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Hot Wings

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and that "I" takes the length into consideration.
If you are referring to my Excel cell in one of the above posts, then no. "I" is independent of the length.
All I did was use the formula for column strength (F = π²EI/L²). Except instead of solving for F that value gets plunged in as a known based on the tensile strength of the tube in question. Then solve for L.
 

Ollie Krause

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Feb 26, 2020
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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.

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.
Thanks for all the great info! Would it be possible to get a copy of your spreadsheet? It seems like everyone here has their own spreadsheet and while we are currently in the process of constructing a series of our own, it would be good to have a solid reference. Thanks!
 

Ollie Krause

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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.
Will do! Matt, our FEA and CFD kid, just produced our first usable flow simulations which turned out to be within +- 0.5% of our initial calculations. I'll send him the YouTube series to watch though before he gets too far into FEA.
 

Ollie Krause

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Feb 26, 2020
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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.
Aircraft Spruce seems to have it in stock for $2.74/foot if we can't find it locally. I'll call some local metal shops to check its availability and price to be sure.
 

Dillpickle

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May 3, 2014
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Piny Woods, Tx
Aircraft Spruce seems to have it in stock for $2.74/foot if we can't find it locally. I'll call some local metal shops to check its availability and price to be sure.
Ollie...since this is a school project, you should have a student solicit materials/software donations and the like from some suppliers. Be sure to show them what you have accomplished so far.
 

proppastie

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Aircraft Spruce seems to have it in stock for $2.74/foot if we can't find it locally. I'll call some local metal shops to check its availability and price to be sure.
seems like a good price in comparison......Until the crisis I was able to save shipping by taking a drive, but they closed the pickup counter for now.

lots of aircraft metal salvage yards in CA/WA might check them out.
 

Ollie Krause

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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.
Thanks for all the great advice! We actually just finished a Euler Formula unit in our math class so this will be a nice way to apply it IRL. We'll continue our brake and beam research and will post our updates.
 

Ollie Krause

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Ollie...since this is a school project, you should have a student solicit materials/software donations and the like from some suppliers. Be sure to show them what you have accomplished so far.
Yep yep. Once our design has progressed some more we'll contact our local metal supplier for a sponsorship. We've been getting a lot of denials from our previous requests but we are in the process of getting fiscal sponsor and hopefully that will help. I just think a lot of companies are worried about sponsoring some high schoolers to go kill themselves...
 

Ollie Krause

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seems like a good price in comparison......Until the crisis I was able to save shipping by taking a drive, but they closed the pickup counter for now.

lots of aircraft metal salvage yards in CA/WA might check them out.
Yeah I like McMaster since they provide the CAD files but I've found that for anything other than bolts, nuts, and washers it's just too expensive. I think I spent $60 on two feet of 2" aluminum ducting once...
 

Dana

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Yes, McMaster has good prices on some things, outrageous prices on others stuff. You pay for the convenience, but they sure are convenient.
 

Rudy Lee

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Apr 26, 2020
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Hi everyone,

My name is Rudy and I co-lead Flight Club’s design team. Recently, I’ve been working on designing the landing gear for our airplane, and I’ve got a couple questions to ask. We were looking around for some information about ultralight landing gear, and the book Landing Gear Design for Light Aircraft by Ladislao Pazmany was highly recommended to us in another HBA forum thread. We are in the process of getting this book, but in the meantime, we were wondering if you all could shed some light on a few basic questions we have. First of all, after seeing several ultralights without suspension (such as the affordaplane), I’m wondering if it is really necessary. If we were able to use a fixed landing gear system, it would reduce design complexity, weight, and improve the ease of construction. Correct me if I’m wrong, but I would imagine this would limit us to paved runways and would require a skilled pilot capable of smooth landings. Maybe this could be mitigated by using larger tires such as the Free Bird Black Max...
If we do end up using a suspension system, we’re currently in the early phases of designing cub style landing gear. However, when mocking it up using Onshape, we ran into some difficulties positioning it based on the method described in the General Aviation Aircraft Design Applied Methods and Procedures textbook we’ve been studying (see attached photo for our design and the textbook’s method). It had us place our wheel well in front of the wing, but this is not something I’ve seen in any other ultralight. Ideally, we would like to have our front wheels located directly below the leading spar of our wing. This would allow us to connect to an existing jury support gusset on the fuselage which is securely attached to a vertical cabin beam. Since our jury struts already attach at this point, there'll also be some really heavy-duty gussets already in place we could just extend a bit.
Another concern we have with our current design is the fact that the angled landing gear beams must pivot so they must somehow attach normal to the fuselage (see attached photo). This puts a lot of stress on two really small gussets (outlined in red below) which look super sketchy. Has this been successfully done before? We noticed this method is used by the Legal Eagle and EMG-6 airplanes but they use welded 4030 fuselages and I’m wondering how a 6061 T6 or 2024 T3 gusset would hold up for such loads. If we were able to place the wheels directly below the leading spar as described above, we could use a single straight beam for the front landing gear beam.
For anyone who's following our project, I’ve also attached some recent screenshots of our airplane. We’ve made some major revisions on the nose, cabin, and have a rough draft of our tail done as well. I want to give a huge thanks to all the HBA members who have been so helpful. You all are great and this project wouldn’t be progressing like it has without you!
123.png1234.jpg
 

Geraldc

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1587982664565.png
What are you planning to do where the tubes shown with red arrows meet?The whole tail boom will hinge around the blue tube where the blue arrow is.
 

Dana

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Regarding rigid landing gear, it's quite common on ultralights. A paved runway is not required. I had the black max wheels and brakes on my Ultrastar, which has rigid gear. Of course, keeping the tire pressure reasonably low helps.

If you put the wheels too far back you run the risk of nosing over on landing.
 

proppastie

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This puts a lot of stress on two really small gussets (outlined in red below) which look super sketchy. Has this been successfully done before? We noticed this method is used by the Legal Eagle and EMG-6 airplanes but they use welded 4130 fuselages
You could have steel welded (or bolted) fittings at these locations....there is no reason why you can not mix steel with aluminum as long as you prime the metals so there is no "mixed metal corrosion" (galvanic corrosion)......
 

tralika

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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.
I disagree with Hotwings on this. I have about four thousand hours in tailwheel aircraft, mostly C185, Beaver and Otters. All had steerable tailwheels. I now fly a Just Highlander that qualifies as an LSA. When I built my Highlander I installed steerable tailwheel that was recommended by the kit manufacturer. That tailwheel, made by a well known manufacturer of wheels and brakes for the experimental market, was terrible. I was never able to get it set up so it would steer at slow speed, even after multiple phone calls and following the recommendations of the manufacturer. I replaced it with a Tundra Lite non-steerable, lockable tailwheel. Having never flown a plane with a non-steerable tailwheel I had a lot of skepticism on how the ground handling would work but I could not be more pleased.
Steering during slow taxi is very easy, use a little power and brake to start the plane turning then use rudder to control it. Many times the rudder is enough to stop the turn but if not another tap on the opposite brake works fine. During take off and landing on a runway the rudder is all that is needed for directional control. Off airport rudder works well for take off, I'm usually on the brakes pretty hard when landing off airport but direction control is not an issue, even when landing in a very confined area. I like a lockable tailwheel from my experience flying in strong gusty crosswinds so I use the locking feature on my Tundra Lite during most take offs and landings. However I've done multiple take offs and landings with the Tundra Lite unlocked (none in strong cross wind) and really can't tell much difference. The only drawback to the non-steerable tailwheel is the risk of a brake failure and loosing the ability to turn in one direction. This is another good reason to keep taxi speed slow.

Conventional wisdom says tailwheel aircraft must have steerable tailwheels. It turns out, at least in my experience, that conventional wisdom is wrong. For the non-believers out there, my old steerable tailwheel is for sale, cheap.
 

Hot Wings

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I disagree with Hotwings on this.
<< >>
Conventional wisdom says tailwheel aircraft must have steerable tailwheels. It turns out, at least in my experience, that conventional wisdom is wrong. \
Specifics please. What was so terrible with that one configuration?
Even LSA weight aircraft respond differently than part 103 weight aircraft. Lower moment of inertia means there is a larger effect from cheap non-linear brakes. Sticky brakes on something as heavy my Aeronca can cause some excitement.
<< >>
Agreed. A landing skid can change the dynamics completely, but eliminates the ability to easily taxi.

@ the OP
Keep it simple. If you decide you need some suspension the simplest - IMHO - is the Wittman style. Just plug a stick of G-10 glass rod in a socket with a matching fitting on the other end for the axle.
There are a lot of ultralights and gliders that don't use any suspension other than what the tire affords. Light and slow lets us do things other plans can't get away with.

Edit - Internet flaky today:
Turn the cluster around at the fuselage joint and it can reduce the loads on the gussets.
cluster.jpg
Looks funny. An alternative is a machined end fitting that plugs into the end of the tube.
 
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Ollie Krause

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View attachment 95926
What are you planning to do where the tubes shown with red arrows meet?The whole tail boom will hinge around the blue tube where the blue arrow is.
Yeah we are in the process of removing that entire T structure as it puts some sketchy shear loads on that vertical beam but just haven't gotten around to it yet. It'll be replaced by two beams like the ones drawn in orange here:
 

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TFF

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On the Legal Eagle, the short stubs are to make it easier for the average builder to make the gear articulate without complex angles. Good or bad depends on skill and knowledge.
 

Rudy Lee

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Apr 26, 2020
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Regarding rigid landing gear, it's quite common on ultralights. A paved runway is not required. I had the black max wheels and brakes on my Ultrastar, which has rigid gear. Of course, keeping the tire pressure reasonably low helps.

If you put the wheels too far back you run the risk of nosing over on landing.
Thanks for the advice! Good to know about tire pressure. We'll look into doing a fixed landing gear system.
 
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