Designing (and hopefully building) the Cheap Air Racer.

Discussion in 'Member Project Logs' started by nerobro, Sep 10, 2014.

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  1. Sep 10, 2014 #1

    nerobro

    nerobro

    nerobro

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    Lets start at the begining: https://www.homebuiltairplanes.com/...-cheap-air-racing-class-promote-aviation.html

    The concept I took from that thread, is that we need a cheap way to go air racing, so we can get air racing done more often, and in more places. People seeing planes do their things, with normal people at the controls, should get more people wanting to fly, and hopefully help re-invigorate the sport.

    Lots of ideas were tossed around. Lots, and lots, of ideas. Ranging from getting Formula Vee going again, to "lets make a class for Cessnas."

    And my mind's gears started turning. I came up with a concept for a Cheap Air Racer. I thought it needed to be a very fast build. I thought it needed to be ridiculously strong. To be practical, everything had to be available new, which means a current production engine. And it needed to be cheap enough that someone with "expensive" hobbies could do it.

    I chose wood as the material, as that's the least common denominator when it comes to construction tools. Metal tools work wood pretty well. Composites people tend to have saws and such. And wood people are already ready.

    The engine I first looked at, was a B&S 25hp. 5hp is enough to fly, but not well. I did a little math, with the help of Raymer, and did my first few revisions.

    I ended up with this as my first "hey what do you think of this?" design.

    [​IMG]

    500 lbs MTOW
    200 lbs Pilot
    25 HP B&S on the front
    48 kts stall
    140 kts design top speed

    Now that's just a drawing. The numbers have some basis in reality, but the design is just "well that seems about right to me" at that point.

    Topaz suggested that I share how I came up with that design, and show how I'll prove if it's viable or not.

    In the next post: Revision history, and changes to the design.

    Future posts:
    can the wings take it? Making wing shape choices and spar design.
    Is the fuselage to heavy?
    Why 25hp? And other firewall forward decisions.

    And there's going to be a lot of posts of just drawings.. showing the bits as I develop them. And showing the math behind those parts.

    Do you have questions? Suggestions? Think I'm crazy? https://www.homebuiltairplanes.com/...p-air-racer-discussion-thread.html#post234758
     
    Last edited: Sep 10, 2014
    Topaz and FritzW like this.
  2. Sep 10, 2014 #2

    nerobro

    nerobro

    nerobro

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    So, that drawing in the previous post.. isn't what the final design is going to be. Many posters here weighed in, and made some really good suggestions and I'll share those here.

    What I drew, has a canopy. Canopies are not all that cheap. And they also require a lot of ancillary components. They need latches, a frame, some sort of ventilation system, and some sort of easy way of getting out of the plane. Those all weigh something. Where a windshield and headrest can be permanently installed, eliminating latches, and hinges. If parts aren't there, they can't weigh anything.. right?

    And added benefit to having an open cockpit, is it lets you see a pilots helmet. Driver helmet paintjobs are a thing in open wheel racing. Since you're not going to be able to express yourself in the airframe, having a fancy helmet would be nice. Also, egress will be as simple as pop your buckle and jump out.

    Another design change, is the landing gear. As drawn, it's a taildragger. That's the lightest landing gear that can be done. It's also got some issues, most pressing is not everyone can fly conventional gear. It also puts the pilot in a bad position to see what's going on in front of the airplane. Attempting to keep the frontal area small, leads to putting the pilots head only just barely above the wing top. The nose and wings will obscure most of the forward vision.

    Going to tricycle gear solves all of that. By integrating the front wheel mount into the engine mount, we can negate a lot of the weight penalty of tricycle gear.

    Tonights project is to re-draw the plane reflecting those plans. And start the the weight and strength analysis of the wings.
     
    litespeed, Topaz, danmoser and 4 others like this.
  3. Sep 13, 2014 #3

    nerobro

    nerobro

    nerobro

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    I spent lunch the other day sketching out the wings.

    [​IMG]

    So now I have some rough dimensions to use for doing the math. Before you go there, yes, the spar design sucks. Hopefully I won't need to optimise the design to make weight. Simple is good. Simple is fast to build.

    I'm also depending on the 1/16" ply skin to handle the torsional forces that the wing will feel. That will probably be a post on it's own. I'd really rather not move the main spar backwards at all, as that starts to impinge on pilot/dashboard space.

    My fanciful design goal for this, is to have the airframe designed for 20g. With a 52kts stall speed you could go 238kts, and still not break the plane with control inputs. For a 140kts plane.. that is a big safety factor. :)

    Next post.. what will those spars ACTUALLY hold up to? To do 20g, they need to hold 11000lbs. And to add to the challenge? I'm doing the math using Fir/Pine and run of the mill marine grade plywood. So, 5.5ksi instead of 13ksi. Thankfully it's only .42g/cm instead of the .67 that birch ply is.
     
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  4. Sep 13, 2014 #4

    nerobro

    nerobro

    nerobro

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    Topaz asked: How did you determine the size of your wings?

    In my case, I constrained the airplane to having 8x4' wings. This is so a single sheet of plywood would cover the whole wing. The area of the wing has an upper limit too, because I only have 4' of material to wrap from the leading edge to trailing edge.

    So I started experimenting with Raymers spreadsheet. (that's a lot easier than sitting an hand calculating the needed wing area for certain landing speeds) I went with a size that "looked good" and fit on the wing material. a 3.5' root chord and 1.5' tip cord. For the initial 500lb flying weight, that lead to a 48kts stall.

    I've since moved to a 550lb flying weight. The stall is up at 52kts at that flying weight, and the same size wing. 52kts is well below the "well that's dangerous" 60kts limit for single engine airplanes set out by the FAA. And that makes me feel pretty good about pilot safety. It's not an ultralight at 24kts, but hey, this thing is supposed to go fast.

    Slow stall speeds are a double edged sword though. A really slow stall speed (provided you aren't using flaps) will limit your planes maneuvering speed. That's ok for a lot of airplanes. One of the design criteria i'm building around on this plane, is to have the Vo greater than the level top speed of the airplane. Ideally, even over the top speed in a good dive. And it builds in margin for faster classes based on the same airframe.
     
  5. Sep 16, 2014 #5

    nerobro

    nerobro

    nerobro

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    Flying in a straight line is one thing. Turning is another thing. If this is to be a pylon racer, we'll need to get some idea of what it's turn behaviors are.

    At a 550lb flying weight, the plane will climb at ~600 feet per minute. Just to stay airborne, a plane has to climb at 1930 feet per minute. (32 feet per second or so..) I think you'll get your best rate of turn, when your airplane is at the speed it will also climb the fastest at. .. at least that's what I'm going with for now. I mention the 1930fpm, because it made the relationship between climb and horsepower make more sense to me. It sets the "x axis offset" if you will. Doubling the horsepower from 25 to 50, takes the climb from 600fpm, to 2600fpm.

    So we know the legs on the triangle. 1920 for "keeping the plane in flight" and then we have whatever the climb rate is to spend turning the plane. Doing a little bit of trig, leaves us with a 17 degree bank angle. And a virtually imperceptible g loading. So, with 25hp, this things constant speed turn won't be "exciting".

    As designed:
    600fpm climb
    2100 foot run radius
    17 degree bank angle
    92 seconds per full turn.

    This left me feeling a little empty inside. 25hp certainly will fly, and even go pretty fast, but doesn't leave a lot of excess energy to use for maneuvering. Then it struck me.. airplanes can store energy, and this is pylon racing, so there are straights. While I can express the concepts, the math to really work all of that out eludes me. For the moment, I need to figure out how to convert velocity, into energy that can be spent in the corners, and figure out how fast this bird will get from the 85kts best climb speed to 140kts fast and level speed.

    Something else that I wasn't aware of... It seems that pylon racing has some pretty strict altitude limits. I was thinking that this racing might just be planes climbing to store energy, then burning that energy cornering. ... turns out, I was wrong. :) And I think that might not be the fastest way to make a lap anyhow.

    With the big block (40hp) engine on the nose of the plane, we'd be talking 2.8g's in the corners. Now that's something to talk about.

    Knocking 50lbs off the plane, get's the rate of climb up a lot. I'm going to go back to figuring out the airframe weight, to see if we can't "race" at a lower weight, and get better performance from the 25hp engine. Just that 50lbs, is worth 120fps climb rate.. Heck, at 450lbs, we're talking 900fpm instead of 600.
     
  6. Sep 23, 2014 #6

    nerobro

    nerobro

    nerobro

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    So, there's been some developments. I.. made a rather big math mistake last time. I built the triangle wrong.

    Here's how (I think) you do it right.

    *and I was wrong again*

    Today

    At lunch today I did some drawing, and math. We'll start out with my assumptions.

    First: Yellow or White Pine is 5000psi, and has a specific gravity of .42.

    Second: 1/16 ply has is also 5000psi, and has a weight of 6lbs per 4x8' sheet.

    Third: The firewall forward weight will be 120lbs.

    The airplane is supposed to have a 55 MTOW. So lets start there.

    Airplane 550 lbs
    Engine -120 lbs
    Fuel -30 lbs
    Pilot -250 lbs
    ----------------
    Airframe 150 lbs

    Seeing that number suddenly makes things feel really tight. Why? Here's why.

    The wing skins are about 16lbs.


    Wing weight:
    Skins 16.00 lbs
    Front Spar 2.00 lbs
    Main Spar 4.56 lbs
    Leading Edge 3.67lbs
    ----------------------
    26.23lbs

    And then I still need to calculate the ribs, aileron hinges and torque tubes. Oh, and the fuel tanks, can't forget the fuel tanks.

    I suppose, that while we're at it, I should take the instruments off the weight budget.
     
    Last edited: Sep 23, 2014
  7. Sep 23, 2014 #7

    nerobro

    nerobro

    nerobro

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    To determine the maximum G loading that this plane can support, I added weight to the design. The plane essentially stops flying at 900lbs. Netting -4fpm, at 90kts. That means the plane can support ~1.6g. And a 51 deg turn. That seems a lot more reasonable.
     
  8. Oct 1, 2014 #8

    nerobro

    nerobro

    nerobro

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    Remember that wing drawing earlier? Here's a graph of the lift distribution, assuming the area in the fuselage is lifting surface.

    [​IMG]

    The 8' main spar, will directly carry 63.8% of the load. Which means the outer 5' of each wing panel will only need to carry about 2000lbs each. That almost feels sane.

    Now i'm off to re-calculate the bending moment on the wing panels. At least twice.. once for the overall wing, and once at the point the wing goes from normal spar, to monocoque.
     
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  9. Feb 17, 2019 #9

    nerobro

    nerobro

    nerobro

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    Wow, I remember sitting at a Chipotle and doing the math here. I didn't know it was 5 years ago.

    SAdly, I think this plane is dead.
     

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