Spitfire vs 109 aerodynamics

[Specter]

Rudel asked a question the other night about why the 109 is such a bad turner and the spitfire is a really good at it.

I tried to answer it, I don't think I really explained it well at the time. In aircraft performance, it really comes down to two parameters. Thrust-weight ratio and Lift-Drag ratio.

The thrust-weight for these two aircraft are so close it doesn't really apply in this case. But the bigger this ratio the more available power you have. For example, He-111 low T/W - Bf-109K-4 high T/W.

The most distinct difference between the spitfire and 109 is the wing. This is what really determines the L/D ratios. The lift generated by the wing is the only thing keeping the airplane in the air. So in level flight Lift=Weight. In heavy combat maneuvering these fighters are capable of pulling 6gs. Meaning that the lift generated by the wing must be 6 times its weight. (This will be important later) If the wing can not achieve this, you get a stall or spin (if only one wing stalls)

Drag is the resistance that is generated by the aircraft moving through the air. This is what the propeller needs to overcome. There are three major components to drag

• Parasite Drag - skin friction and pressure drag. These are related to the shape of the body only. Skin friction is air molecules being stopped on the surface. You can think of it like sand paper on wood and is roughly proportional to surface area exposed to the airflow. Pressure drag is due to a low pressure area that forms behind a body. Basically if its flat behind, its bad and will create a lot of drag (think C-130, or bullet). This is why airplanes are long and pointy. And almost everything that flies has a distinctive tear drop shape.
  
  
• Induced Drag - This drag is caused by vortices rolling off the wingtips. Sine at the wing tip it is still producing lift, the forces have to be balanced by the air. Newton can be a real pain sometimes (For every action there is an equal and opposite reaction). This creates the corkscrew effect you can see a lot, especially when you're looking at an airplane near fog.
  
• There's also compressibilbity drag, but that only happens at transonic speeds. >.7 Mach. Not within the usual flight envelope of a WWII fighter.
  

In aerodynamics we normalize everything with respect to the wing area(S) and dynamic pressure(q). Wing area should be obvious, dynamic pressure is 0.5 (pressure) (velocity)^2. Since we're concerned with competitive performance, velocity is the only difference (pressure changes with altitude).

(Lift [lbs]) L = qS (CL)
to make things easier lets substitute nCw for CL. n is the load factor (g's) and Cw is the 1g weight.

Drag is the same way, but the drag coefficent (CD) is broken into its two components, CD0 is parasite drag.
CD = CD0 + (nCw)^2 / (e pi AR) <-- Induced Drag Coefficeint (CDI)

I'm really trying to make this simple, but its kind of hard to explain this if you don't know the relationships. e is something called oswald efficiency and is the efficiency of the wing compared to an elliptical span wise lift distribution. A theoretical elliptical wing produces zero lift at the wing tips, so induced drag is minimized. AR is aspect ratio for a WWII airplane you could just say span / chord. They are roughly equal for the spit and 109.

Lets compare the two aircraft.

• S: spitfire > 109
  
• CD0: spitfire > 109
  
• e : spitfire = 1 (it is an elliptical wing) 109 is something less than 1 (lets just say .7)
  
• Cw : spitfire < 109, remember what I said above about lift = weight in level flight. nCw = L / (qS) Since the spitfire has bigger wings CL is smaller.
  

In the game when a 109 is out maneuvered by a spitfire it is because the 109 looses more airspeed than the spitfire due to drag. To illustrate how this works I'll look at two cases. High speed, gentle maneuvering. and low speed aggressive maneuvers.

High speeds, gentle maneuvers (>400kph)
q is very large
n < 2
CD0 >> CDI (CDI drops out of the equation

D = qS (CD0)

Since the spitfire has a greater parasite drag and bigger wing area, this is good for the 109 bad for the spit. This is also why the spitfire has such a low top speed compared to the german fighters. If you notice too this is a linear function, so even though the 109 has the advantage its not as dominant as you may think. So at low speeds there's not that big a difference.

Low Speed, Agressive Mauevers:
q is small
n > 2
CDI >> CDO

D = qS ( (nCw)^2 / (e pi AR) )

This is really bad for the 109, notice the load factor is inside the square. So induced drag at 5g's is 9x greater than at 4gs. Wing loading for the spitfire: 28 lb/ft^2, 109: 36 lb/ft^2. To put this in perspective in any maneuvering engagement the induced drag is 2.4 times greater on the 109 than is on the spitfire. So at low speeds and a high G turn, you bleed energy incredibly fast.

The one saving grace for the 109 though is that it has such a high T/W, once you unload the aircraft you can accelerate it fairly quickly. Which is not possible in the 190.

It all comes down to, the spitfire is built to turn, the 109 is built to go fast.

Hope this helped, feel free to ask any questions or for better explinations.

[Stachl]

Yea, but how do the slats figure into this? I have noticed the 109 has a decrease in low speed performance since 4.08. In fact it doesn't seem to have much, if any, advantage anymore as it flips over and into a spin much easier now. And speaking of spins it is much harder to get the 109 out of a spin which I'm suspicious about. Also I read in Flight magazine about an experienced Spit pilot test flying that recently rebuilt 109E (Marseille's old plane) and he said it's roll rate was 50% quicker than any Spit he had ever flown. I don't think that's the case in game. Don't get me wrong though, I love the speed advantage we have in 42.

[Specter]

Slats and flaps change the airfoil geometry.

Flaps decrease the critical Angle of Attack (angle of attack at stall), but increase the maximum CL. So you can fly slower without falling out of the sky. This lets you turn tighter, also. Angle of attack is the angle between the flight path and the nose of the aircraft (or more specific zero lift angle)

Slats increase the max CL slightly, but the biggest benefit is that it delays the stall separation for the wing. The slot energizes the upper surface flow on the wing. So it counters the decreased angle of attack created by the flaps. Which allows you to turn even sharper before you stall.

[Schulz]

I thought it had to due with the wingloading. The Spits larger wing leaves to a lower wing loading.

[Specter]

wilhelmschulz said:

I thought it had to due with the wingloading. The Spits larger wing leaves to a lower wing loading.

You're exactly right. CL is proportional to Wing Loading (Lift / Wing area). The majority of my first post was really proving what you said. The induced drag is the physical phenomena that makes the high-wingloaded aircraft poor high-g turners. Wing loading is a good rule of thumb for this, and probably why you've seen it on forums and such.

[Heiden]

Wing loading, wing loading. :-) Specter, this sort of post is right up my alley. I do flight dynamics for PSS and AlphaSim addon aircraft for FS2004 and FSX, I deal with this sort of stuff all the time. Good post.

[Felt]

Good post Specter...I needed a good refresher.

S! Felt