Vertical stab is washed out....

[airmcn_3]

Hello all,

I have run into a design issue, I have a very large fuselage with a aft mounted nacelle similar to the seawind, at just about any angle of attack if I pull the throttle back I get flow seperation over the vertical stab, it gets very mushy and the tail end of the bird begins to osolate back and forth, add this to a very short coupling of the stab to the wing and you have a stability issue. I am sure it is due to the fuselage washing out the flow. I know adding vertical height will help but I wonder if there may be anything else I can do....


Any suggestions?

Thanks in advance,

Chris

[Brandano]

Oscillate sideways or in pitch? Are you sure you aren't describing a series of mild stalls? in any case adding a small ventral fin under the tail will do much more for high AOA than adding a lot of vertical fin on top of the existing one, and will also help reduce a bit the engine side thrust

[airmcn_3]

Oscillate sideways or in pitch? Are you sure you aren't describing a series of mild stalls? in any case adding a small ventral fin under the tail will do much more for high AOA than adding a lot of vertical fin on top of the existing one, and will also help reduce a bit the engine side thrust


Oscillating sideways, defiantly not a stall issue as much as yaw instability, with the short coupling it causes the wings to rock as well. Thank you for the tip, I will give that a shot. How far back?

I will be changing the design in Solidworks but at this particular time do not have time to cut a new bird. I need to solve all issues in the prototype before I continue on.

Thanks,

Chris

[bwalt822]

Pictures would help to diagnose the problem. Is it a T tail, conventional tail, how big is the fuselage in relation to the tail?

AOA doesnt really have anything to do with yaw stability or the vertical stab stalling. If you are getting separation over the back of the fuselage (possibly caused by high AoA), that would definitly be a possible culprit. If that is the cause the only ways to stop it are to stop the separation or get the vertical stab out of the separation. Adding more surface area to the fin could help to depending on the situation.

[biber]

Pictures! ;)

biber

[Brandano]

How far back?
YMMV. The farthest back the better, but you might be limited by the asset of the plane in landing. Take for example the fins on an F16:
http://www.ml.afrl.af.mil/mcrg/images/tt-ventralfins01.jpg
you can see how their position is dictated by that of the undercarriage and tailpipe, and this meant they had to be placed further forward and be made bigger. Just don't place them ahead of the CG :)

[bwalt822]

I meant pictures of his ship but this helps anyone who doest know what a ventral fin is. I believe some of the falcon corporate jets have them too.

[Brandano]

They are a bit of a trademark on the Learjet series of planes, where they are named "delta fins", and also appear in the Piaggio P180 Avanti that was developed in collaboration with learjet. Learjet has also retrofitted these to older planes, and their scope is (hear, hear..) improving the handling at high AOA. Since the increase in drag is probably noticeable i assume they must really be beneficial, or they would have been sacrificed for an extra 50 km useful range.

[bwalt822]

In cruise they are not beneficial but the plane cant get to cruise if it isnt stable at low speed, high AoA attitudes, such as takeoff.

[airmcn_3]

Thank you guys for the help, I added ventral fins along with increasing the stab size vertically, just got back from the field and wow what a difference.

I unfortunately can’t show pics at this time due to restrictions from work, should be around 3 weeks until I am allowed.

FYI, the ventral fins I added are similar to the F 16.

Chris

[JetPlaneFlyer]

What you are describing is dutch roll... Sounds like you have fixed it by adding more vertical stab. The other possible solution to dutch roll is to
reduce dihedral.