Elevator balancing for tube and fabric tail

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ScaleBirdsScott

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I don't see why the early sketches that just extend the tip a few inches of the elevator past the horizontal with some forward extension of the tube would not work for a pure mass balance. If some aerodynamic surface balancing is desired then extend it further than just a few inches, and bend the trailing edge tube back inwards towards the horizontal, then add a short section of tube or formed rib back to the spar tube. Use gussets of similar construction and thickness as elsewhere in the horizontal. Add a weighted mass to the forward edge and secure. At this point that's the sensible approach to tackle the job.

Now the tricky bit is engineering to validate loads and forces involved should be applied as required; and I can't really contribute on that end.
 

stanislavz

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My attempt to fly with weights removed completely, turned into very short and very exciting event. Not something I want to repeat again.
I do remember similar "problem" on other airplanes with aluminum tube tail, but issues was fixed by going to the next tube on size - 125x2mm to 125x2.5mm. Could you see an visual notable flex on tail boom ?

Also - please name that low wing airplane with tail boom :) Looks interesting. Any more structure photo ? Do tail boom is main tube as in c42, or only short for tail itself ?
 

Eugene

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I do remember similar "problem" on other airplanes with aluminum tube tail, but issues was fixed by going to the next tube on size - 125x2mm to 125x2.5mm. Could you see an visual notable flex on tail boom ?

Also - please name that low wing airplane with tail boom :) Looks interesting. Any more structure photo ? Do tail boom is main tube as in c42, or only short for tail itself ?

 

stanislavz

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Thank you more info on that GRYF.

And yes, it is definitly look like sibling of ikarus c42, but low wing.

1592913986126.png

But - i think it would be even better with main spare below knees - non need of forward swept wings.

C42 for comparision 1592914358105.png
 
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Eugene

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But we do know that it is fine with mass balance.
I can't find an airplane that used aluminum tubing and fabric for horizontal tail with mass balance. All of them slow ultralights and have no need for it. That's why no examples for me to see how to do it right. Maybe I'm not looking in the right places?
 

wsimpso1

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Looking for a good way to provide 100% counterbalancing for tube and fabric elevator construction. Nobody seems to worry about it for 100 MPH aircraft. So, I can't find good examples to follow

View attachment 98313

I can think only about one option where elevator would be made out of aluminum tubing. Trailing edge tube can be made longer to except lead weights.
Add a hinge near the tip and this looks fine. Eugene, what what is the problem with this?[/QUOTE]
 
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wsimpso1

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I can't find an airplane that used aluminum tubing and fabric for horizontal tail with mass balance. All of them slow ultralights and have no need for it. That's why no examples for me to see how to do it right. Maybe I'm not looking in the right places?
Eugene,

  • You know your ship needs mass balance from flight test results;
  • You have scheme that will mass balance fine structurally and aerodynamically;
  • Just because you can not find another airplane in your speed class that has mass balance does not mean you can not do it;
  • The only downside to mass balancing a surface is it weighs a little more.
Now as to why your ship needs mass balance, let's talk about how mass balance works. Start by assuming the control surface is at its equilibrium position with steady state air flowing over it, and is pulling downward as it must in flight.
  • Then we fly into a gust, the air you are flying in for just moment is moving upward more than before, the tail shoved vertically by this gust.
  • Then you leave the gust while the tail is still moving up from the gust. Momentum is up, tail boom and lift are pulling down, and the tail slows, then starts back down.
  • The unbalanced control surface lags behind the stabilizer, making the horizontal tail pull even harder downward, which causes the tail to overshoot the equilibrium position.
  • The tail eventually is stopped, the elevator now lagging inertially behind the stabilizer and is down, making for extra power upward.
  • This cycle repeats and grows in amplitude, and is called pumping.
  • This pumping will occur at the resonant frequency of the tail to vibrate up and down on the boom, and will occur at the resonant frequency of the entire airplane to oscillate in pitch.
And you know it happened with your tailplane when you removed the balance weight.

This unbalanced rigid body flapping of a control surface pumping the elastic tail boom is known and usually does not require mass balancing in such a slow airplane. I suspect, but will not attempt to prove, that the low stiffness of your tail boom and perhaps other parts of the system brought the resonant frequency lower than most other airplanes, and into range for your airspeeds.

The solution is to mass balance the elevator. It then does not lag behind the stabilizer when disturbed, and thus does not pump the resonant frequencies of the system. It is quite reliable in fixing this mode.

The only downsides to it are a bit of weight, a bit of drag, and bit of fuss building it. So we usually put the mass as far forward as we can conscience, which allows less mass. Oh, and couple it to the elevator stiffly. A flimsy connection is bad. You want the resonant frequency of the mass on the arm to be well above the frequency of the tail on the fuselage.

I do suspect that the really soft tailboom is what puts this all into range in a 90 knot airplane. How do you stiffen the boom? That will both reduce the range of tailplane lag and raise the resonant frequency some. Natural frequencies go with the square root of stiffness divided by mass. You have to quadruple the ratio to double the frequency. The boom on your ship is straight tube. Increasing straight tube mass, you add mass everywhere along the tube. Raise the wall thickness and it gets stiffer faster than it gains weight, but the ratio improves slowly while your airplane gets heavier and CG shifts aft. Raise the OD (same wall thickness) and it gets stiffer faster than it gets heavier, and the ratio improves more quickly, but it does get heavier and shifts the CG aft. But make the tube wall thinner and bigger in diameter at the engine and then taper it down as you go aft, and you can make it lighter and a lot stiffer and raise the resonance frequency quite a bit while not shifting the CG aft. This is why conventional fuselages start big and taper toward the tail. Light, way strong and stiff, and no one worries over balancing the tail surfaces until the airplane gets a lot faster.

Now your ship, you can build either a tailored shape composite tail boom (lots of examples) or a three tube aft fuselage (lots of examples) with some jury struts. Both will be way stiffer and you can probably save some weight aft. They will do a better job of supporting the tailplanes, even with them enlarged, reduce worries over resonance, and make for a better flying airplane.

And given that you have already cheated death on a flutter event, let's leave the mass balance on the elevator even if you do greatly raise the fuselage resonant frequencies. No more tempting the gods on flutter, OK?

Billski
 
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Eugene

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Eugene,

  • You know your ship needs mass balance from flight test results;
  • You have scheme that will mass balance fine structurally and aerodynamically;
  • Just because you can not find another airplane in your speed class that has mass balance does not mean you can not do it;
  • The only downside to mass balancing a surface is it weighs a little more.
Now as to why your ship needs mass balance, let's talk about how mass balance works. Start by assuming the control surface is at its equilibrium position with steady state air flowing over it, and is pulling downward as it must in flight.
  • Then we fly into a gust, the air you are flying in for just moment is moving upward more than before, the tail shoved vertically by this gust.
  • Then you leave the gust while the tail is still moving up from the gust. Momentum is up, tail boom and lift are pulling down, and the tail slows, then starts back down.
  • The unbalanced control surface lags behind the stabilizer, making the horizontal tail pull even harder downward, which causes the tail to overshoot the equilibrium position.
  • The tail eventually is stopped, the elevator now lagging inertially behind the stabilizer and is down, making for extra power upward.
  • This cycle repeats and grows in amplitude, and is called pumping.
  • This pumping will occur at the resonant frequency of the tail to vibrate up and down on the boom, and will occur at the resonant frequency of the entire airplane to oscillate in pitch.
And you know it happened with your tailplane when you removed the balance weight.

This unbalanced rigid body flapping of a control surface pumping the elastic tail boom is known and usually does not require mass balancing in such a slow airplane. I suspect, but will not attempt to prove, that the low stiffness of your tail boom and perhaps other parts of the system brought the resonant frequency lower than most other airplanes, and into range for your airspeeds.

The solution is to mass balance the elevator. It then does not lag behind the stabilizer when disturbed, and thus does not pump the resonant frequencies of the system. It is quite reliable in fixing this mode.

The only downsides to it are a bit of weight, a bit of drag, and bit of fuss building it. So we usually put the mass as far forward as we can conscience, which allows less mass. Oh, and couple it to the elevator stiffly. A flimsy connection is bad. You want the resonant frequency of the mass on the arm to be well above the frequency of the tail on the fuselage.

I do suspect that the really soft tailboom is what puts this all into range in a 90 knot airplane. How do you stiffen the boom? That will both reduce the range of tailplane lag and raise the resonant frequency some. Natural frequencies go with the square root of stiffness divided by mass. You have to quadruple the ratio to double the frequency. The boom on your ship is straight tube. Increasing straight tube mass, you add mass everywhere along the tube. Raise the wall thickness and it gets stiffer faster than it gains weight, but the ratio improves slowly while your airplane gets heavier and CG shifts aft. Raise the OD (same wall thickness) and it gets stiffer faster than it gets heavier, and the ratio improves more quickly, but it does get heavier and shifts the CG aft. But make the tube wall thinner and bigger in diameter at the engine and then taper it down as you go aft, and you can make it lighter and a lot stiffer and raise the resonance frequency quite a bit while not shifting the CG aft. This is why conventional fuselages start big and taper toward the tail. Light, way strong and stiff, and no one worries over balancing the tail surfaces until the airplane gets a lot faster.

Now your ship, you can build either a tailored shape composite tail boom (lots of examples) or a three tube aft fuselage (lots of examples) with some jury struts. Both will be way stiffer and you can probably save some weight aft. They will do a better job of supporting the tailplanes, even with them enlarged, reduce worries over resonance, and make for a better flying airplane.

And given that you have already cheated death on a flutter event, let's leave the mass balance on the elevator even if you do greatly raise the fuselage resonant frequencies. No more tempting the gods on flutter, OK?

Billski
Bill,

Thank you! Just want to make a few corrections, so there is no misunderstandings.
The "flutter event" video was flight with loose connections "A" and "B" that you can see on picture below.
IMG_4277.jpeg
This bolt was removed from connection "B"and replaced with new larger one.
IMG_0308.jpeg
This video is connection "A"
I was flying with this loose connections for about one year before I found them. My trip to Oshkosh was this way. And my experiment on flying without counterweights as well. With loose connection this tail boom was "much longer" and pumping started below 80 MPH. On the way back from Oshkosh first time, going 75 MPH against the wind turn into long trip home. After everything was fixed and elevator balanced with 100%, pumping is not coming back even at 5500 RPM, but you feel small resemblance of it with your hand. Make me think that tail boom with 912 engine is pumping up and down all the time and this is what makes those "A" and "B" connections loose over time. Perfect recipe for making new tail boom.
IMG_2522.jpegIMG_2518.jpeg
Pictures above is my attempt to make elevator larger. Obviously by doing this I lost my hundred percent balancing. As a result "pumping" was back immediately right after takeoff. So I turned around and landed immediately and removed them right away. This pumping directly related to engine RPM. You can glide down 80 MPH with no problems at all, but as soon as you advance power = control stick starting dancing in your hand. Same thing was happening on my short flight with no counterweights at all.

So, balancing on elevator is needed for sure. Maybe it will be different with a new tail boom but I don't think I ever want to experiment that again. At this point makes me pretty nervous even when I look at somebody else's airplane with no balancing on elevator. So for rest of my life I will be probably flying with counterweights. That's OK.
Screen Shot 2020-06-21 at 15.14.05.png
Above is original, large elevator that was installed with 50 hp engine and no balancing. Trailing edge was simply band tube of 2024T3 aluminum.
I was thinking possibly to copy this elevator but run trailing pipe longer and use it as counterbalance. But smart people telling me that this pipe possibly will be too long and too weak, like small "tail boom". That's my picture below.
9041C645-C396-4B65-931F-789727F05281_1_201_a.jpeg
Elevators that installed on all airplanes with 912 engines about 20% smaller and made different. Only one pipe is used for leading edge. Rest of elevator is bent from flat aluminum. You can see on side view that this approatch made trailing edge very thin, this is the way I like it. This way I can make stabilizer a little bit thicker and as a result I will get poor man's symmetrical airfoil. So I would like to make new and larger elevator using exactly same technique, but there is really no good way to add balancing to this design. Our designer simply used a few rivets, but he was not planning on 100% balancing and his bracket was shorter and ballast was lighter.
22815474_1831521673543007_1022811236804753685_n.jpeg22788973_1831518316876676_6312720441249685640_n.jpeg22814255_1831517726876735_6995596844915423882_n.jpeg
Some Skyboy owners telling me that their airplane flying just fine without any issues at all, but later in conversation you would find out that they usually cruise around 4300 - 4500 RPM. They would tell you that this is their comfort zone. I wonder why. They doing this on engine that was designed to work between 5000 - 5500 RPM. I am convinced that this flexible tail pipe is doing pumping all the time at high power setting. You simply have to get down to original power to avoid this.

I would never guess that tail on airplane plays such important role. I was always thinking that it's just flying behind just in case you decide to use it to go up or down. I am getting smarter every day! Thank you!

I wish I can say, thank you very much, and promised no more questions in the future. But I will be lying to you. Sorry, not that smart.

Eugene
 

wsimpso1

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Keep faith, you are converging on good answers. Someday you will decide that a three boom tail is really a better scheme....

Bill
 

Eugene

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Here is an example of 3 - boom tail. One tail pipe and two support cables. They probably had similar issue. Otherwise why did they install two cables just like I did on my experiment. Why nobody used 2 solid pipes versus flexible cables? One large main pipe is already in place! But technically attachment points on the wings should be positioned much closer to the propeller. Yes or no? This would be very quick fix. No questions this kind of approach is very appealing to me. Then my fuselage fiberglass will be just for airflow and drag reduction non-structural. But they tell me that all this airplanes with exposed tubing like that end up being very very draggy.

IMG_2753.png
 

Eugene

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Eventually with 912 engine factory made conclusion that 100% balancing needed and longer bracket was made and larger ballast installed. You can see the difference

IMG_5525.jpeg

 
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rotax618

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The supporting cables on the Boorabee were to control airframe flexing during taxying and bad landings, they were never very tight. A kind of belt and braces approach.
 

Heliano

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Interesting subject. Aeroelasticity is no simple matter: it is a complex combination of aerodynamic loads, loose connections and several vibration modes. The tail boom may be a factor, but control cable elasticity may also contribute, and many other things. One thing I can say: be careful when you add a balance weight at the tip of the elevator and the command cables are attached at the root, in an elevator that has low torsional stiffness. It may induce some torsional oscillation modes. Eugene, the video you posted on monday is scary. Such type of oscillation needs immediate research and fixing. Certified aircraft are required to have a huge flutter speed margin over VNE (or VMO). As a matter of fact, flutter speed has to have a good margin over VD, which is about 10% above VNE (or VMO). Want to make this matter even more complicated? flutter speed is not exactly function of indicated airspeed. It also bears a relationship with TAS.
 

Eugene

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be careful when you add a balance weight at the tip of the elevator and the command cables are attached at the root, in an elevator that has low torsional stiffness. It may induce some torsional oscillation modes.
I have herd this before from Skyboy designer. He designed this 50 HP aircraft, but later approved 912 engines installations. Different guy owns production rights and company "INTERPLANE" was making this airplanes and ship to US with 912 engines about 1999 year.

Designer and "INTERPLANE" had many disagreements and he left them in 2001.

"INTERPLANE" was trying to fix "flutter" problem and in 2004 issued AD for all Skyboys to remove small counter ballast on short bracket and replace it with new longer bracket and longer heavier bracket.

In 2008 "INTERPLANE" got out of business.

I purchased my Skyboy in 2016 and found another Skyboy owner who sold me new ballasts with AD paper work. I was ready to install them, but our local mechanic stoped me by saying " this doesn't look right, doesn't feel right for this flimsy fabric elevator"

I found original designer and send him picture of new and improved ballast. His response was " My God are you really going to install this long and heavy thing on your soft elevator???!!! I am flying with same tail and no weights at all! Try to remove ballast completely , maybe you don't need them at all!!!"

I tried and it was not very good idea. Don't want to do it again.

So, I used solid tube filled with lead. But don't really like anything about it and looking for better way to do it on new elevator.

IMG_1542.jpegIMG_1544.jpegIMG_1543.jpeg
 

wsimpso1

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I tried that already but didn't get approval from anybody.
What do you mean "approval"? You are intent upon redesigning your airplane. Your effort, your benefit, your risk, and you have to accept that to go forward. What kind of "approval" do you need to feel like it will work? Do remember that you will need a new FAA inspections and revised set of operating limitiations for any of these things, but the FAA is mostly interested in paperwork.

And what you included pictures of is a single boom tail that is heavy in exchange for being small in diameter, but the designer pushed it too small, and ended up with a tail that flexes and works the fasteners on everything involved with it.

The light way to build the aft fuselage is usually a rather larger section at the aft edge of the wing, where the bending moments are large. It then tapers down in cross section all the way to the tail, where it almost vanishes. It is mostly just a single layer of thin aluminum with some longitudenal stiffeners. Now imagine, you drive the skin layer material onto the longerons as tubes. You now have an open frame that you can blow air through. Nice and light, supports the tail.

Another way to make a light tail is to make an aluminum space frame that starts out large at the aft end of the wing, tapers down to almost noting at the tail, covered with fabric. Skip the fabric so you can blow air through it, then condense some of those tubes onto other tubes to reduce the drag and you have a light low drag aft fuselage that you can blow air through and support your tail.

Billski
 
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