onewasp

Ben, Bruce, Dick, John, Lou, Paul, (alphabetical order)

At what point does the horizontal stab stall due to elevator deflection?

A. Assume NACA 0009 section with elevator surface @40% of total.

B. Assume a flat section with elevator surface @ 40% of total

This would be an R/C monoplane (think CAP planform).

I realize airspeed has plenty to do with the answer-----my thought is that at SOME point elevator deflection HAS to create a circumstance where drag overcomes lift----i.e. stall. It also would seem that this would occur regardless of airspeed at some deflection point.

I'm no real fan of 3D but their surface deflections seem absurd to me yet they keep on flying----(well, let's say keep on staying airborne) ---- hence the question.

Tall Paul

For those extreme deflections, although the surface may be stalled... lift diminished... the drag is still there and keeps the back end at the back end.
The deflction itself with power on serves more as a "turning vane" than a control surface is expected to behave.

rmh

the flat/9% shape means nothing except for stiffness.
but the RESULT- is where things get muddy.
IF--the horizontal stab is still doing the job intended- what does it matter if it is/is not stalled?
I do not disagree with the statement Paul made .
However -I gave up -long ago looking at models and their behavior -relative to the rather strict operating guidelines NECESSARY for most full scale behavior.
I have proven to myself how the "stalled elevator works -using the small foam models and adding to /changing the size of the elevators and the deflections.
It really does become a simple deflector plate -which does work - when it is angled up to 45 degrees .
I cannot find the point at which it changes --from being a "proper working surface "-to the deflector plate status .
So I just look at it as rather unimportant which it is -in the opinion of others .
If it works correctly I accept it.
Now then - I still recall rather vividly - a NASA write up from way back -which noted that the tailplane functioned more effectively in a very high wing AOA--if the effective AOA of the horizontal stabilizer /elevator was CLOSER to the flight path.
Anybody remember this ?
If so --
Then would not the true elevator deflection -relative to the flight path be a better way to look at it's deflection angle?

BMatthews

The answer is that parts of the sections are well stalled at those deflections and often are stalled at what we consider normal deflections. However stalled sections still produce lift in large amounts. But high drag goes with this lift when the deflection angles are steep. But the tail is small enough that it's not an issue unless you're a racer where every bit of drag counts.

The movable elevator produces a variable camber airfoil which sounds good at first. But that airfoil has sharp transitions that will easily generate bad separation bubbles at even low angles. Add to that the fact that the tail is rotating towards the "top" side of this cambered airfoil and you can get a situation like the sketch below. Here the elevator is up and is forcing the tail down so the air is coming up from "below". Note that the high angles involved is producing separation bubbles in TWO spots. The "underside" of the cambered airfoil on the top of the stabilizier portion and the rear of the elevator because the air cannot possibly follow such a sharp kink. But such a tail will still make the model try to bite off it's own tail.

LouW

Before answering the question the term “stall” must be defined. I don’t know exactly what “drag overcomes lift” means. The stall is conventionally associated with maximum lift coefficient and is the angle beyond which lift drops off significantly with further increase in AOA. Separation may begin well before the maximum lift angle at sharp bends as described by Bruce. This adds drag but considerable lift is still being developed and the wing is not “stalled”.

Because its position is fixed relative to the wing, during any fly-through maneuver the angle of attack of the stabilizer will be the same as the wing (plus or minus any small difference in mounting). If the wing isn’t stalled, neither will the stabilizer be. The condition will be similar to the flaps on the old Cessna 172, and the Cessna 150. With their 40 degree deflected flaps the wing still developed significant lift but with the separation described by Bruce, they also had a lot of drag. It was still possible to stall the Cessna wing but only by increasing the AOA, which for a stabilizer fixed behind an unstalled wing is not very likely to happen. If the surface should stall during a maneuver the result would be a sudden loss of control effectiveness.

A separate effect takes place with a full flying tail (as opposed to a conventional flapped elevator). Such a configuration is subject to stalling at high deflections. Such installations are usually limited to moderate deflections to avoid stalling problems.

Bax

My Feedback: (11)

From LouW:

"A separate effect takes place with a full flying tail (as opposed to a conventional flapped elevator). Such a configuration is subject to stalling at high deflections. Such installations are usually limited to moderate deflections to avoid stalling problems."

Early Cessna 177 Cardinals had a problem with the stabilator stalling at high deflections. This happened during a full-stall landing. The stabilator would suddenly lose a lot of its downforce and the aircraft would very suddenly pitch nose down. It was cured by adding a slot in the leading edge of the stabilator that prevented/delayed flow separation when the stabilator was operating at high AOA. The stabilator then remained effective through all flight regimes.

onewasp

From LouW: "I don’t know exactly what “drag overcomes lift” means."

Nothing more than a 'ham-fisted' description of what I thought I saw with the "3D" A/C.

Mentally, I could not (then) visualize how the stab could still generate lift with the extreme deflections. As usual, you all are indeed answering my question (i.e. I feel I am gaining a better understanding) and for that----many thanks.

I am very much in the dark when considering this-- "If the wing isn’t stalled, neither will the stabilizer be."

Am I right in reading this as meaning that the wing will always stall prior to the stabilizer??? I wouldn't have thought this to be the case------but then that is why I'm asking the questions and you are answering.

rmh

Those Cardinals were killers - lousy engineering and limits on angle assured a disaster.
I guess nobody understood my comment or RELATIVE angles .
In a deep stalled condition -or as we stupid 3D flyers call it " Harrier" - the plane is descending --so --the effective AOA of the wing is extremely high - it is fully stalled .
BUT- the angle of the horizontal tailplane is more closely following the flight path.
So -it's EFFECTIVE angle is far less .this keeps it working
The apparant high deflection is what is making it seem impossible .
Just because it is not common on full scale stuff does not mean it isn't generating a good controllable force.
If this theory is not true - then what do you think is happening?

Tall Paul

Without power to provide the airflow along the fuselage (reference line), a plane in this condition will not be able to maintain this attitude.
The prop blast provides the airflow over the horizontal which generates the downforce on the horizontal which keeps the nose up.
The same condition power-off results in a stall or a series of mini-stalls, with a full loss-of-control most usual.

rmh

BUT- that is not the case - power is being applied - hence the controlled deep sink
Your pic must be Frak's plane --and I note the reflexed ailerons which also greatly decrease AOA of the wing.
My new IKARUS F3A electric will sink perfectly straight -No wobble -no reflex in ailerons (Tell Tony this is "must have" model)
Also I can simply add power anytime during the sink -and simply increase AOA and slow descent -tho forward speed increases of course.
Or add more power and simply rotate and recover.
So we can call the stab stalled --but due to the conditions being flown -it is perfectly controllable and functional.

Johng

My Feedback: (4)

Before we get to the quote, we need to be clear on one thing - If a surface is stalled, that does not mean it is zero-lift. If we can treat lift and normal force as the same thing for this discussion, since we are talking about much larger angles than usual lift is considered, the lift falls off after stall, then the lift(normal force) curve flattens and may even increase as angle of attack approaches 90 deg. In that case, normal force and drag are in the same direction - up off the wing.

So, the wing can stall and still make lift, that's what makes 3d possible. Same for the tail. Since tails on our models are usually relatively low aspect ratio the lift curve will tend to be relatively shallow sloped and rounded, without a sharp stall break. More likely a gradual fall off of lift.

Actually, I just had a flight that disproved Paul's statement this week. I have a Funtana 90 with a .91 that has idle mixture troubles. So I was in an elevator maneuver at idle throttle, elevator stick full back, and the engine quit. THe nose may have come down a degree or two, but it stayed in deep stall and flat attitude without prop blast. I rode it down a little ways, then let the nose drop, flew to the end of the runway, turned and landed.

rmh

John - unless one has flown this stuff and seen how it acts - I don't think it makes sense - relative to scale type or full scale sport planes or 737's etc..
We have a room full of small 3D designs and 3D ARFS -which we use to try all kinds of flight .
Also our new VP prop setup is about ready to go -to make the plane back up -as you know they will do-- although momentarily- it is flight direction reversal

adam_one

One of ways to prevent troubles with stab stalling is building models like those shown below.
One day all planes will be build that way… VTOL [8D]

onewasp

And-------that will be the day I STOP flying (and stop asking questions).

I didn't even like box kites as a kid!

adam_one

onewasp,
Don't worry, I was just kidding, you'll still be allowed to fly your conventional planes…
And provided they're not nose-heavy, the stab stall won't be likely to occur.

BMatthews

John, I suspect your model had a very rearward CG location at or perhaps even slightly behind the neutral point?

Adam, obviously great minds think alike... A few months back I did some sketches for a cross winged model just as you've got there but for glow power. The biggest worry was how to color it so that I could do some fast rolling maneuvers and still figure out if I was upright, inverted or knife edge when I stopped. This is NOT a model design to be flown at great distances where the color degrades to just a grey silhouette! ! ! ! Are those yours and if so how do you manage to fly them?

adam_one

You are right Bruce, those ones require a "little" more concentration when flying them, fortunately they are not mine, I prefer the conventional ones myself…
Maybe it's something for Dick to enjoy...

rmh

The flying cross and the box --are fun designs .
Interesting thing tho -the cross is actually same thing as conventional monoplane with lots of side area , located at the cg.
If you fly aerobatics -this is obvious.
There is a model on the market -a biplane with side force generators, it is essentially same thing.
It was designed by an active aero engineer, George Hicks.
these are easy to fly but have some unusual reactions .
when rolled , they neither turn or pitch.
The "cross" makes visual reference dicey .
The box tho -a bit different -tho not as bad as one which would have circular sections.
A real problem with these setups -- torque reaction.
contra rotating props would fix this -
When we fly our electrics -we see and use much of the character exhibited by these two old time classic toys.
The missiles we worked with in 1958 were controlled much like the cross.
two vanes provided pitch -two provided roll- very old design the US copied from German WW11 efforts.
Our current small electrics will back down to the ground quite nicely -tho they look like conventional models.
It all has to do with power /weight . and some fast inputs to the controls .
When we get our adjustable prop going -I expect to see the models back up in level flight and back up when pointed down. Doubt it ?
Models doing this, already exist.
The reverse flight can not be sustained for more than a very short period -
This will change.
Adding a prop/motor at the tail will fix this problem and if the "box " airframe is used - it will be easy to make it go up/down back /forth.
All powered flight has been ,is and will be fundamentally changed by advances in powerplants- (hence the term, powered flight)- then structural improvements and lastly - aerodynamic improvements - all required but the power to weight leads the development -the rest is refinement.
You don't need a degree in anything to see this - just look at history.
It is wonderful to see how many of the old time "what if" ideas now can easily be brought to life -simply thru use the extremely light powerful motors now available.
You guys who have not taken advantage of these new tools are missing out on a lot of fun.

Johng

My Feedback: (4)

Probably slightly ahead of the neutral point. Still needs a little weight off the nose to be neutral inverted.

LouW

I was careful to qualify my comments as applying to “fly-through” maneuvers by which I mean normal forward flight where the forces acting on the airplane are predominately aerodynamic. Under those conditions the tail is not likely to stall as long as the wing is not stalled.

On the other hand many 3D maneuvers are flown such that the primary forces are inertia and thrust. Under those conditions the wing and control surfaces may well be stalled, or in case of hover, have no airflow at all except prop wash. As Dick continues to remind us, given enough power and a way to deflect it, any flight path or condition is possible limited only by the eye-thumb coordination available at the transmitter.

rmh

Lou -I keep bringing it up simply because it is the HOT thing in model merchandising at the moment - the stuff literally flies off the shelves.
I watch and participate in this industry so I am not just making a guess on this .
Also - In full scale the US taxpayers have sunk a sizable piece of change in a POS called the OSPREY-which is an attempt by the aircraft industry to make a craft which operates in a similar fashion>
I deride this craft -simply because I personally think the entire approach is very bad .
I am not alone in this and I am not one to poo poo technological advances.
But a bad idea is a bad idea .
If they had double the power and half the weight - they might make it 1/4 as practical as proposed. .

Tall Paul

I saw the first Osprey arrive at Pax river in 1990... (that's 15 years ago.. and the plane is -still- a POS).. too many parts! Parked next to an HH-53, the Jolly Green looked more practical!