CAR-251: The Cheap Air Racer Discussion thread.

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Victor Bravo

Well-Known Member

Oh no you don't... speaking for myself and assuming the (un-invited) responsibility to speak for one or two others here with some experience... we are NOT trying to beat down or beat up or beat sideways. There's a guy here who is trying to actually take an internet discussion thread, wrestle it out of cyberspace and into the real physical world. Remember the great music video for that Norwegian band A-HA, "Take on me", where the girl is trying to rescue the imaginary race car driver from the pages of a comic book and get him into reality? This is the aviation version of that.

What nerobro is attempting to do is a very honorable and difficult goal. I for one want him to succeed, whether I agree with his design details or not. A decent, safe airplane that would allow a lower cost entry into air racing is a good thing.

What I am trying to do, and what I believe at least one or two other reasonably educated people are trying to do, is to somewhat steer the process towards a final product that has a chance of achieving the purpose. For my part, having been through this from the race pilot and team owner perspective, I have a pretty good idea of what race organizers, FAA race inspectors, and race pilots are going to want and need. When it comes to running detailed structural calculations, I can't add much... but I believe there are people here who have that knowledge and experience.

So please don't think that I'm beating on anyone. I won't do any of that unless someone is suggesting something really unreasonable that is going to put lives at risk.

nerobro, please forgive me if anything I've said here sounds or feels like it is mean-spirited, pejorative, or even rude. Absolutely positively not intended. I will stand by anything I've said, or apologize if I have a change of heart, but this is sincerely intended to be constructive and supportive. Anything sounding like "criticism" is only because there's something that I think I know that you don't, or that you are under-prioritizing.

nerobro

Well-Known Member
Log Member
Don't worry, we're all still friends here.

Victor Bravo

Well-Known Member
Thank you.

As far as the plywood goes, I still very much disagree with "just paint". However, I do have a compromise. It's a compromise I don't like and would not do myself, but it represents the bare minimum you could do to prevent a structural weakening of plywood from sun, water, etc.

Monokote it.

Although not by any means a "legitimate" covering for full sized aircraft, Monokote will deliver a 100% water seal and 75% UV barrier. It's definitely the lightest thing you could do, even lighter than a few coats of primer and paint.

Now... when you try to remove it from the plywood for repair or inspection... especially if it was applied properly using that Balsarite surface prep..... you'll hate me for making this suggestion.

RJW

Yesterday I made about $500. I started in the morning thinking of ordering the material for the spar on the little airplane I showed a few posts back. The spar was based on a very loose analysis (I wanted to save design and build time). The material and some hardware came to about$800. Since I like doing things on the cheap, this seemed like way too much money. So it motivated me to do a proper analysis of the spar in an attempt to save some dollars. To make a long story short, by the end of the day, I had found that my original “math shortcuts” spar was way too big for the job. I ended up with a spar that weighs only half of the original design and will cost less than $300 in material (all aerospace stuff btw—2024, 4130, NAS bolts, etc.). I even reduced the build difficulty. Rob nerobro Well-Known Member Log Member Monokote it. *snip* Now... when you try to remove it from the plywood for repair or inspection... especially if it was applied properly using that Balsarite surface prep..... you'll hate me for making this suggestion. I already know that pain. The stuff does stick with some serious tenacity though. What's it work out to? Something like$0.75 a square foot.

Lets say that the thought did cross my mind.

Yesterday I made about $500. *snip* So it motivated me to do a proper analysis of the spar in an attempt to save some dollars. To make a long story short, by the end of the day, I had found that my original “math shortcuts” spar was way too big for the job. I ended up with a spar that weighs only half of the original design and will cost less than$300 in material (all aerospace stuff btw—2024, 4130, NAS bolts, etc.). I even reduced the build difficulty.
For the moment, the structural wood is cheap wood. Going to Spruce would make a rather large difference in the size of corner blocks and spars. That's another "If I need to..."

BBerson

Well-Known Member
HBA Supporter
The vinyl graphic material from sign shops, Oracal (not Oratex) is probably the cheapest.
The Belite ProCub was covered with Oracal. Just stick it to the plywood.

Apollo

Well-Known Member

Oh no you don't... speaking for myself and assuming the (un-invited) responsibility to speak for one or two others here with some experience... we are NOT trying to beat down or beat up or beat sideways....
Sorry, I should have added a smiling emoticon after that sentence . I think you've been very constructive in your comments and don't view them as beating up anybody. As stated, I'm in complete agreement with virtually everything you've written.

I agree that it's very courageous of Nerobro to participate in this thread and subject his every thought to intense scrutiny. I'm not sure how this "experiment" in collaborative review of Nerobro's design process is going to turn out, but I hope it results in a nifty little cheap air racer someday. I'm thankful you and a few other designers are providing checks and balances during the design process.

And NOW the beatings, er, lessons, may continue .

nerobro

Well-Known Member
Log Member
I agree that it's very courageous of Nerobro to participate in this thread and subject his every thought to intense scrutiny
Sometimes it makes me break out in a rash, but I've got a topical ointment to help with the burning and swelling.

I'm thankful you and a few other designers are providing checks and balances during the design process.
Me too.

nerobro

Well-Known Member
Log Member
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
----------------------
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.

Well-Known Member
Nerobro, did you read the build article about the Arletty II? Some great lessons and some great advice too

And note that easily 2/3rd of the airframe weight EXCLUDING the engine is in "stuff", not the major structure. Ailerons, a hardpoint here and there, a stringer in that corner and you're suddenly 3 times heavier. That's the really hard part about weight estimates.

nerobro

Well-Known Member
Log Member
Nerobro, did you read the build article about the Arletty II? Some great lessons and some great advice too
Nope. Do you have a link handy?

And note that easily 2/3rd of the airframe weight EXCLUDING the engine is in "stuff", not the major structure. Ailerons, a hardpoint here and there, a stringer in that corner and you're suddenly 3 times heavier. That's the really hard part about weight estimates.
Yeah, I've noticed. Stress concentrations are the detail that devils us all.

Affirmative.

BJC

Well-Known Member
HBA Supporter
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
----------------------
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.
I'm impressed with the accomplishments of the Windward Performance group. Their Sparrow Hawk is a 155 pound ultralight glider.

See Specifications | Windward Performance

nerobro

Well-Known Member
Log Member
I'm impressed with the accomplishments of the Windward Performance group. Their Sparrow Hawk is a 155 pound ultralight glider.

See Specifications | Windward Performance
Me too. That's a gigantic airplane for very little weight. And it's slick as all get-out.

Autodidact

Well-Known Member
Affirmative.

What an accomplishment. Everything; even a prop hub and blades for a ground adjustable propeller. Wow.

Autodidact

Well-Known Member
Nerobro, in your latest C.A.R. project post, you state that the plane can sustain a 63 deg turn, and base that on climb performance. I think, though, that in a 63 deg turn you're pulling about 2.2 g? In a climb, as in a glide, as well as in level flight, the lift coefficient is virtually the same (almost no difference, because of the shallow angles involved), but in a 2.2 g turn, the lift coefficient has more than doubled, and induced drag increases with the square of the lift coefficient, so the induced drag increases by a factor of 4.84. I don't think you can relate climb performance to turning performance. You need to analyze the wing characteristics more for that.

Victor Bravo

Well-Known Member
This may be a worthwhile idea for comparison sake.

Take one or more good example of each existing construction method that could be a legitimate competitor for the CAR-251 mission, and compare the various parameters and structural efficiencies.

The KR-1 represents a good example of "simplified wooden construction"... figure out what all the stuff weighs and adds up to to achieve its empty weight, speed, G loading capability, etc. with the VW engine and retractable/fixed gears, etc.

If you're feeling froggy, you can also try to weigh and analyze the Chilton Monoplane as an example of the non-simplified "million sticks" method of wooden construction. The Chilton was a very very successful "Cheap Air Racer" from the 1930's that did really well on 32 or 35 HP. This airplane is also a valid data point because it represents a tremendous amount of old-school engineering from the best of the best of the best old-world wooden airplane engineers and craftsmen (DeHavilland Aircraft).

Then take the Moni and the DA-5, and perhaps the Onex, and do the same (Moni engine is 45 pounds plus 40 pounds of lead needed to balance the CG), and you should arrive at an estimate of how well metal does the job for a similar size airplane.

Then take the Arnold AR-5 and do the same as a really good example of a small fast plastic airplane lighter and smaller than Formula 1. The Rutan Quickie 1 is also an excellent data point because it can be scratchbuilt, can be built fairly cheap as plastic goes, and it achieved high speed on 18-20 HP.

Perhaps use the Sonerai 1, or another similar airplane, to represent the steel tube method. Anyone who can point out a very small light steel tube design for 25-30 HP feel free to throw that in for consideration.

The SD-1 and Luciole should also be evaluated the same way, as a separate "carbon reinforced wood" category.

Although this will eat up some time, I am guessing it is valuable data that will inform nerobro and the entire discussion as to which materials might offer the best solution to the weight issue, and show some demonstrated results, performance, etc. that is directly in the ballpark where we're working.

It doesn't matter where a T-18 or Long-EZ or Cassutt or RV-6 or MacCready flying wing would wind up for the purposes of this comparison.

nerobro

Well-Known Member
Log Member
I'll take another stab at it. I think you may be right.

Excuse me, time to make some scrap paper.

Topaz

Super Moderator
Staff member
Log Member
Nerobro, in your latest C.A.R. project post, you state that the plane can sustain a 63 deg turn, and base that on climb performance. I think, though, that in a 63 deg turn you're pulling about 2.2 g? In a climb, as in a glide, as well as in level flight, the lift coefficient is virtually the same (almost no difference, because of the shallow angles involved), but in a 2.2 g turn, the lift coefficient has more than doubled, and induced drag increases with the square of the lift coefficient, so the induced drag increases by a factor of 4.84. I don't think you can relate climb performance to turning performance. You need to analyze the wing characteristics more for that.
+1 on this. Without being critical in the slightest, I'm not really following the full detail of the method you're using here, nerobro. It may be simply that your design process is that much different than mine. At any rate, Autodidact is right that the induced drag increase needs to be taken into account. I guess you could do a full power-available/power-required curve, just like you'd do to figure out top speed or maximum climb rate, but with the aircraft loaded at 2.2g instead of 1.0. That should do it too, I would think.

Autodidact

Well-Known Member
Nerobro, you should download and read through this short NACA report; it gives you a way to estimate all of the relevant characteristics of just about any wing, and it is mostly graphs and charts with a couple of simple equations. There is some heavy math in there, but it is only to show how the graphs and charts were generated and checked, and doesn't need to be worked through:

NASA Technical Reports Server (NTRS)

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