Forum: Technology

What I understand high wing design gives an aircraft increased lateral stability. That’s something you don’t want your fighter aircraft to have.

Still there are many especially naval, such as F-14, A-7, F-8 and F-111 which have high wing design. Are there aerodynamic reasons for high wing construction? I know many decisions were made because of practical reasons – handling on the deck, was easier to construct, more room for bombs, etc. And does anyone the reason for MiG-25 to have high wing?

Cheers!
Icer

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Yes, a high wing is one way to increase lateral stability...dihedral is another. However, positive lateral stability does not impact maneuverability.

As I have understood it, lateral stability affects your roll rate. Rolling Eyes

Elite 1K Joined: May 26, 2005 - 08:39 PM Posts: 1496

Yes, but so do big honking all-moving horizontal stabilators. Very Happy

Icerman wrote:

As I have understood it, lateral stability affects your roll rate. Rolling Eyes

Kinda...
Airplanes need their directional and lateral stabilities to be balanced (see discussion below). Supersonic aircraft needs lots of directional stability to overcome the aftward shift in the center of lift (reducing the effectiveness of the tail)..this is why the many modern fighters have either twin or oversized tails. Lots of tail though is bad at subsonic speeds...so they crank in lots of "dihedral effect" to counter it. High wing, sweep, and of course dihedral all promote positive lateral stability.

http://www.geistware.com/rcmodeling/art ... bility.htm

For non-aero types, an aircraft with positive lateral stability can be put in a bank angle and if the stick is released it will roll back level. How this works is a roll will tip the lift vector to that side, and the plane will then start sliding that way, generating a sideslip. The sideslip reacts with the wing and body to create a roll moment in the opposite direction (See lilnk above).

However, consider that if this same plane plane is banked with strong directional stability (big tail), it will immediately begin to turn into the bank as a result of that very same sideslip. The resulting yaw rate will make the high (outer) wing move through the air faster than the low (inner) wing (bigger circle), generating more lift, and hence more roll in the direction of the original bank angle (countering the positive lateral stability discussed in the preceding paragraph). This shows why big tailed aircraft need enhanced lateral stability.

As for roll rate, positive lateral stability may effect the initial rate, but positive directional stability helps it (both as described above), so hopefully they both cancel out.

Clear as mud, right? Cool

Forgot to say sweep is a really big contributer to positive lateral stability, so sometimes negative dihedral (anhedral) is designed in to weaken the effect. F-104 is a good example. Lowing airplanes weaken lateral stability and will typically have more dihedral to counter that (Light twin) whereas high wing aircraft promote lateral stability and will typically have less dihedral (Cessna 172)

Forum: Technology

Reason for fighter aircraft high wing design?