# How much do chin radiators drive tail sizing?



## spiralcopter (Sep 2, 2022)

Putting the radiator under the engine in a single-engine fighter seems to have a lot of technical merits, such as keeping the radiator and engine closer together. On the other hand, surely having a big, deep chin radiator like on a P-40 or Typhoon increases the side area in front of the CG of the aircraft, and requires the vertical stabilizer to be larger? Wouldn't that, holistically, add a lot more drag?


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## Shortround6 (Sep 2, 2022)

spiralcopter said:


> deep chin radiator like on a P-40 or Typhoon increases the side area in front of the CG of the aircraft


 Yes, no, maybe????

The P-40 gets a bit of a bad rap. 





When they changed the reduction gear on the engine the propeller moved up about 6 inches but the radiators/oil cooler stayed in the same place. The air scoop looks bigger on the P-40D/E and later but the curve at the bottom is pretty much the same size and the actual side area of the nose is just about the same. Since this was a P-36 with a V-12 instead of a radial the nose got longer by about 3 feet. Of course to was pointier 

On the P-40 the radiators actually didn't hang below the bottom of the fuselage. Remember that the main fuel tanks were in the wing under the cockpit. 

Typhoon is a bit different.




The radiator/oil cooler is hanging below the engine and does increase the side area. 
however when they changed to wing leading edge radiators on the Tempest I 




They didn't bother to change the fin and rudder.

You do need a certain amount of fin and rudder at low speed just handle the torque. 
You also need a certain amount of vertical fin and rudder to keep the plane pointed straight at high speed. 
The later P-40s got a 20in longer fuselage to move the tail fin back (elevators stayed at original location/distance from the wing). 
The Merlin powered P-40s had larger scoop (bigger radiators? or air intake moved from the top to the bottom with little change in actual side area ?) but the later P-40s and Merln Powered ones also had several hundred horsepower more. 

Propeller wind stream is doing a spiral down the fuselage and over the wing roots. Since the vertical fin is dealing with different amounts of pressure depending on the side of the fin and the speed you don't want to cut things too close.

Spitfires grew larger fins and rudders as power increased and their radiators as they grew larger, were always behind the CG. P-51 H got a taller fin.

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## Tom Fey (Sep 5, 2022)

I would expect it is all lever arm length x area. If you add 1.5 square feet of side area 10 feet ahead of the CG, you are going to need the equivalent of one square foot of side area located 15 feet behind the CG to keep the areas balanced. When air racers put propellers of greater diameter/blade area on the nose to accept racing horsepower, they often have to enlarge the vertical stab area to tame the yawing. Same goes for adding a larger spinner.


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## FLYBOYJ (Sep 5, 2022)

spiralcopter said:


> Putting the radiator under the engine in a single-engine fighter seems to have a lot of technical merits, such as keeping the radiator and engine closer together chin radiator like on a P-40 or Typhoon* increases the side area in front of the CG of the aircraft*, and requires the vertical stabilizer to be larger? Wouldn't that, holistically, add a lot more drag?


You might mean weight of the structure? The C/G, based on weight and balance is about weight and distance (arm) from the calculated center of gravity.


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## gumbyk (Sep 5, 2022)

FLYBOYJ said:


> You might mean weight of the structure? The C/G, based on weight and balance is about weight and distance (arm) from the calculated center of gravity.


I don't think so. He's talking about stability about the yaw axis. Too much surface area ahead of the C of G relative to the tail will reduce stability.

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## spiralcopter (Sep 6, 2022)

Yes, I am talking about yaw stability. I thought of it when I read an article on the F-35 SWAT weight-reduction program. One of the things Lockheed Martin engineers switched from the X-35 to the F-35 was that on the X-35 the nose landing gear is covered by a single door while on the F-35 there are two doors. This reduces the amount of side area in front of the CG, which in turn drives vertical tail sizing.


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## gumbyk (Sep 6, 2022)

spiralcopter said:


> Yes, I am talking about yaw stability. I thought of it when I read an article on the F-35 SWAT weight-reduction program. One of the things Lockheed Martin engineers switched from the X-35 to the F-35 was that on the X-35 the nose landing gear is covered by a single door while on the F-35 there are two doors. This reduces the amount of side area in front of the CG, which in turn drives vertical tail sizing.


I think the dynamics will be quite different. The slipstream from the P-40 prop will change things, as will torque from the lift fan in the F-35. I'm not sure what the moment arm is for either of the noses of the aircraft, but I think that the F-35 C of G is further aft than you would expect, given how far aft the main gear is.

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