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How do tail strakes work on a cub - Does any one know?

Presumably, you are referring to the stol kit strakes...if so, theory says it would be to stabilize airflow over the lift producing surface(underside....negative lift) of the horizontal stab to avoid premature stall of the h stab...probably due to the turbulence from the flaps...




In theory, theory and practice are the same. In practice, they aren't....
 
Thanks fob

So if I have underside VGs on my stabiliser and have never lost elevator authority at stall speeds then they wouldn't be of use in my case?
 
I wouldn't think so, but they do have a slight additional effect...they add somewhat to the horizontal stab area, therefore they help to keep the nose down,(since they are fixed angle) and thus less likely to let the aircraft pitch up hard enough to stall the wing or tail...???...all these benefits would be most likely at forward CG and high pitch angles....
 
When I'm flying around slow (trimmed) with no flap I feel like I'm tail heavy even though I usually have forward C of G. I have to give it a bit to get it "up on the step" then it's ok. Maybe the strakes would help in this instance? Or is this a solution looking for a problem?
 
The stock Cub can be flown fully stalled in level flight. Every variant of VGs can also be flown the same way. We no longer have any Super Cubs without one or the other kinds of Vgs, but they all still fly just fine.

The one that surprised me was Micros on a C-180 with the Sportsman kit. I did not like it.
 
I would ask you to explain how you "feel like you're tail heavy....?..." Does it feel like it wants to drop the tail lower?
 
When I'm flying around slow (trimmed) with no flap I feel like I'm tail heavy even though I usually have forward C of G. I have to give it a bit to get it "up on the step" then it's ok. Maybe the strakes would help in this instance? Or is this a solution looking for a problem?

Colin,
I think that I know what you are talking about. I do not have VGs on mine and to date have found no reason to install them. However, when flying around slow with the stabilizer trimmed enough to let go of the stick, it does "feel" tail heavy even though there is adequate elevator authority. I assume that it is just "feeling" uncomfortable because you do not need to apply any pressure on the stick and in your mind you are thinking that you are on the edge of "no control" due to the high nose up attitude. To my thinking it is just an uncomfortable "squishy" feeling in the seat of your pants because you are not applying any force on the controls. You feel as though you are just going along for the ride. If you were in the same position/speed/pitch attitude without being completely trimmed but holding up elevator pressure, you would not have this uncomfortable feeling.

I do understand that there are at least two certified versions of VGs for Cubs. One of them has the strakes ahead of the stabilizer on the fuselage sides. I suspect that during the certification tests that there was some stabilizer shake which the FAA did not like. The strakes solved the dilemma so therefor became part of the approval. I'm assuming that the strakes solved the shake question, but do nothing else.

I do not believe that the addition of the strakes, in your case, will make any difference at all.

OK guys, throw daggers, prove that my theory is wrong.
 
I try not to turn around and look at the empennage when at full power and slow. Weren't early AMD kits supplied without the empennage accoutrements?
 
I try not to turn around and look at the empennage when at full power and slow. Weren't early AMD kits supplied without the empennage accoutrements?

I don't believe so. I visited him when he first approved the kit. Asked about the strake. He shoed me video of a tufted tail of his cub with flaps deployed and slow high AOA. Lots of mixed up air. In fact lots of the tufts were pointed forward..... The strikes were not intended to increase tail area, but to reorganize flow over the tail at high AOA with flaps deployed. That's my understanding at least.

The he video he showed me of the "after" with the strake was remarkably different.

are they necessary? Obviously not....been flying decades without.

Can they improve SAFETY? Yes, I believe they can. Forget STOL for a moment and consider high AOA in flight.....

There are a number of folks no longer with us who stalled a cub when they obviously didn't intend to. Could strakes have helped? I have no idea, but I can tell you that those videos were pretty dramatic.

MTV
 
The strakes operate quite differently than the underside vg's. They complement one another.
 
When I'm flying around slow (trimmed) with no flap I feel like I'm tail heavy even though I usually have forward C of G. I have to give it a bit to get it "up on the step" then it's ok. Maybe the strakes would help in this instance? Or is this a solution looking for a problem?

Check your lower longerons to see if they're flat. Take a straight edge and hold it to the bottom longeron from the tail post forward. Should lay flat.

Maybe that 0-375 on the front might have something to do with it also. Lift it from the front spar mounting points (float lift tabs) TOTALLY EMPTY. How's it hang?

Do the same with a PA-11 and you'll get the picture....

Take care,

Crash
 
I had the strakes on mine CGoldy and it still flew on and off "the step" a bunch. On long (cross country) flights it is easy to tell when it is doing it. one minute it would have a tail low plowing kinda feel and 5 minutes later be singing along tail high
 
It seems to me like the stakes would simply make vortices like a wing tip or vortex generator, and keep the air attached better because of it.
 
If I remember correctly, the FAA test pilot for the BLR approval for the PA-14 told me that the purpose of the strakes was to prevent a tail stall at the rear CG and at the reduced airspeed permitted by the VGs. Apparently at very slow airspeeds under certain conditions it was thought that the tail could "stall" and drop resulting in the nose pitching up uncontrollably/irrecoverably with some ugly outcomes. The purpose was not to give the elevator more up authority as much as to prevent the tail stall. Cubcrafters must have the test data and the reasoning for the strakes. BLR didn't develop them just for sport and to get in the way of our tail handles.
 
When I'm flying around slow (trimmed) with no flap I feel like I'm tail heavy even though I usually have forward C of G. I have to give it a bit to get it "up on the step" then it's ok. Maybe the strakes would help in this instance? Or is this a solution looking for a problem?

Put a notch of flaps in when you are trimming for slow flight.
 
Found this picture of a PA18 with strakes.

http://rareaircraft.com/sales/aircr...cubcrafters-pa-18-super-cub-n392cc-sn-9960cc/

They look too small to increase damping by that much, in case of a flat spin - the Chipmunk and Tiger Moth strakes designed to steepen spin characteristics are three feet long.

Dragging in on the back side of the power curve increases potential gyroscopic effect (left yaw on a right handed engine will result in a pitch up), which combined with increased downwash and an aft CG, might result in an inadvertent flat spin if you try and pick up the wing drop with out of spin ailerons. Strakes would increase damping, and the PA18 fuselage with a flat bottom and rounded top, does suggest a potential need for anti spin damping.

Tail plane stall is normally associated with ice contamination, and with a variable incidence tail plane the PA18 might not be the first candidate for tail plane stall. But the strake in the picture does appear to possibly be there to act as a slat to re energise flow.
 
If I remember correctly, the FAA test pilot for the BLR approval for the PA-14 told me that the purpose of the strakes was to prevent a tail stall at the rear CG and at the reduced airspeed permitted by the VGs. Apparently at very slow airspeeds under certain conditions it was thought that the tail could "stall" and drop resulting in the nose pitching up uncontrollably/irrecoverably with some ugly outcomes. The purpose was not to give the elevator more up authority as much as to prevent the tail stall. Cubcrafters must have the test data and the reasoning for the strakes. BLR didn't develop them just for sport and to get in the way of our tail handles.


The horizontal tail surfaces provide what might be referred to as "negative lift". They provide a downward lift vector in normal flight, in contrast to the wing's upward lift vector. Thus, if the tail stalls, the aircraft suffers an abrupt nose DOWN pitching moment, not a nose UP pitching moment.

As I described earlier, keeping the airflow from separating over the inboard section of the stabilizer will help to delay the stall of the tail.

MTV
 
...and the tail plane has to work harder at a forward CG, an aft CG is requiring less negative lift.

If the tail plane did stall recovery is counter intuitive, you need up elevator which is opposite to standard stall recovery.
 
The horizontal tail surfaces provide what might be referred to as "negative lift". They provide a downward lift vector in normal flight, in contrast to the wing's upward lift vector. Thus, if the tail stalls, the aircraft suffers an abrupt nose DOWN pitching moment, not a nose UP pitching moment.

As I described earlier, keeping the airflow from separating over the inboard section of the stabilizer will help to delay the stall of the tail.

MTV

I had the tailplane stall on me in a Murphy Rebel once. It was a non-event as it abruptly pitched nose down and then everything was flying again. I wouldn't want it to happen close to the ground though. Only happened once and I was unable to replicate it.

Andrew
 
Since the conversation is beginning to drift in this direction.

During check out in my CC-18, (has the strakes)as you would expect, stalls were not a big deal. Lots of warning through buffet with all sorts of stalls and configurations, even accelerated. Installed gap seals same instructor came back and now stalls felt significantly different. What was explained to me is that likely since primary training I have only experienced the tail stalling, that this is a dynamically stable design feature. Tail stalls first, nose drops, mostly recovers on its own. This is much like the built in under steer in almost every car. You turn the wheel traveling too fast for the grip available , car doesn't turn, and your natural instinct to lift off the power and/or hit the brake. Both have the same effect of putting more load on the front tires and now they steer at a speed that the car is capable of handling, dynamically stable. If the back end lets go first it takes much more skill to recover or the condition worsens, dynamically unstable.

When I got to thinking about it I could really tell the difference in the tail vs the wing stalling. When the tail stalls first it feels as like it moves up and the plane rotates around the cg (presumably). When the wing stalls it feels as if the front of the plane drops out from under you. Subtle differences, and in the end your still sort of looking at the ground. Having not thought of it before, I thougt this might be good for others to consider if they ever install gap seals or other tail enhancements. If you begin to venture into back side of the power curve approaches, when near the ground, it can be the difference in wham, and WHAM!!!
 
Someone referred to negative lift generated by the tail. It is actually positive lift in a negative or downward direction. We all know that if the aircraft has a forward CG. The tail flying upside down creating “negative lift” or positive lift in a downward direction balances the nose down tendency that the aircraft has. The the more forward the CG the greater the downward lift that must be generated by the tail to balance the forces. The strakes or the VG's under the tail work the same way the VG's on top of the wing work.


Many of us concentrate on saving ounces to make the aircraft as light as possible to improve performance. THE FURTHER FORWARD THE CG THE MORE DOWNWARD LIFT THE TAIL MUST GENERATE TO KEEP THE SYSTEM BALLANCED. The wing must create enough lift to carry not only the weight of the aircraft and the stuff in it ,but also the weight (NEGATIVE LIFT OR DOWNWARD LIFT THAT THE TAIL IS GENERATING). Many keep the CG forward so that they can carry a heavy load. An aircraft of the same gross weight with an aft CG will fly slower than the same aircraft with a forward CG since the wing is carrying less weight since the tail does not need to generate as much downward force. I'm not talking about adding weight to fly slower. If for example you were to relocate your battery from the front to the back, the tail would not generate as much downward lift and the wing would be flying less weight and the aircraft would be able to fly slower. One could do the same thing by flying in a very tight situation from the back seat as long as he did not go out of legal CG limits.


The catch of course is that with an aft CG the aircraft becomes more unstable and can generate a dangerous situation including the potential of a flat spin.

reliableflyer (stu)
 
Little wing (avatar in honor of Hendrix?) - the nose drops because the centre of pressure on the wing moves back at the point beyond the critical angle of attack at the stall, creating a stronger nose down couple.

With a variable incidence trimmable tail plane I am still thinking the Super Cub shouldn't be prone to tail stall, although STOL mods, full flaps and power behind the drag curve may create downwash conditions that need strakes or VGs to keep the tail plane flying. The same trimmable tail plane should mean there is always elevator force through the stall, unlike the Cherokee's limited all flying stabilator.
 
L18C-95

Spot on except that we have added VG's to the wing, and most run near the forward GC limit when empty, so we have created a situation where we need VG's under the tail, gap seals, and sometimes thrust (artificial airflow over the tail) in order to drive (or hold) the wing at the high AOA we are asking for. You are correct; at that high AOA there is a strong "nose down couple" as you say and it takes a lot of tail authority to get there and keep it there. Thus sometimes we see Cubs with oversized tail feathers. The downside to all this is when it lets go it can be more abrupt.

Bill
 
Strakes: Checking out pictures of Cub Crafters Top Cub and Carbon Cub, it appears that the Top Cub has Vortex Generators on the wing and strakes. The Carbon Cub, however, seems to have vortex generators on the wing (looks like possibly the same pattern as on the Top Cub) but no strakes and no vortex generators on the underside of the horizontal tail. Anyone know why?
 
Because of the basic law of aircraft design:"Simplicate and add lightness" (Geoffrey DeHaviland) .....if there isn't a sufficient reason to add something, don't do it...
 
Coyote, pterosaur fingers 1 through 3 could also be useful on the tail, working as extensible snags (snags operate differently than alulas).
 
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