elevator vs. stab area

[target]

Hello all,

My question is this-
Is there a way to determine if an elevator is either too small or too big relative to the area of the stabilizer? And what are the characteristics. The tail surface in question is a "flat plate", with radiused LE, and tapered to 1/32" thick from the hinge line to the TE of the elevator.

The question stems from my attempt of a scracth built low wing design. The elevator response seems sluggish. We have moved the cg back to where it seems tail heavy, then moved it a hair fwd. Incidence is set to 0%-0%.

It is a sloper with a minimum wing load of 16oz/sq ft, and ballastable to higher loadings.
Span is 50", area 300 sq ", and the Le of the wing is straight, with a strongly fwd tapered TE on the wing.

Any input is appreciated.

Thanks, Target :confused:

[jofo1]

Looks like a CSD Vortex wing...are you getting enough throw on the elevator? Are the linkages stiff? Do you notice a difference at high or low speeds?

[shoe]

Hey Target,

The plane looks great! Not sure what the cause of your sluggish elevator is.

Out of curiosity...you mentioned your decalage was 0deg, when the model is on step how does it sit? Nose high, low or level? How's your elevator trim at this setting?

Just eyeing it your elevator to stab ratio is a little lower than I like to use. I also like to go with true airfoiled tails.

Other things that could dampen pitch response I suppose would be an overly heavy tail with corresponding extra weight in the nose.

Just throwing out suggestions, I'm sure someone smarter than me will straighten it out.

[BMatthews]

And good ones they are Shoe.

The elevator looks fine. You say you moved the CG back until it "seemed" tail heavy. What made you think it was tail heavy? The only real sign is when the model doesn't want to pull out of a dive by itself or is very reluctent to pull out. After that it's all about elevator throw angle. And in my experience the model you show there should be fine with the balance point back to the fine edge. The elevator sould be nicely sensitive without being too sensitive.

I suspect that you misread your balance point issue.

[target]

Jofo1,
The plane I built for my brother, so it's not here, but we did just get back from flying it.
As I recall elevator throw is @3/8", which is enough to stall it. The plane has a kevlar hinge for the tail, and it's fairly free moving. The difference that I DO notice is that it does great when the wing is knife edge-it'll do great, tight radius, pylon turns of about 15-20' diameter.

Shoe-
I am not certain what the answer to your first Q. is, I'll look more closely next time it's up.
Elevator TE is right in the center of the imaginary plane of the stab.
The tail IS built a little heavy- it's got CF tubing for LE' on both the stab, and fin. And it has uni carbon on the sides of both.

I'm going to probably cut off the kevlar hinge, and experiment with the size of the elevator, using some TE or aileron balsa stock. If I get it where I like it, I'll V-bag the tail again making it both thinner, and lighter.

Like my funky overspray paint job? I'll see if I can attach another pic.

Thanks for the interest,
Target

[target]

BMatthews,
I'll do more dive tests, but that is exactly how we did it; when it started getting squirrely, we put about 1/8-1/4oz back into the nose. More dive tests wont hurt.

I guess the question that I'm trying to pose, is:

If you take a plane, and make several different identical outline horizontal tails, with just differing proportions of stab/elevator, what will be the effects at the extreme ends of the spectrum??

Will a too-small elevator need so much deflection, that it acts more like a brake?

Target

[target]

Here's another pic showing proportions.

[Ollie]

The things that affect elevator control power most are tail moment arm length, horizontal tail area, elevator throw and to a lesser extent elevator chord and horizontal tail aspect ratio.

To me the the tail moment arm and tail area seem on the smallish side. this combination leads to weak elevator response. I would replace the horizontal tail with one of the same chord but larger span and make the elevator area about 25 to 30% of the horizontal tail area. Increase the stab span until the horizontal tail area is 15 to 18% of the wing area for such a short tail moment arm.

[target]

Thanks Ollie
Seems that you are the "guru" on such matters. I'm going to look at the area ratio as compared to the wing.

But what says you to that other question about the ratio of elevator-to-stab, and the effects of a too small elevator?

Thanks again,
Target

[Ollie]

Elevators produce lift by changing both angle of attack and horizontal tail airfoil camber. The maximum coefficient of lift that a horizontal tail can produce doesn't increase very rapidly for elevator chord widths beyond about 20%. An angle of attack increase doesn't produce much additional lift after the elevator stalls. When the elevator begins to stall has a lot to do with how smooth the hinge line is and how much the elevator is deflected.

BTW, it is very important that no air flow through the elevator hing line. Any air leakage through the hinge results in reduced elevator control power.

[Tim Green]

Target --

You say you moved the CG back until it seemed tail heavy. I wonder, did you determine this while flying, or on the bench?

Because if you did it on the bench you could very well be wrong, specially with a scratch built. And you may have more adjustment to make to the CG. Note that a CG back as far as it can go should provide a very twitchy plane - with too much response to elevator.

But if it turns out you really do have the CG back as far as possible, and keeping in mind you're happy with the amount of elevator (you liked your pylon turns) and are only unhappy with the response, then you've probably got either a wimpy control arm, or slow servo, attached to the elevator. And if not that, then your tail's just too far from the wing for your liking.

[BMatthews]

Tim, as per his reply to me he did it using the dive test.

Target, sounds like you're on the right track. The first pic made the tail look plenty big but the second one makes it look too small. What % of the wing is the current tail area?

I still stand by the idea that you've got a nice proportion of elevator to stab but if the overall tail area is on the smallish side then you may not have the best balance point. A smallish stab will force the "best" dive test balance point to indicate as further forward than it may be best at. Some of the stab size is based on the pitch moment of the airfoil but I suspect you're using a thin low camber section. As such you can get away with a smaller stab for basic flying but if you're trying to perform some hotter aerobatics then you may just not have enough overall area to give the control authourity you want.

And I like the color sheme. It looks like a small dog walked aound with wet paint on it's paws... :D

[Tim Green]

Bill,

I suppose I could have taken the trouble to read all the posts before I replied :D

And I like your idea - that a too small elevator/stab caused the dive tests to indicate a CG too far forward for good elevator response.

[target]

Tim-
The control horn is layed up ply, CF, and glass; most definately NOT whimpy! And the linkage is a 1/4" CF tube, with walls of sufficient wall thickness to almost be a snug fit of the 2-56 threaded rod that I glued into one end. I definately try to "go big" on things like that.
I am beginning to think that Ollie is correct, that my stab(and elevator) are too small.
It's much easier to slap a larger elevator on there, without removing the tail(complete with fillets), first, so that is what I'll try first.
The pics that I have of the completed plane with painted fuse are too big to post; I'll take some smaller file pics today, and post them tonight.
Thanks for the input gents!
Target

[target]

Well no pics yet.
But, I do have some area ratios for all the aero-scholars out there.

Wing- S6062- 8% thick, 1.58 camber
root- 7.75"
tip-@4"
span-@50"
Wing Area...............302 sq."

Horizontal-
Span 14"
root 4.5 "
tip 1.5"
Area........................43sq."

elevator-
root- 1.25"
tip-0.5"
elevator area.........12.25"

Ratios-
Tail : Wing Area = 14.2%
elevator : Stab = 28.5%

My intent is to cut off my original elevator, and replace it with a strip aileron stock that is 1.5" wide.

That will bring my elevator area to 21 sq",

New Ratios will be :
Wing:Tail =17.1%
elevator:Stab = 40.5%

What do you all think? I this too much elevator?

[Ollie]

The size of the original stab and elevator are not so far off as to result in a serious lack of elevator response in my opinion. Therefore, please disregard my original advise until you try other simpler cures.

Remove 1/4 ounce, or less, of lead between test flights until the pitch control starts to get a bit squirrely. If the elevator response is still not strong enough, increase the elevator throw. If after increasing the elevator throw up to plus or minus about 20 degrees, the elevator response is still not enough to suit you, then consider replacing the horizontal tail with one of about 20% of the wing area by increasing the span (not the chord) for a higher aspect ratio. The higher the aspect ratio of the tail, the more effective it will be for a given elevator deflection.

I don't advise just replacing the elevator with a wider one because that would lower the horizontal tail aspect ratio which would offset the effect of increasing the elevator area.

I think you have over built the tail and pushrods so that too much mass is in the ends and this results in a high moment of inertia that takes a lot of control power to move. What ever you can do to lighten up the tail while maintaining sufficient stiffness and strength will improve both pitch and yaw response.

Another consideration is that control response increases as the square of the airspeed. Flying the model faster will increase control response dramatically. The S6062 airfoil does not have a high maximum lift coefficient. To turn tightly with low lift coefficient requires a low airspeed. That's another clue to suggest that you are not flying fast enough.

If you have a computer radio, you can mix elevator to flapperons to increase maximum lift coefficient of the wing when doing a bank and yank type turn. This may be what you are looking for.

In other words, you may be asking more from the S6062 than it can deliver without flaps. The S6062 is a low drag airfoil that will be fast in the straight away but can't deliver a tight high speed turn without full span flap deflection of about three to five degrees.

With the flaps drooped a little and the wing operating at a high coefficient of lift in slow flight or in a tight high speed turn, aileron deflection will produce adverse yaw. The adverse yaw will require rudder deflection to overcome the undesired yaw. So now a simple plane becomes a complicated plane to squeeze out more performance. That is the price to pay.

It is a truism that in designing an aircraft, everything affects every thing else, either directly or indirectly. That's what makes design such a deliciously complex activity. Every design decision has a multiplicity of consequences.

"An aircraft is a set of compromises flying in formation."

[target]

Thanks, Ollie.

I haven't yet cut the Kevlar hinge from the stab, so, your amended advice has reached me in time!

Another interesting clue that I have been thinking about, is that the elevator response seems much more positive while the plane is on its side. Can it be that the simple fact is the thing really is nose heavy still, and that when highly banked over, it isn't "fighting" gravity as much?

We'll be more aggressive with the CG, and let you know what happens. Also, I forgot to tell you where the CG is. I'll get that for you.

Thanks for all the advice.

Target

[Ollie]

The CG location won't be of much use without the tail moment arm measurement from 25% of the average wing chord to 25% of the average horizontal tail chord.

If a 30 degree dive test results in six or more seconds from the time the controls are neutralied until the plane stalls, then I wouldn't judge it to be nose heavy.

If the plane is flown in a knife edge with the spanwise direction vertical and in a straight line, then the wing isn't doing the lifting but the fuselage and vertical tail are doing the lifting. If the elevator response is good under those conditions, then I don't think the tail area or CG is the problem. This would suggest that the lack of "elevator response" is really due to a fairly narrow range of coefficients of lift associated with the wing's airfoil. If that is the case, the range of lift coefficients can be extended with full span flapperons.

[target]

Ollie,
I'll get the 25% to 25% measurement along with the CG location currently.

If I am understanding you correctly in the last paragraph, then you are suggesting that I get going on mixxing in a few mm of down ailerons when the elevator is up, and up ailerons when the elevator is down.
Sounds like also, we need to get motivated to take the plane to a higher lift slope, like Point Fermin, where the S6062 will be happier going a little faster.
Stand by, I can get the measurements here in about an hour.

Thanks, Target

[target]

Ollie,
Im unsure of the "25% of average chord" of the wing and stab,
but if you explain further on how that works, I'd be happy to do the math...
Here's some measurements so that you can proceed if you want to do the math...
Please refer to the pic of the plane incase I'm being unclear.

The wing has a "straight line" LE, root chord is 7.75", tip chord is @ 4".
Stab(including elevator)root chord is 4.375", tip is 1.625".
The sweep back at the fwd tips of the stab is 2", the sweep fwd at the TE tips is .75"

The CG is 2.25" aft of the LE of the wing.
The TE of the wing to the LE of the stab is 10.5", and from the TE of the wing to the TE of the stab is 14.875". I guess you could have deduced the last one!

OK thats what I've got for now...
Thanks again.

[Ollie]

Based on the tail moment arm, wing area, wing chord and horizontal taii area, the tail volume coefficient is about 0.45 which is more than adequate. that means that you have no reason at all to change the horizontal tail. The CG is at 2.25 inches back from the leading edge of the average wing chord (38.3% of the average wing chord of 5.875 inches). I think you could move the CG back about 1/8 of an inch but not much more without introducing pitch instability. I would estimate that your static margin is only about 2 or 3% of the average wing chord which is little enough.

Just fly faster and use flapperons coupled to elevator and I am sure you will notice improvement. With maximum up elevator the flap should be down no more than five degrees and with full up elevator the flap should be reflexed no more than about two degrees.

[target]

Thanks,
We've toyed with the CG some, and the 2-1/4" spot seems about right; any further back, and the plane has trouble losing energy.
I think a litle fiddling with the Tx and a trip to the booming lift zone of Point Fermin are the solution.
I'll set up the flapperons to switch on and off, and give you a flight report next weekend, unless the wind comes on today. I doubt it will, seems like a little heat wave has arrived-good fuse painting weather!
Ollie, thanks alot for the help, I appreciate it.

Thanks again, Target

[target]

Here's the late update:

We took the plane to Point Fermin, put in the heaviest ballast slug(9oz stainless steel-3/4" dia x 4.5" long), bringing the AUW to 44ounces, and let 'er rip in about 25-30mph of SW wind.

The result was awesome; at the speeds the plane cruised at in level flight, the elevator response was excellent. Responsive, but not twitchy.

The other planes that were in the air that day weighed considerably more, but I was very happy with the almost "frightening" performance that new design gave. The plane came away from its first "Fermin(across the street)Landing" without a scratch.

If anything, I think the next version will have a slightly larger span on the horizontal, to give a little extra pitch stability.
Here's on last pic. Thanks to all who helped!

Target