Forward Swept Fins

[accooper]

Here is my latest design that my wife calls Switch Blade. It is an experiment with forward swept wings at close to transonic speeds, although you will notice it is only going to go .44 speed of sound with an e28 reload. Just wait till I build her and use the 95mm long 24mm re-loadable.

Some of my flying buddies think fin flutter may be a problem.

Andrew

 

[powderburner]

Getting to transonic speed is difficult enough, getting past it is even tougher. You would stand a much better chance of getting there if you used aft-swept, low aspect ratio fins.

If you want to experiment with forward-swept fins, that's fine too. That may call for a different, slower design that stays away from transonics/supersonics

I just don't think you will successfully end up with that style AND supersonic flight in the same package. I think you have a very high probability of some fins that flutter to failure (and separation) if you try to push forward-swept fin geometry (with any significant aspect ratio) to those speeds.

Hey, try it....prove me wrong!

 

[SMR]

It may come apart when fast, but will definitely be a nice looking rocket. Just keep it slow.

 

[JRThro]

Here is my latest design that my wife calls Switch Blade. It is an experiment with forward swept wings at close to transonic speeds, although you will notice it is only going to go .44 speed of sound with an e28 reload. Just wait till I build her and use the 95mm long 24mm re-loadable.

Some of my flying buddies think fin flutter may be a problem.

Andrew

Since that design only has a stability margin of 1.02 calibers with the E28-4, you also may have stability problems if you don't add any nose weight with the 95 mm long 24 mm engine.

It's a nice looking design, though. I like forward swept fins and use them from time to time just to do something different.

 

[cjl]

Unintuitively to most, forward swept fins are actually just as good as rearward swept fins as far as transonic and supersonic drag is concerned. They are significantly worse structurally though, so it might be challenging getting them strong enough to not flutter at high speed.

 

[BobCox]

I believe the problem will not be flutter (which is a sinusoidal oscillation) but divergence (an monotonically increase). I have a program called FinSim and it makes a clear distinction between the two. Forward-swept fins are much more susceptible to divergence, since once they start flexing in one direction, they keep grabbing more air and deflecting even farther in the same direction.

 

[cjl]

That's possible as well. The fins will definitely need to be extremely stiff to survive transonic and supersonic speeds.

 

[kramer714]

cji,

I'm not sure I agree with your comment about forward sweep drag the same as rear swept. I agree that the drag due to the fin in a free stream super sonic flow would have the same drag but by sweeping the fins forward the tips will exit the shock cone from the nose at lower speeds than an aft swept wing of the same span.

Plus look at the shock wave formations at the interface between the fin and the fuselage when the airflow becomes transonic. If my aero memory is correct, the concave root transition gives higher drag than an aft swept one.

 

[cjl]

cjl,

I'm not sure I agree with your comment about forward sweep drag the same as rear swept. I agree that the drag due to the fin in a free stream super sonic flow would have the same drag but by sweeping the fins forward the tips will exit the shock cone from the nose at lower speeds than an aft swept wing of the same span.

I honestly don't think that's a problem. Almost all rockets would require extraordinary speed to have the nose shock come anywhere close to the fins (way past mach 3 for most designs, including this one just from looking at it, though I didn't actually measure the required shock angle).

Plus look at the shock wave formations at the interface between the fin and the fuselage when the airflow becomes transonic. If my aero memory is correct, the concave root transition gives higher drag than an aft swept one.

That may be the case - I'll have to check my aero textbook to be sure, but IIRC, it's not a big difference. In addition, if the shock angle exceeds the fin sweep angle, I don't think it makes any difference at all (though in this case, that would take somewhere around mach 1.7-2 or so, again without actually bothering to measure anything). My aero is a tad rusty though, so I'd need to double check to be sure.