I have a Speedy Bee by Clancy Aviation. It is a shoulder wing short coupled plane. The wing cord is very large and is a flat bottom design (some say semi semi semetrical).

When flying at very slow speeds the plane is pretty stable. However, when the speed is increased the plane will suddenly dive (Bee Tuck). The dive can be recovered with the elevator. This dive is unpredicatable and occurs without warning. Any theories on why this happens?

Two things occur to me. One's the radio, the other is the wing mounting.

If your radio isn't given to twitching the elevator, then I would suggest you secure the back of the wing so it doesn't lift off in flight. Perhaps a screw, or tape, or additional rubber bands.

We see a similar phenomena on high wing planes with rubber band mounts. With only four rubber bands, the front of the wing will lift, and the plane will climb abruptly (usually during a maneuver.)

In a similar fashion, the lift from your wing could be causing it to lift at the trailing edge, causing the plane to dive.

Good luck,
Dave Olson

SCAR, thanks for the quick reply. As a matter of fact, the picture I posted is not my plane. I modified my plane to allow mounting the wing with bolts instead of rubber bands. The wing s secured by two bolts - one in front and one in the rear of the wing.

Other modelers who experience this "Bee Tuck" have attributed it to flying too fast. They say the plane was designed as a slow flyer and at high speeds, the main wing turbulance reduces the effectiveness of the horizontal stab and elevator. That is somewhat beleivable, but I thought I would try for a more technical explaination from the experts here on RCU.

Try moving your CG maybe 1/4 inch forward.

My two cents,I am not an "expert" ,so take my thoughts lightly. Anyway this is what I think. I would say Scar,s last statement is what is doing this. Just think ,a very wide wing so close to the stabilizer,it seems to me at high speed would create a lot of vacuum on the upper side of stabilizer,thus making it dive,more so if the stabilized is a "lifting type". Cure would be to raise the stabilizer above the wing or make a longer fuselage. It looks to me this model is not meant to go fast,but slow and easy and good for stunting,etc. Keep in mind "air" is a substance and as such acts much as water does on an object,only at a much higher speed. Example,, say you are going 1000 mph in a jet plane,if you could indeed stick your head out into the wind,what do you think would happen to it? I suppose that it is hard to think that air would be so violent,cause we live in it all the time and can't see it per say.
If this model was to be into a wind tunnel and tested at higher speed [where you could add smoke to show effects] I think you would see that that is what is going on[lifting stabilizer at higher speeds]. Well,guys and gals out there,what do you think?

It's related to the close coupling and the high camber of the airfoil. Basically the strongly cambered wing has a strong pitching moment that is related to flying speed. At lower airspeeds the stabilizer has enough authourity to control this strong nose down pitching due to the airfoil's camber. As the speed builds the pitching moment force builds faster than the stabilizer's ability to control it and the result is the tuck. Part of this mismatched rise in pitch damping may be due to how the downwash off the wing impacts the stabilizer as the speed changes as well as the general lack of pitch authourity of the stabilizer due to the ultra short moment arm compared to wing chord.

I saw this same issue with a friend's Slo-Poke, another very short coupled "cute" model. It's the price you pay for a design that is so extreme. If the various 'Bee's and 'Pokes used an airfoil with a lower pitching moment or had longer tails then this would not be an issue.

You can try moving the CG ahead a bit but then it'll tend to have other "issues" like wanting to nose up strongly with the addition of power. Adding more downthrust can control that. But my own feelings are that the tuck will still occur but just at a slightly higher airspeed. If a 'Bee or 'Poke was made with a pitching neutral or positive pitch stability airfoil then this tuck issue would not occur since it would sort of be like a plank style flying wing with an added pitch control. Basically with that sort of setup the wing would be stable without the stab and elevator and that surface would merely be there for pitch control.

Bruce,

I really learned something from your answer. That's what's great about RCU, no matter how long you have been doing this, you can always pick up something.

As I was reading your answer to the tuck, a few things from way back clicked. First, Ed Westwood designed a couple of flying wing or partial flying wing seaplanes back in the 1980s. The first was the Beast, a low wing, single float seaplane. It has a reflex airfoil similar to Evans' Simitar airfoil except much thicker. The Beast also had a small, fixed stab mounted atop the twin fins. He (Westwood) said it aided pitch stability.

His second design was the 2 Ugly, a shoulder wing, short coupled, twin float plane using what looked to be the same airfoil except this time he used a very small, low mounted stab and elevator. I just scanned the plans (which I never built) and it appears to be about 10% on the wing area. Both planes were reputed to be good fliers. Both had reflex airfoils and small horizontal tails.

My 3rd item that clicked was my own experience in the 1980s with flying wings. Mine were rectangular plan form with symmetrical airfoils. I had started with elevond, but didn't like the roll characteristics, so I switched to separate ailerons and elevator. After testing and rigging, I found the best set-up was with the ailerons reflexed upwards and the elevator neutral. If I didn't use the aileron reflex, the plane would pitch down if I lost power as the elevator wasn't large enough for total pitch stability and control. I assume that the prop blast was aiding the pitch control.

Getting back to the original question, if the Speedy Bee were built with reflex in the wing, it might be that the small stab would be enough.

I experienced what sounds like exactly the same behaviour with one control line speed model, eons ago, and more recently, one pattern R/C model. I must have built at least 30 control line models with their wings and stabs in line, with zero incidence, and only one acted up. The pattern job would suddenly nose down while flying straight and level at speed, losing about ten feet of altitude, then behave perfectly for ten minutes or longer.

What is happening, I think, is that the narrow airflow stagnation zone behind the centerline of the wing suddenly flips from the top to the bottom of the stab, or vice versa, exerting a very noticeable influence on the lift or downforce developed by the stab. I removed the gremlin in the pattern ship by simply shimming the trailing edge of the wing about 1/8 inch down, effectively adding about a half degree of positive incidence, putting the wing slightly out of line with the stab. I don't put the stab exactly behind the wing any more, and the problem has not recurred.

That airplane has a lot of draggy stuff below the thrust line and the thrust line is above the wing C/L. Drag is not linear, rather a function of velocity squared.

Quick test - fly it without the landing gear. I bet the problem goes away or is muted dramatically. With the motor above the wing, its already trying to tip it on its nose, as is the landing gear's drag from below.

Larger fixed horizontal stab area might counteract these tendencies some.

The tendency might also be a function of the wing downwash. At some speed it changes it's angle enough at the LE of the stab to do the deed. A tail with a longer moment could be in a less radically changable environment. And/or the tail might be so close it's getting greatly disturbed air from the wing in one situation (and it's enough to change radically the tails ability to control the a/c's pitch) and only slightly disturbed air in the other. Or both situations.

Also, the drag on poorly streamlined gear, like just round wire and wheels would be, usually equals the drag numbers you'd get from the airplane without gear. So gear drag is definitely significant.

It's pitching moment. The Bee's airfoils will have a strong nose-down moment.
At speed, this overcomes the down lift from the horizontal, and the plane tucks.
I've have some slopers that will do this on command.
Moving the c.g. forward helps.
The COX foam Bee had this "amusing" feature (feature in the MicroSoft definition).
Too much of a dive, and it tucked. And then the wingtips would meet on the top of the plane.

Wouldn't you agree that it's the pitching moment overcoming the ability of the tail to stabilize the pitch of the airplane.

Pitching moment changes with the generated CL. And is stabilized by the horizontal tail. And the tail does the job through most of the envelope but doesn't at some condition (basically a certain speed).

So we've got a certain speed or condition where the airplane suddenly pitches. Since it didn't pitch just one or two mph before reaching that speed is it just the pitching moment change that does it, or has there been a more radical change somewhere else in the aerodynamics of that model? The CL will have changed very, very little as the speed changed. And the pitching moment will have changed only slightly as well. But the airplane suddenly tucks. Which is a relatively large change in state.

What causes sudden changes in pitch most times? Stalls. Wings stall, and what else can stall all by it's lonesome? The tail? And what would happen aerodynamically when the tail stalled? Well, if the wing was generating any pitching moment there would be the sudden removal of it's stabilizing force from the tail and the model would suddenly go with the pitching moment.

Can the moment slowly build until it suddenly overpowers the stabilizing force. I'm not sure there are any examples of that happening and resulting in massive pitch change. Unless the tail were operating very close to it's stall angle and was pushed into it. And that's a function of the wing's downwash and the tails incidence.

But I'm just musing..............
Never read anything that describes sudden pitching and the reasons. Anybody got a reference?

An interesting point no doubt.

While I considered that it could be the downwash on the stab I'm starting to wonder. At lower speeds the wing is operating at a higher lift coefficient. Hence the downwash angle is steeper and will hit the top of the stab more strongly so the stab appears to be operating at a more negative angle of attack. As the speed rises the lift coefficient for level flight lowers and the wing moves to a lower angle of attack. This does two things as I see it. One is that the pitching moment has more airspeed to work with so the pitching torque increases. More nosedown.... At the same time the downwash off the trailing edge of the wing isn't angled down as much since the Cl is lower so the stabilizer appears to be operating at a lesser negative angle of attack. So less tail down force at the stabilizer to counter the nose down torque of the airfoil's pitching moment. All of this will sort of balance up to a point where the pitching moment increasing and the stab download decreasing suddenly crosses the line and it's LAWNDART TIME! ! ! ! Unless the pilot steps in with some up elevator to pry the nose up and slow the model down to below the critical speed range where the balancing act runs afoul.

It's very quick!
My CR Turbo with wingerons would fly quite happily, until the speed built up, and then down it would go, with the pitching moment force stalling both servos trying to move the wings.
I have a couple of foamie electrics that tuck that way, but don't get going as fast so the moment is opposable with elevator.

There aren't many charts of pitching moment. But there are many that show the CP (center of pressure) movement. Either one shows that the change in values of both do not change suddenly or with any real variance of change in rate that would suggest any sudden force that would provide the necessary result, a sudden change in pitch.

To get a sudden pitch change should take a suddenly increased or decreased force from somewhere.

What is interesting about this sudden tuck behavior is that it occurs as the wing needs to provide very little lift and then less and less. At higher speeds, the needed CL lessens and lessens. Same lift, more easily produced with increased speed, less CL needed. And if you look at plots of cambered wings that show CP movement along with the CL slope. The slope is almost flat and also constant.

What does seem worth pondering is that the CP movement is usually around where we often locate our model CGs and moves aft as the CL is decreased. And the NP of the model often is in the same area just a bit aft the CG (or the sucker ain't up there flying).

Interesting stuff, this theory.

Well..............

What lines to cross do we have in aerodynamics?

We have stall. And that's sort of a line or sudden change that's described quite clearly with the plots. And the plots don't show any sudden changes for much of anything else.

I guess I got lost reading this, "One is that the pitching moment has more airspeed to work with so the pitching torque increases." because pitching moment is torque, isn't it. And it doesn't have more or less to work with from the air, it simply is whatever it is depending on the airspeed.

You know, the written word is a lousy means of communication. [:@]

But you know, the plots do show drag buckets. I'm going to declare that there is also a "pitch bucket" that noone has identified yet, and that causes these strange sudden pitch sightings. That's it..... pitch bucket!

CP as an aerodynamic function was abandoned by NACA in the '30s when the existence of the pitching moment was confirmed.
Taking CP to the limit as is needed in manuvers such as a zero lift descent, as Martin Simons explains, results in a CP infinitely aft of the wing.
Once the force moves off the wing, it has no more effect on the wing, if one follows the math generating the CP rigorously.
It's only a mathematical contrivance, and can't be measured in a wind tunnel.
Cm OTOH follows the rules of physics and is a parameter measureable in a tunnel, and is present on every NACA airfoil profile since the '30s, and the modern aerodynamicists also include it in aircraft polars.

In the tucking case for these designs it's the point where the overall pitch forces swap direction from wanting to move nose up to wanting to go nose down. As you know "normal" models are flying with a CG that is trying to nose them down while the aerodynamic trim of the model is trying to rotate the model nose up. Steady level flight occurs when the moment of the CG being in front of the neutral point perfectly matches the aerodynamic trim's tendency to try to rotate the nose up. Now if the trim of the model alters and over a rather narrow speed change the trim forces switch from nose up torque to nose down torque we suddenly have a tucking issue. Apparently the unique planform of the Speedy Bee and the Great Planes Slo-Poke that I flew share this odd behaviour. I should also admit that it didn't happen like a finger snap. It actually had a small transition speed range of what I remember to be in the 5 to 7 mph range where it went from a docile easy flier to a nose tucking terror.

Like you say the terms can mix us up a lot. I'd have to go and read up on it again but I'm sort of assuming here (and we all know what happens when you assume right? ) that the pitching torque is dependent on airspeed. The faster it flies the more pitching force or torque. In all the airfoil info I've seen about pitching the value was always given as a single number like a "coefficient" and that leads me to believe that you then match that pitching moment value with the airspeed to find the rotational force of the airfoil for any given speed. I'll have to look into this aspect when I'm not so tired. I've been playing hard today and my mind is mostly mush right now.

I can't remember where I saw the chart on the pitching coefficient, but from what I remember it actually goes the other way. Not remembering for sure has kept me silent about that, but I really think that the pitching moment of a wing is basically a function of the movement of the center of pressure (CP) fore and aft.

I believe one of the pitching forces in this Bee Tuck deal is the location of the CP relative to the NP of the airplane. Quite a few original NACA plots show the location of the CP and show it's movement with AOA changes. Since the guys who've seen tucking have all described it as happening as the airplane has been increasing speed, it would appear that it's something that happens when the CL is small and the AOA is moving from a shallow angle to a shallower angle. And what shows in the CP plot is that the CP hardly moves at all until the AOA is close to those smaller CLs, but when it approaches the zero lift AOA it begins to rapidly move forward.

I've not read much about CP but have seen it tied into pitching moment. And it seems logical that it would have measurable affect on the pitch attitude, especially when pitch trim is fixed while airspeed is moving in the range that would affect the CP movement most.

So I was hoping (I always hoped it was safer to hope than to assume someone who knew about CPs would wade into this discussion. because.........

It looks to me like what might be doing the sudden pitching was the CP suddenly starting to shift, which it does at the lesser AOAs.

On the Clark-Y series, the CP basically is around the .3C to .4C until the very smallest AOAs but when those airfoils start into the zero and negative AOAs, the CP starts aft fast.

I'm guessing the tuck comes from the CP realtively fast move aft feeding in a moment or torque the tail can't handle anymore.

But it's just a guess. NOT AN ASSUMPTION

CP can be used for pitching computations.. up to the point where it forces the location of the force -off- the wing. Then of course it can't affect the wing, while the airplane is still needing -something- to explain what it is doing.
Cm is there all the time, as a function of the camber, and its effect changes with airspeed for all flight regimes.

CP of the wing used in pitch computations can certainly move the neutral point of the airplane "off the wing", but it doesn't push any forces around. It's a force that moves with different AOA. And for a model that is only gaining speed, not weight, the AOA will flatten very slightly during an acceleration when the airplane is already at speed.

The Cm of a wing is quite another thing than the Cm of the airplane. And it's the airplane that suddenly pitches.

I guess I don't understand what you mean about a CP being able to "force the location of the force off the wing"? What force? Pitch stability is a balance between the lift force and vector of the wing versus the lift force and vector of the tail (if we ignore the contribution of everything else).

So I guess I'm not following your argument.

Interesting what can be found in books...........

Center of pressure is considered to be where the lift generated acts.
Pitching moment is considered to be the lift force times the distance.
The coefficient of moment is the pitching moment reduced to be dimensionless the same way the coefficient of lift is.

The Cm of a wing changes value as the CP moves. It will decrease as the CP moves aft, as when airspeed increases. The lift remains the same, but the lessened angle of attack changes the CP aft, increasing the distance the force acts upon, and the moment decreases.

However, the lift required of the system is constant, since the airplane isn't losing weight. And when it moves aft, it obviously still works on the entire balance system of the entire airplane. What it does to the wing is interesting, but what it does to the airplane moreso.

Actually, it appears that the Cm gives the answer for what additional work the horizontal tail has added to it's primary job, maintaing the pitch of the airplane. The amount and sign of the Cm describe this extra effort. But the tail's task is to keep the pitch constant until we throw elevator into the job. And where the CP is would affect that task.

Tail moments are usually figured considering the convention that the distance from wing to tail is from one fixed location in one to a similar location in the other. 25% MAC is often used. But CP locations are also used for finer predictions. Which of course are also far more tedious to figure. And tail moments are used in pitch stability estimation because they are a primary player.

Speed a Speedy Bee up and what happens? The AOA is lessened. The CP moves aft. The tail moment is effectively shortened. The tail needs more area or AOA to do an increasingly difficult job of holding the airplane's nose up. It won't find any more area and it doesn't control the AOA, it's own or the airplane's. The wing does. And the Bee speeds up a bit more........

Exactly, it becomes a runaway self feeding sort of deal due to the extreme design elements.

Center of Pressure isn't used anymore for varous reasons. The Cm pitching moment took over from it since, as I understand it, the results are more consistent. With the old CP deal there's a point where for each airfoil at it's zero lift AoA the CP is infinietly to the rear of the airfoil. And for some reason engineers hate things like infinite values....

Interesting information, because the Cm is a product multiplying CP times it's location. After all a moment is the force times it's distance from something.

Whatever......... I personally think the Bee tucks because it gets tired.