Ventral fins to prevent deep stall?

[Steve Innova]

I know that a properly built aircraft flying within correct weight & balance should not encounter a deep stall condition. That said...

 

What would be the effect of adding ventral fins (similar to lear jet or the Diamond D-Jet) at the rear of the aircraft below the cowling?

 

Diamond Aircraft's D-Jet uses them to prevent a deep stall condition. The idea is that in a deep stall, with the aircraft dropping rapidly at nose-high attitude, the ventral fins create enough lift to raise the tail, bringing the nose of the aircraft down and allowing normal recovery. Under normal flight conditions, the fins are aligned with the direction of flight and have minimal effect.

 

Could this work to prevent a deep stall on a canard?

 

 

I got the idea from a Q&A on Diamond's website: http://www.diamondaircraft.com/enewsletter/djet/2007_09_25/djet_flyer.pdf

I noticed that the D-JET has grown downward slanted fins on the rear fuselage, as seen on many Learjets. What exactly are they for and how do they work? Dave Forwell, Ontario

The fi xed ventral fins on the lower rear fuselage of the D-JET are commonly referred to as ‘V-Strakes’ or ‘Ventral Fins’. They are designed to prevent the aircraft from entering a deep-stall at certain flap and center of gravity combinations.

A deep-stall is a post stall phenomenon that occurs when the low energy wake of the stalled main wing blankets the horizontal stabilizer, reducing the elevator effectiveness and preventing a recovery from the stall. A deep-stall is a stable, wings level fl ight condition, characterized by a very high angle of attack and a high rate of descent. Although predominantly an effect seen with T-Tail confi gurations, experience has shown that deep stall can also be found for some Mid-tail and V-tail configurations.

The ventral fins are placed low and aft on the fuselage, such that in normal flight they are aligned with the airflow, but at high angles of attack, these fins are now below the wake of the stalled wing and in clean air. The lift that these triangular fins generate at high angles of attack raises the tail – thereby lowering the nose of the aircraft and preventing deep stall.

Ventral fins also increase directional stability. The optimum size, position and orientation for the ventral fins are found from a combination of Computational Fluid Dynamics (see D-JET Flyer Issue 1) and Wind Tunnel testing, with final finetuning done by actual flight test.

[SAF_Zoom]

Hi Steve,

 

I’m no aeronautical engineer but my first observation is that this needs to be aft of the main wing… So I don’t think that it would work on a canard with no tail boom…

 

But what about installing retractable spoiler type devices on the canard to destroy its lift in case of a deep stall? Would that bring the nose down in case of an emergency? I don’t know…. And have no idea how one could fabricate such spoilers...

[Aiman]

I recall reading somewhere that deep stalls are possible (however unlikely) to recover in these aircraft... something about trying to 'throw' the aircraft over using combined rudder and aileron inputs. I could be wrong though.

 

The lesson from all this is: Pay CLOSE attention to your CG limits, stay within them, and fly inside your tested 'flight test' envelope.

[SAF_Zoom]

I recall reading somewhere that deep stalls are possible (however unlikely) to recover in these aircraft... something about trying to 'throw' the aircraft over using combined rudder and aileron inputs. I could be wrong though.

 

The lesson from all this is: Pay CLOSE attention to your CG limits, stay within them, and fly inside your tested 'flight test' envelope.

Yes this is very true...

 

But what about a WHAT IF scenario...

[Edge 513]

Would add some weight- but add a deployable drag device to one wing outboard of the aileron. Pop it out to get a wing up on the situation.

Or the right seater could just start grabbing the ballast out of the baskseat footwells [that was placed there because the ballast was moved since flying solo] and start putting in his seat to get some weight forward.

Or....just keep track of you w/b....

Hmmmmm, somebody said that.

[Cozy1200]

Yes this is very true...

 

But what about a WHAT IF scenario...

I can't say I'm against anyone wanting to try something new to improve the bird AS LONG AS it's tested. That being said. In order to further make the canard more stall resistant, someone will have to stall it. Now you are putting the airframe at risk and would probably want to have a movable ballast design like Nat. IMHO, that's a lot of work for little benefit. Also it's bound to add weight.

 

just my 5 cents. (I would have said 2 cents, but Australian's don't have a 1 cent piece).

[Steve Innova]

Before everyone jumps on board the bandwagen to tell me to "build it per plans", I just want to say that I don't plan on adding ventral fins to my project.

 

I am however interested in what the aerodynamic effects would be. Saf Zoom's comment about it needing to be aft the main wing is interesting.

Assuming ventral fins could have any effect canard-type deep stall, would they need to be aft the main wing, or just aft of aircraft's center of lift, when in a deep stall configuration?

[Edge 513]

Just speaking out of my proverbial hindquarters here. The ventral fins would just be achieving some draggy bits aft in order to hoist it skyward and would seem that it simply needs to be behind CG. But the farter aft the better. Just my idea.

[Marc Zeitlin]

Before everyone jumps on board the bandwagen to tell me to "build it per plans", I just want to say that I don't plan on adding ventral fins to my project.

Just build it to... wait... nevermind.

 

Assuming ventral fins could have any effect canard-type deep stall, would they need to be aft the main wing, or just aft of aircraft's center of lift, when in a deep stall configuration?

An aircraft gets into a deep stall when the CG is not far enough ahead of the Aerodynamic Center to ensure that the front wing will stall first. With T-tail aircraft, especially business jets that tend to have the main wing pretty far back on the fuselage, a normal stall can cause the horizontal stabilizer to be in the turbulent wake of the stalled wing. Since the stab is no longer acting like a wing, the AC of the aircraft effectively moves forward and the CG is not far enough ahead of the NEW AC to be able to get out of the stall.

 

What the ventral fins do is create lifting surfaces far enough aft so that the AC of the aircraft will remain far enough aft of the CG, even when the horizontal stab is blanked, so that the aircraft won't deep stall. So, in this case, you need the "new" lifting surfaces to be aft of the normal AC of the aircraft so that they'll have an effect during a stall.

 

Canard are no different - the front wing must stall first, and not allow the rear wing to stall. IF the rear wing stalled, because the CG was not far enough ahead of the AC, then you'd want to move the AC rearward (as stated above), and whatever lifting surfaces you were putting on the aircraft would have to be to the rear of the normal aircraft AC, and the more rearward the better.

 

If you can ensure that the AC of the aircraft will always be far enough behind the CG, then you either won't be able to deep stall, or you'll be able to recover if you get into an incipient deep stall.

 

As to the question of whether ventral strakes on the lower cowl would work for this purpose, my GUESS would be not unless they were huge, as the airflow there is pretty messed up (that's the technical term), the boundary layer is thick and they wouldn't be very far behind the normal aircraft AC, which is somewhere around the FS-104 - FS-106" point on a COZY MKIV.

[Edge 513]

Just build it to... wait... nevermind.

 

An aircraft gets into a deep stall when the CG is not far enough ahead of the Aerodynamic Center to ensure that the front wing will stall first. With T-tail aircraft, especially business jets that tend to have the main wing pretty far back on the fuselage, a normal stall can cause the horizontal stabilizer to be in the turbulent wake of the stalled wing. Since the stab is no longer acting like a wing, the AC of the aircraft effectively moves forward and the CG is not far enough ahead of the NEW AC to be able to get out of the stall.

 

What the ventral fins do is create lifting surfaces far enough aft so that the AC of the aircraft will remain far enough aft of the CG, even when the horizontal stab is blanked, so that the aircraft won't deep stall. So, in this case, you need the "new" lifting surfaces to be aft of the normal AC of the aircraft so that they'll have an effect during a stall.

 

Canard are no different - the front wing must stall first, and not allow the rear wing to stall. IF the rear wing stalled, because the CG was not far enough ahead of the AC, then you'd want to move the AC rearward (as stated above), and whatever lifting surfaces you were putting on the aircraft would have to be to the rear of the normal aircraft AC, and the more rearward the better.

 

If you can ensure that the AC of the aircraft will always be far enough behind the CG, then you either won't be able to deep stall, or you'll be able to recover if you get into an incipient deep stall.

 

As to the question of whether ventral strakes on the lower cowl would work for this purpose, my GUESS would be not unless they were huge, as the airflow there is pretty messed up (that's the technical term), the boundary layer is thick and they wouldn't be very far behind the normal aircraft AC, which is somewhere around the FS-104 - FS-106" point on a COZY MKIV.

I still thing the deplorable drag device has not only a catchy ring, but would definably set your aircraft apart from all others on the ramp. This is not to encourage transgender dressing in any fashion. That kind of drag device is not appropriate in the Cozy atmosphere.

However Dame Edna has been discussed on the other forum before and hopefully will not become enamoured with the Cozy lifestyle.

Ahemmm...

YMMV.

[CBarber]

Wow, and I thought some of MY post were whimsical, irrelevant and distracting.......thanks, I need the competition to stay sharp

 

All the best,

 

Chris

 

P.S. I would like to nominate Marc Zeitlin for the most improved poster award. I am enjoying his remarks much more now that his humor is shining through. Now, not only is he strikingly accurate, mindful and thorough, he makes me grin as well. I knew you had it in you Marc.

[steve]

i want one millon in cash or marc gets it !

[mfryer]

Now, not only is he strikingly accurate, mindful and thorough, he makes me grin as well. I knew you had it in you Marc.

Oh, is that what generated that giant Grin you show?!!

 

I add my thanks to Marc and others also. It is good to know that there are some really knowledgeable people willing to take the time to help out all of us who are trying to complete these relatively complex, and potentially dangerous projects.

[CBarber]

Actually, what generated the grin was just sitting my MY plane and having the rotary engine I built make sweeeeet music:cool:

 

All the best,

 

Chris

[Jon Matcho]

Could this work to prevent a deep stall on a canard?

While not a true canard aircraft, the Piaggio Avanti has delta fins. According to this Wiki article, a result of working with Lear on the design (the Lear jet also has them).

 

http://en.wikipedia.org/wiki/Piaggio_P180_Avanti

[Marc Zeitlin]

While not a true canard aircraft, the Piaggio Avanti has delta fins.

But it also has a T-tail, which can become blanked by the wake of the main wing in a stall attitude.

[Cozy Girrrl]

Drag devices can be a new source of problems as the test pilot for Lancair found out after he got one of their prototypes in a deep stall or spin and pulled the drag chute to stop it, that part worked, the drag chute release that was supposd to let go of the chute once the plane was straightened out did not work and he had to bail out of the plane as a result of deploying the drag device.

Each additional level of complexity adds new failure modes.

...Chrissi

[Vortal]

one solution i can see is an all flying canard to make it stall if the wing has stalled and maybe push down the nose to come out of the stall. but then you have other problems like being more prone to stalling the wing before the canard. but at least, in that case you can do something about it

[SAF_Zoom]

What about a single (or maybe a pair) of of vertical "stabs" below the aft cowling that can act as a speed brake (picture the rudder airbrake of the space shuttle... its split down the middle to act as a brake) (basically picture one or two winglets below the cowl)....

 

This could maybe (and its just an assumption) pitch the nose down... you would get something that looks like the ventral fins mentioned before but installed in a very steep angle... when the airbrake is retracted... it (they) could act as a stator... cleaning up the airflow to the prop... but when the airbrake is deployed it would hopefully pitch the nose down...

 

Other benefit... no more tipping on the prop as they should be long enough to hit the ground before the prop... they would also prevent the prop from hitting the groung on over rotarion of "stalled landing"...

 

But I think they would cause lot more problems that they would solve...

[TMann]

.... try more airspeed. I hear that works!

[SAF_Zoom]

.... try more airspeed. I hear that works!

Not a deep stall.... not even a F-4 will get out of it...

[TMann]

Not a deep stall.... not even a F-4 will get out of it...

Point being ......... how did you get there in the first place?

[Spodman]

Was at the attached talk by Dave Wheatland, and copped many more stories over a few rums at Bundaberg a while ago:

 

http://www.airwaysmuseum.com/Dave%20Wheatland.htm

 

His story of the crash of the prototype Airvan during stall testing for FAR23 made me think a bit, and the above discussion has reminded me. The scenario was the aircraft was loaded 10% over MTOW, 10% aft of the c.g. limit, then stalled. I don't remember the configuration. It went into a flat (0 airspeed) spin and was not recoverable by any combination of flight controls tried. Dave then remotely released a weight at the rear of the aircraft, which brought it back within c.g. No effect. He then deployed a spin recovery chute which half inflated above the aircraft and did squat.

 

When all seemed lost he bailed out, then looked around for the aircraft before pulling the cord. He found it vertically above him. He said it looked like he was lying on his back in bed looking at a tropical ceiling fan... After some cartoon swimming to get out from under he popped the chute & survived. The aircraft became a smoking hole.

 

Relevance? Your call what you bolt to your aircraft in the name of safety. Extra security provided by such? Debatable. Extra risk by boldly going where no man has gone before? Significant. Likely effect of using your new safety blanket to push the operational envelope? Quite mysterious.

 

Manage your risk, let us know how you get on. Try and survive to tell us what went wrong, if it does. Or, at least hit 'save' on your data logger, like Dave didn't think to...

[mfryer]

one solution i can see is an all flying canard to make it stall if the wing has stalled and maybe push down the nose to come out of the stall. but then you have other problems like being more prone to stalling the wing before the canard. but at least, in that case you can do something about it

I seem to recall the mention of an all flying canard on a Varieze that resulted in a fatality on the first flight.

[SAF_Zoom]

Point being ......... how did you get there in the first place?

Exceeding the critical angle of attack of the main wing... not dropping below a certain airspeed