, maintaining a higher overall pressure under the wing, and a lower overall pressure above the wing. I've illustrated this in the image to the right.


It's clear that winglets and endplates should improve a wing's efficiency greatly. This has been proven by airline manufacturers like Boeing, who's winglets typically improve fuel savings by 4-6%.(NASA)

Winglet Shapes

There 6 main winglet shapes and types, each of which have differing effects on drag and lift. I'll explain the main ones below, and then we can determine which shape is ideal for my case.


Blended Winglet - Blended winglets amoothly curve upwards at a gradual angle. They reduce drag by up to 5-7% and are common on Boeing's planes. The blended winglet's gradual curve reduces drag at the wing-winglet junction. The blended winglet is pictured to the right.




Sharklet - The sharklet is Airbus' version of the blended winglet. It is very similar to the blended winglet, but features aerodynamic enhancements from Airbus, including slightly more rake. The name "sharklet" refers to the winglet's shark-fin shape. It's efficiency and function is comparable to that of the blended winglet.




Wingtip Fence - The wingtip fence is comprised of small vertical fins placed above and below the wingtip. It is fairly efficient and most common on smaller and slower aircraft including the A320. The wingtip fence works just like any winglet - by controlling vortices above and below the wing.




Split Scimitar - The split scimitar is a newer design, released by Boeing in 2013. This wing shape combines a blended upward curve with a smaller, downard pointing sharkfin. Split scimitar wings are one of the most efficient, reducing drag by 2-3% more than blended winglets. These are most common on the Boeing 737 and work by cancelling more vortex flow both upwards and downwards.




Raked Wingtips - Raked wingtips extend backwards and out in a sharp sweep. They act as a winglet, but are part of the wing. These are excellent for aircraft with long wings, and for high speeds. They are common on the Boeing 777 and 787, and reduce drag in a simpler system.




Canted Winglet - Canted winglets are more traditional, and replicate traditional vertical winglets, just angled outwards. These are less efficient than modern winglets, and are used on older Boeing 747's. Canted winglets offer improvement over no winglets, but are outperformed by most other winglet shapes.



From this list, the split Scimitar wing is considered the most efficient. It blends advantages from the blended winglet with even more efficiency from the extra downward fin.

Car Wing Endplates

Car wings have 6 common endplate designs and features. Just like winglet shape, the shape of an endplate can have considerate impact on a wing's characteristics. Like above, we'll dive into the main types of car wing endplates below, and determine what's best for me.


Flat Vertical - Flat and vertical endplates are simple flat, rectangular plates that attach perpendicular to the wing. They are easy to manufacture, and light. Flat vertical plates are common in formula and cup cars.




Tapered - Tapered, or raked endplates are wider at the bottom, and taper or angle near the top. They are designed to direct airflow inwards(to reduce wake) or outwards,(to increase tire wake clearance) and are very common in pre-2022 F1 cars. Tapered endplates are known for helping balance airflow off the wing and down the sides of the car.




Vortext Generating - Vortext generating wings feature cutouts, notches, or fins at the rear upper portion of the endplate. These purposefully create vortices to add energy and keep airflow attached. They are used in advanced vehicles like modern F1 cars, and are highly efficient but complex to develop.




Curved - Curved enplates curve inward or outward in an arc. They create smoother pressure transitions between high and low pressure zones, minimizing drag. They are common on high performance time attack builds, and are efficient.




Ground Following - Ground Following endplates extend downward to reach close to the ground. These seal the bottom airflow, increasing ground effect. These are also most common on time attack cars.




Wickered - Wickered endplates have a small lip at the tail edge, pointing outwards or inwards. These lips help to stabilize flow beyond the wing, reducing drag. They are common on a plethora of sports cars.



From this list, vortex generating endplates commonly increase downforce and reduce drag the most, while curved, tapered, and wickered plates also make big differences.

Based on what's currently out there and efficient, I'll blend several elements from botha airplane winglets and wing endplates to create a high-lift, low-drag solution. Specifically, I'll use the split scimitar style, include a wicker, and add some type of cutout for a vortex generator.

I will attach an image here when I model the wing in CAD.

What I Learned

• How wings create lift and how vortices can help or hurt lift
• How airplane wings differ from motorsports wings
• Airplane winglet shapes and their effects
• Motorsport endplate shapes and effects