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BJC

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I've been running loads on my aerobatic ultralight napkin sketch, and because the vertical stab/rudder extends above the tail the loads end up being higher on the top than the bottom (air loads). My worst case so far has been outside snap rolls which is probably a loading the aircraft being discussed won't see, don't know what the standard part 23 loadings are.
A friend, who formerly competed in the unlimited category, quit doing snaps after watching the deflection of the horizontal stabilizer on his Extra 300 while snapping.
I haven't considered landing loads, but they also put the top in compression, which is often the critical state for a truss. Makes me want to put the single pole on the bottom.
The other considerations are how the wings, wing struts, and landing gear attach. In many examples, the wide part (bottom of the triangular truss) helps with the torsional loads for minimal structural weight. Think of pinned spar attachments up top plus pinned strut attachments at the bottom, as well as the need of the landing gear to deal with side loads.

As usual, though, everything is a trade-off, but the “two on bottom, one on top” configuration appeals to me.


BJC
 

robust

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Sorry could you clarify a bit please? We don't have any flaps but we might wanna extend our ailerons since we'll be flying at such low speeds they might not have enough of an effect at their current size.
Aileron should be the extreme point of the wing. Then its efficiency is higher. Then its moment, relative to the X axis, is greater. The aircraft will have better handling. Angular rotation speed dω / dt will be higher
 

Ryan Collier

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Feb 13, 2020
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Howdy, I'm a member of this project and I've been trying to design the landing gear and taxi systems for this project and I know that a lot of Cessnas use differential braking to steer. I would think this would be a really nice way to do the steering because I feel like it would be easier to control than a steerable tail wheel. However, I haven't seen it done much on ultralights. Is there a reason for this?
 

Ryan Collier

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Another question I have is if the dimensions of our fuselage tubing are strong enough. The Legal Eagle uses 4130 steel that has a diameter of 5/8 of an inch and a wall thickness of 0.035 inches. We're planning on using 6061 t6 aluminum with a 1 inch diameter and a 0.035 wall thickness. Multiplying the yield strength of either material by the cross sectional area of their respective beams gives me very similar results. We do plan on doing finite element analysis of our frame at some point, but because our frame is inspired by the legal eagle we've been using their specs as a starting point. Do you guys think that 0.035in is too thin for 6061?
 

Hot Wings

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Is there a reason for this?
Tail wheel airplanes are inherently unstable on the ground during taxi. This requires precise and effective control. Airplanes with the center line wheel in the front are typically stable on the ground because the fixed wheel is aft of the center of gravity. The classic analogy is a shopping cart. They work fine when pushed forward but take some concentration to push backward.

Also the typical brakes found on ultralight airplanes are not the best in terms of modulation and predictability. This makes using them to steer a tailwheel airplane using brakes even harder. Brakes are useful on a tail wheel airplane for directional control but are generally reserved for very slow speeds or when the rudder just isn't enough to keep things straight.:eek:

Also if you are trying to make part 103 weight brakes are heavy.

Long story short: Differential braking works well with nose wheel airplanes but a steerable tail wheel is the accepted method for airplanes with the center of gravity behind the main gear.

Edit: Tube strength: My spread sheet says that 1" x .035 6061 has the same strength in compression and tension at around 17 inches. That means that if the length of the tube between the joints is more than 17 inches the failure will likely be in compression. The 5/8" x .035 has a cross over length of around 14 inches - or about3 inches less than the aluminum you are contemplating.
 
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Ryan Collier

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Tail wheel airplanes are inherently unstable on the ground during taxi. This requires precise and effective control. Airplanes with the center line wheel in the front are typically stable on the ground because the fixed wheel is aft of the center of gravity. The classic analogy is a shopping cart. They work fine when pushed forward but take some concentration to push backward.

Also the typical brakes found on ultralight airplanes are not the best in terms of modulation and predictability. This makes using them to steer a tailwheel airplane using brakes even harder. Brakes are useful on a tail wheel airplane for directional control but are generally reserved for very slow speeds or when the rudder just isn't enough to keep things straight.:eek:

Also if you are trying to make part 103 weight brakes are heavy.

Long story short: Differential braking works well with nose wheel airplanes but a steerable tail wheel is the accepted method for airplanes with the center of gravity behind the main gear.

Edit: Tube strength: My spread sheet says that 1" x .035 6061 has the same strength in compression and tension at around 17 inches. That means that if the length of the tube between the joints is more than 17 inches the failure will likely be in compression. The 5/8" x .035 has a cross over length of around 14 inches - or about3 inches less than the aluminum you are contemplating.
Thank's for the quick reply, that's very helpful. Something I'm having trouble wrapping my head around is the shopping cart analogy. A shopping cart is definitely harder to push backwards but it has the steerable wheels in the front, just as most cars do. That's why my original line of thinking was to have differential breaking because i thought that it steering from the back would make things more difficult.
 

Hot Wings

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can you explain the "crossover" idea with numbers? Thx.
I set a cell in my spread sheet that calculates the length of the tube where the buckling limit* is the same as the tension limit - solve for the variable being the length.
If the tube is longer than "X" then it will probably fail in compression. Shorter than "X" then tension. Right at the goldilocks point we, in theory, get the lightest structure.

* depends on the end conditions.

edit: Shorter than "X" then tension is not strictly correct. The limit in compression for the short tubes is still limited by is yield strength. The formula for for buckling can give misleadingly large numbers for short tubes.
 
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pwood66889

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"I set a cell in my spread sheet that calculates the length of the tube where the buckling limit* is the same as the tension limit - solve for the variable being the length. "
You gotta formula for that cell, Hot?
Thanks in advance = Percy
 

poormansairforce

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I set a cell in my spread sheet that calculates the length of the tube where the buckling limit* is the same as the tension limit - solve for the variable being the length.
If the tube is longer than "X" then it will probably fail in compression. Shorter than "X" then tension. Right at the goldilocks point we, in theory, get the lightest structure.

* depends on the end conditions.
Aha, I gotcha. I told you I'm dull! Nice feature to have. Thx.
 

mcrae0104

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We do plan on doing finite element analysis of our frame at some point...
To analyze a fuselage truss like this, FEA is like bringing a gun to a knife fight. Learning the underlying statics is the prerequisite for reliable FEA. Don't put the cart before the horse. Check out the youtube series Hot Wings posted.
 

Hot Wings

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You gotta formula for that cell, Hot?
Yes, but it doesn't do much good without the rest of the sheet.

=SUM((PI( )^2*H15*H16)/H25)^0.5

H15 = Youngs mod for the material...............the factor I forgot to change in my sheet twice before posting a number here 😞
H16 = Tube "I"
H25 = yield of the tube in tension

Both "I" and tension yield get figured elsewhere.


edit: Shorter than "X" then tension is not strictly correct. The limit in compression is still limited by is yield strength The formula for for buckling can give misleadingly large numbers for short tubes.
 
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jedi

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Another question I have is if the dimensions of our fuselage tubing are strong enough. The Legal Eagle uses 4130 steel that has a diameter of 5/8 of an inch and a wall thickness of 0.035 inches. We're planning on using 6061 t6 aluminum with a 1 inch diameter and a 0.035 wall thickness. Multiplying the yield strength of either material by the cross sectional area of their respective beams gives me very similar results. We do plan on doing finite element analysis of our frame at some point, but because our frame is inspired by the legal eagle we've been using their specs as a starting point. Do you guys think that 0.035in is too thin for 6061?
Have you checked the availability of 0.035. It can be difficult to get (as in expensive). It sounds light to me but I have not done any calculations. Great if you can find it and don't break it.
 

TFF

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Same with 4130. Some sizes are made in bucket loads and some are rare. I know the difference in some of the steel can be double or triple in price on just one size difference. And the next the price drops. Sometimes ideal does not exist off the shelf.
 

JohnB

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5/8x035 is a very common size, buy LOTS of it every year
 

proppastie

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Your FEA (properly constrained and loaded) will give you raw numbers but you need to check against the column loading in compression with the charts. In tension it should be PSI yield......The key word here is "properly constrained and loaded" Need to know how much to load it and how you are going to hold it in your load tests and your FEA. Load test is critical, it is too easy to screw up a FEA.
 
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Ryan Collier

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To analyze a fuselage truss like this, FEA is like bringing a gun to a knife fight. Learning the underlying statics is the prerequisite for reliable FEA. Don't put the cart before the horse. Check out the youtube series Hot Wings posted.
Thanks for the advice, I checked out the series and a bunch more on statics and truss frame analysis. I'm having a little bit of trouble analyzing indeterminate structures because most of the videos I've found only cover determinate. However, my research is not over. Throughout this project I'm trying to learn as much as possible and that's actually one of the reasons why we're using FEA. Because It's a tool that could be useful in the future and on more complex projects, It' helpful and interesting to learn how to use it.
 

Ryan Collier

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Have you checked the availability of 0.035. It can be difficult to get (as in expensive). It sounds light to me but I have not done any calculations. Great if you can find it and don't break it.
Thanks for the warning, I've checked the availability of 0.035 and it's actually one of the cheaper options, at least when using 6061t6.
 

pwood66889

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So you're saying:
The square root of:
{Pi squared times Youngs Modulus for the material times "I" for the tube;
All that divided by tube yield in Tension.}
and that "I" takes the length into consideration. Said length can effect buckling
in that short lengths give larger than real measures.
Appreciate!
 

Dana

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Some random comments:
  • In a truss structure like a fuselage, tensile or compressive strength is usually not the limiting factor, column buckling strength is, look up "Euler formula". In column buckling, the material's elastic modulus controls, and aluminum is about 1/3 that of steel even if the tensile strengths are similar.
  • .058" wall tubing is commonly used in ultralights, because the next (1/8" smaller size) tubing is a slip fit inside it for splicing or reinforcement.
  • Many ultralights have brakes, some have differential brakes, my Ultrastar did. I would definitely suggest them, especially for taildragger.
  • The issue with a taildragger is not that you're steering from the back, it's that the center of gravity is behind the main wheels, so once it starts to swerve, it wants to keep on going.
  • Most light planes don't use differential brakes for primary steering; even with differential brakes they usually have a steerable nose or tail wheel (there are exceptions). But the steerable wheel usually doesn't give a very tight turning radius; that's where differential brakes help, e.g. making a tight turn at low speed.
 
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