CoosBayLumber

My Feedback: (1)

Need an estimate as to how much torque will be required to turn an elevator, so that I can obtain the appropriate servo.

The elevator is also the Hoz-stab. It all rotates, and has an area of 61 S.I. with a 3 7/16" chord. It is full symetrical, and the pivot center is one inch back from the leading edge. This is on one of those slow flying 1/2A powered A/C that flies at about 20 MPH or less, and as such would maybe need like a +/- 30 degrees deflection.

How many Oz/In of torque is required?

Wm.

FHHuber

The load a "Flying Stab" puts on the servo is usually much smaller than the load you would get form the same size aircraft at the same speeds with a fixed horizontal and having the elevator hinged off the back end.

since you have a VERY low speed airplane and assuming a passably balanced pivot line (25 to 27% MAC of the moving surface is the ideal range for the pivot line. I can't tell for sure about yours being in that range... without knowing the shape...) You should be able to use a "suprisingly" small servo. One of the sub-micros should do it.

I have had a flying stab type model that had appx 15 sq in stab area, pivoted on the 25% MAC line of the stab, and the plane was capableof in excess of 140 mph. I used a 27 in-oz servo, and had plenty of authority at any speed.

With your lower speed... you will be in a LOWER required power range than I was... if that stab is pivoted on the correct line.

if you have it pivoted too far forward (with sweep... its possible with the described dimensions...) you might need as high as 40 in-oz...

Which specific aircraft are you doing? if its a home design... got a picture? (put a stripe of blue or tan masking tape on at the pivot line so we can see where it pivots for sure...)

CoosBayLumber

My Feedback: (1)

The shape to the rotating surface is basically a rectangle. It is about 19 inches long by 3 7/16" wide. The center for pivot is one inch back from the leading edge, or at about 29% of the chord.

Went to the full swinging elevator design so that the A/C could climb and dive radically within the confinds of a backyard.

Wm.

jettstarblue

HS-55 or similar would be more than adequate, you agree Mr. Huber?

Jetts

CoosBayLumber

My Feedback: (1)

The HS-55 servo is a pretty small one. It doesn't have a whole lot of torque according to the box specs. I would assume something in the 30+ range as a minimum.

Wm.

Tall Paul

A 29% pivot point is unstable!
The surface will seek one extreme of its possible movement or the other, and may oscillate between the two, if the servo gears don't strip which is probably what will happen with any mini plastic- geared servo.
The pivot point MUST be no further back than 25% to maintain control of the surface.
A small amount forward of that will provide some degree of streamline centering.
Also a full-flying tail has -less- authority than a hinged surface.
It stalls at lower Cls than the conventional type.
The full-flying style is more suited to planes which don't manuver a lot; TD, electric gliders... something where trim drag makes a difference in the end performance.

CafeenMan

I'd think an HS-85 would be good. It's light and has a BB.

jettstarblue

O.K. let's see- it's a little surface, at very low speed, then, uh, Well alrighty then....

CoosBayLumber

My Feedback: (1)

Does anyone have an idea as to a numerical value for the torque? Thought here is that next guy trying to copy my A/C may have Airtronics of some other bizarre radio. It becomes simplier than to indicate a minimum torque value.


Wm.

Tall Paul

Theory and practice state that a symmetrical airfoil, pivoted at 25% mac has no pitching moment.
In the wind tunnel such a profile can be placed at any angle below the stall, and it won't move with airflow.
The torque therefore is zero.
As a true symmetrical shape can be difficult to construct, and the pivot point difficult to position precisely AT 25% mac, fudging a little forward won't hurt anything.
The torque required goes up though.
Probably not much.
But aft of 25%, the profile as stated will attempt to find a stable angle other than streamlined; this might be full up or full down, or all points in between..
I've seen an otherwise well-designed plane with such a flying tail crash due to the loss of control created by the oscillation (not flutter) of the horizontal around the pivot from stop to stop because the pivot was aft of 25%! Don't do that! .
I put the pivot a bit ahead of 25% on my airplanes with flying tails, both horizontal and vertical.
Haven't lost a servo yet.
I have had a conventional rudder strip gears in an HS-81 though, in flight.
Some equipment is truly made down to a price.
For your purpose, most small servos should be OK. My GP Yardstick uses the Hitec Feather servos, which don't complain about the large surfaces they have to move.

FHHuber

The number I was quoted for stability was 27%... not 25. (25 being easier to calculate... its normally used for the model's pivot line.. as it ensures a margin of stability.)

I agree that being back at 30% is a bad idea in general. You may have trouble getting a stable trim setting for level flight with the small, slow aircraft. If that pivot line is changeable... I'd change it.

The slow speed airplane... you're torque required is going to be VERY small if the pivot line is very close to 25%. The larger factor to worry about will be the "hanger rash" forces put on the servo. Witht he 30% line... you need twice the power as if the pivot line was ahead of the neutral force line the same amount. (you're on the edge of the power range of the 27 in-oz HS-81 in my opinion.) Move that pivot line... and I'd be comfortable putting a Tower TS-5 in it. (appx 12 in-oz... about 1/3 the weight of the HS-81 and slightly under half its power.)

Note... the flying stab is not necessarilly the best choice for maximum maneuverability at low speed. Deflection above appx 18 deg... you can stall the stab. You get MORE control authority from the same size (total area) 50% fixed, 50% moving stab/elevator setup at low airspeeds.. and you'll probably never fully lose control authority of the stab (before the main wing stalls) with 30 deg deflection on the elevator.

Flying stabs are virtually essential for tran-sonic and supersonic flight. They are GREAT for precision trimming with minimal drag. (why the speed design I flew used it... minimal drag.) They are rotton for aerobatics at low speed.

Tall Paul

The information in this last post from Huber is SO wrong, follow it at your own risk!

CoosBayLumber

My Feedback: (1)

OK, OK.

Checked and over the small chord, it means I have to move the center for rotation about 1/8 inch forward. This places it smack at 25%. That also incorporates a whole bunch of revisions to the plans though and will involve an extra hour or so.

Wm.

FHHuber

Where is it wrong?

If he were putting a .61 in the nose (which is the size I'd expect with that large a stab..) then he'd need more servo power. but for the SLOW electric... he's not going to have much load.

Tall Paul

./
The enormous amount of aeronautical ignorance in your posts is appalling.
The messages are composed of random words and numbers, neither of which you comprehend in context, and are presented as reality.

FHHuber

Instead of just BASHING... tell WHERE it is wroneg with numbers and calculations...

Oryx

Hmmm, I have to agree with Paul...

The part of the discussion that I do agree with, is that by placing the pivot at 25% MAC, and assuming the airfoil is symmetrical, the torque required from the servo would be very small (theoretically it is zero, but with friction, inaccuracies and other real-world effects there will usually be a slight load). From an "aeroelastic" point of view (not really the right term since we are not really talking about structural elasticity) - the control surface is neutrally stable. That means you can put it at any angle relative to the flow and the moment remains essentially zero.

If you use a more forward pivot point, you are erring on the safe side. Now you require more load from the servo to pitch the surface, but the aerodynamic load on the surface is trying to move the surface back to neutral. Essentially, if you disconnected the pushrod the control surface will point itself into the flow.

Moving the pivot point anywhere behind 25% is extremely risky. Basically, whenever the horizontal tail produces some load, the moment on the pivot point is in a direction that will want to increase the pivot angle. Bigger pivot angle increases the aerodynamic load, which increases the moment, which wants to pivot it even more and so on. In other words, for any non-zero load the servo will be fighting the control surface from running away, or if you disconnected the pushrod under any flight condition the surface would slam to either the full-up or full-down stop.
Although the servo may be strong enough to counter this constant load at low speed, near zero tail load the surface will want to oscillate, which in turn could develop into a flutter or at the very least make the airplane difficult to control. At higher speeds the servo may not be able to cope with the load anymore and instead of being forced back towards neutral, it will be forced to full up or full down elevator!

The best place to put the pivot point is just slightly ahead of the 25% MAC.

FHHuber

In the md 1980's... I was flying mostly slope gliders.

There was a guy I flew with (Can;t remember his name exactly...J Calderon?) who designed and kitted the "Swift 46" This was a 1 lb 2 channel swept wing T-tail (well the stab wasn't quite at the top of the fin) glider using ailerons and flying stab. Top speeds of appx 140 mph in ridge lift.

We used the Futaba mini servo of the time (same size as the HS-81...) with THIN metal cable in sheath to control the flying stab. The pivot point for the flying stab was... 25% MAC of the swept stab.

If a smaller servo had been available...we'd have used it. The mini of the time was as small as we could find. (and didn't have the power of the HS-81.. )

I did misquote ithe HS-81 power.. its rated 37 in-oz... and the TS-5 is rated 17 in-oz. (I looked them up.) But underestimating a servo's power is erring on the safe side... definitely not dangerous.

Oryx

I don't think anyone had a problem with the 25% location. It doesn't leave much of a safety factor, but it should be fine, and indeed it would require the smallest torque from the servo. The problem was with the suggestion that placing it at 27% to 30% is a good idea.

FHHuber

No one but the originator wanted it at 30%... I noted that you need much more power if its behind 27% than if its ahead... because behind is less stable and less safe.

Oryx

I responded to your earlier comments:
"25 to 27% MAC of the moving surface is the ideal range for the pivot line" (post no. 2)
and:
"The number I was quoted for stability was 27%... not 25" (post no. 11)

I would not place it behind 25% at all. Not even 2% behind 25%.

Tall Paul

And that's just one blunder among many.... the lack of knowledge about where the pivot point must be, despite claims of experience and expertise with flying tails.
There's a looong history of such bad information because of ignorance, from that source.
Some of the bad stuff is humorous, others like this can lead to crashes.
The totally mythical "9 degrees of downthrust on the Me-262 to cure the poor engine out handling" illustrates several problems with information from that source.
First, the "downthrust" didn't exist.
Second, it wouldn't fix the problem.
Third, the idea of it was said to be proper for two completely different configurations, neither of which could benefit in any way and had actually been accepted as fact by the guys with questions about engines.
Fourth, when all this is pointed out beginning with the invisibility of downthrust on the Me-262, the "9 degrees" is said to be "subtle"...
Which points out the obvious unfamiliarity of that source with downthrust in the first place, and what 9 degrees of it would look like on the Me-262 had it existed.
On the humorous side, is the "200 mph control-line speed model" using an motor that could only get past 100 mph by boxing it up and airfreighting it someplace.
As I say, it's mostly a collection of random technical terms and numbers, which when looked at reveal an ignorance of both what they are, what they mean and how they're used.
This is the 'net though, and "free advice" is worth what you've paid for it.

rmh

If you are serious about doing this project -- the servo which works best -will influence how successful you will be .
Go to the JR Servo site and look at the very small digital servos.
These have metal gears and being digital types, will have far better holding power than analog of greater torue rating. Not theory - fact.
(Has to do with on/off power inputs on digitals).
These servos , setup for maxixmum servo output rotation, will give you the most control power in the smallest lightest package.
Course - that's just my opinion.
We tested these smaller ones in Gas powered aerobatic models -on elevators --just to see what the probable failure rate would be.
They did not fail tho eventually the gears got a little loose in the second model we setup.
There are a few sizes to choose from -
If these are simply too large --look at the model 281 metal gear sub miniature servos .
Tho not digital ,they are quite strong.
We use these on our tiny electric acrobats.
Flying stabs are not anything new - they work well and as long as your setup does not "self excite" you should be fine.
Err on the stable side.
Whatever you do tho - set up geometry for max servo movement to required surface deflection. also max out travel adj in your TX --on JR that's 150%
any linkage slop or bending is deadly on these designs.

rmh

I just realized I was throwing out a vague term - "self excite"
I should have said "stick snatch".
I like that one - it makes some think you are talking dirty.

FHHuber

Tall Paul...... get good 3-views of a ME-262 that show the thrust lines of the real thing. The downthrust I quoted for the particular design is there. The photo you used to refute it just plain is a bad angle for seeing it. Its just too bad my 3-view set DOES NOT FIT my scanner. (it would take about 24 pieces to scan it all...)

I never said that you ALWAYS used that much downthrust on a twin... I said some designs used UP TO 9 deg, to reduce the amout of side thrust needed. becuase it works.) I said that a PARTICULAR MODEL of a P-38 for FF used 9 deg downthust and it cured the thing from being forced into a spn if one porp quit providing power before the other. (not that the full scale had anything near the same thrust angle...)