FlyingMonkey Posted April 4, 2016 Report Share Posted April 4, 2016 Hey all, this just occurred to me: Is the ventral fin on CTs serving some essential purpose? I assume it's there to give a little added directional stability, but is that really true (or maybe, not the whole story)? Has anybody here ever flown a CT without the fin installed, and care to comment on how it flies in that state? Just curious... Link to comment Share on other sites More sharing options...
Anticept Posted April 4, 2016 Report Share Posted April 4, 2016 Protects the stabilator from impact. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted April 4, 2016 Author Report Share Posted April 4, 2016 Protects the stabilator from impact. Hmm...that be, I've never hit the fin but I know some others have. I think you'd have to just about be in a tail slide over the runway to bang the stab. Is that a guess or deduction, or info you got somewhere from FD or others? Link to comment Share on other sites More sharing options...
Anticept Posted April 4, 2016 Report Share Posted April 4, 2016 Hmm...that be, I've never hit the fin but I know some others have. I think you'd have to just about be in a tail slide over the runway to bang the stab. Is that a guess or deduction, or info you got somewhere from FD or others? I've taken mine off and lowered the tail to the ground. It will touch. It also protects the tail itself too. The fin will break and absorb the energy. Link to comment Share on other sites More sharing options...
Roger Lee Posted April 4, 2016 Report Share Posted April 4, 2016 It is part of the aircraft's aerodynamics and it must be there. It is called the underfin. It is listed on the maint. checklist for inspection. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted April 4, 2016 Author Report Share Posted April 4, 2016 It is part of the aircraft's aerodynamics and it must be there. It is called the underfin. I know legally it's required, I'm just wondering exactly how it works and how the airplane might fly without it. Just wondering. Link to comment Share on other sites More sharing options...
Anticept Posted April 4, 2016 Report Share Posted April 4, 2016 It may help counteract the roll tendency when applying rudder by acting as a weathervane. Link to comment Share on other sites More sharing options...
Tom Baker Posted April 4, 2016 Report Share Posted April 4, 2016 The fin is required for directional stability. If you try flying with it removed you might find the aircraft uncontrollable. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted April 4, 2016 Author Report Share Posted April 4, 2016 The fin is required for directional stability. If you try flying with it removed you might find the aircraft uncontrollable. Really, uncontrollable? I could see it being more yaw-happy than it already is (which is a lot!)...but it's doesn't seem *that* critical to safe flight. I always assumed it was there to tame some annoying tendencies, but not critical to flight control. I concede that I could very well be wrong. That's why I'm asking, it's something I don't think we've had a thorough discussion on. Link to comment Share on other sites More sharing options...
Tom Baker Posted April 5, 2016 Report Share Posted April 5, 2016 Somplace I remember a warning to not fly with the sub fin removed. Link to comment Share on other sites More sharing options...
FastEddieB Posted April 5, 2016 Report Share Posted April 5, 2016 I will echo Bill and others. Things like extra fins and strakes and do-dads often come about from flight testing, that reveals idiosyncrasies the designers did not anticipate. Let's take some hypotheticals... Hypothetical one: They fly the plane with no fin, and find it has an unpleasant and cyclical yawing motion when displaced around the vertical (yaw) axis. Hypothetical two: In spin testing, they find recovery slower and more problematical than they like. In both cases, they could have redesigned the entire vertical stabilizer, maybe making it larger or extending it forward. But, since the cg is trending a little towards the nose (let's say), they decided a ventral fin which could also protect the tail and provide a more easily replaceable structure should a tail strike occur. Those are just guesses. My point is, it's safe to assume the fin is there for a reason, and to fly with it removed is to take on test pilot status without test pilot pay. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted April 5, 2016 Author Report Share Posted April 5, 2016 With regard to flying without the underfin: Besides a nose heavy CG shift, the absence of the fairing could very well influence the flight control characteristics of the stabilator and vertical stabilizer. Another consideration is how it would affect the "Spiraling Slipstream." I think it has additional purpose besides just protecting the tail structure. Oh, I'm sure it has aerodynamic purpose...I am just curious exactly the purpose(s) is/are. If they just wanted to protect the stab a simple metal skid hanging down 6-12" would be lighter, stronger, and have less drag. The rear of a CT almost looks dart-like with that big underfin, so there must be a reason for such a large area surface. To be clear, I would never fly with it removed. It's just such a prominent feature, I want to know how it works! Link to comment Share on other sites More sharing options...
Tom Baker Posted April 5, 2016 Report Share Posted April 5, 2016 Oh, I'm sure it has aerodynamic purpose...I am just curious exactly the purpose(s) is/are. If they just wanted to protect the stab a simple metal skid hanging down 6-12" would be lighter, stronger, and have less drag. The rear of a CT almost looks dart-like with that big underfin, so there must be a reason for such a large area surface. To be clear, I would never fly with it removed. It's just such a prominent feature, I want to know how it works! Andy, what happens if you remove part of the fins from a dart? Link to comment Share on other sites More sharing options...
gbigs Posted April 5, 2016 Report Share Posted April 5, 2016 The fin is hollow and will crush easily. On the new planes they added a teflon block on the bottom of the fin for tail strikes. The fin is not small and is is no doubt there for stability. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted April 5, 2016 Author Report Share Posted April 5, 2016 Andy, what happens if you remove part of the fins from a dart? If you remove enough, the dart will swap ends in flight because the CG of the dart is very far forward and it would not have enough stability. But a dart doesn't have wings, and many airplanes have very forward CG and no under fin, yet don't swap ends. My *guess* is that the CT has even worse vertical axis (yaw) stability without the fin than it does normally. The CT is pretty tail happy anyway, and without the under fin it might be a real handful when the air is not smooth. It might also have very poor crosswind control in landing without the fin, and every little gust would knock the nose around and mess with runway alignment. Just my speculation though. Link to comment Share on other sites More sharing options...
bitten192 Posted April 5, 2016 Report Share Posted April 5, 2016 Dorsal fins and "underfins" are common of aircraft with skinny bodies (boom tube fuselage) because there is little cross sectional area to help stabilize. Link to comment Share on other sites More sharing options...
Doug G. Posted April 5, 2016 Report Share Posted April 5, 2016 Of course the downside us weathervaning. Especially when taxiing. Also probably a pay off why we need to be more active on the pedals than a lot of other similar planes. I do agree that it is necessary or it wouldn't be there. Link to comment Share on other sites More sharing options...
Tom Baker Posted April 5, 2016 Report Share Posted April 5, 2016 The fin is hollow and will crush easily. On the new planes they added a teflon block on the bottom of the fin for tail strikes. The fin is not small and is is no doubt there for stability. The whole airplane is hollow, and the construction of the sub fin is the same as the rest of the airplane. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted April 5, 2016 Author Report Share Posted April 5, 2016 The whole airplane is hollow, and the construction of the sub fin is the same as the rest of the airplane. So you are saying the CTs crush easily, OMG!!! Link to comment Share on other sites More sharing options...
Ed Cesnalis Posted April 5, 2016 Report Share Posted April 5, 2016 The whole airplane is hollow, and the construction of the sub fin is the same as the rest of the airplane. I think its fiberglass My eye says the fin is unimportant for stability. Link to comment Share on other sites More sharing options...
gbigs Posted April 5, 2016 Report Share Posted April 5, 2016 The whole airplane is hollow, and the construction of the sub fin is the same as the rest of the airplane. No it's not. Push on the sides of the fin...it's constructed like a child's plastic toy. The rest of the plane is nothing like it. Link to comment Share on other sites More sharing options...
Rich Posted April 5, 2016 Report Share Posted April 5, 2016 Remember the flight testing of the C-162? There were two crashes due to stability problems. The under fin was added for stability control and most importantly, stall recovery. It may be the same for the CT Link to comment Share on other sites More sharing options...
Tom Baker Posted April 5, 2016 Report Share Posted April 5, 2016 I think its fiberglass My eye says the fin is unimportant for stability. The SW is fiberglass, the LS is carbon. Link to comment Share on other sites More sharing options...
Tom Baker Posted April 5, 2016 Report Share Posted April 5, 2016 No it's not. Push on the sides of the fin...it's constructed like a child's plastic toy. The rest of the plane is nothing like it. Having repaired a couple different under fins I know how they are constructed. Link to comment Share on other sites More sharing options...
FlyingMonkey Posted April 6, 2016 Author Report Share Posted April 6, 2016 My underfin is made of wishes and dreams of unicorns. Link to comment Share on other sites More sharing options...
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