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Thread: Small fast 'fun plane'

  1. #1
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    Small fast 'fun plane'

    Hi everyone,

    I've been reading here as a guest for quite a while and seeing all the wisdom here I thought I'd ask a few questions.

    I'm sure most of you will be aware of the little french contraption the cri-cri. Tiny little thing

    Wing area of 3.1m2
    Weight 75kg
    Crusing speed 185kmh

    All the pertinent information is here.Cri-Cri Homebuilt | Technical description of the Cri-Cri aircraft

    What I'm wanting to know is, if the seating position was improved so the pilot was more reclined, and the canopy much more aero-dynamic. The twin engines replaced by a simple single in the front with a similar amount of power 30hp-ish.

    Would the top speed be significantly improved?

    After these steps, which seem to me relatively common sense. What further could be done to improve the cruise and top speeds without adding more power, sweeping wings, changing profiles etc?

    Perhaps i should have mentioned earlier. I'm wanting to build something along the same lines as this. in terms of size and weight. It will be scratch built.

    Also as a side note for those with the knowledge, the whole thing dry ends up at only 75kg in Aluminium, it there any chance it could be significantly lighter in composite? Would it even be worth the additional work?

    Sorry it turned into so many questions.

    Thanks,

    Reuben.

  2. #2
    Registered User wsimpso1's Avatar
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    Re: Small fast 'fun plane'

    Philosophically, you need to make a transition: You are talking about designing a new airplane. Yes, it might be based on the Cri-Cri, but at some level, most airplanes are based off of other airplanes... The reason it is a new design is that you will have to check out everything.

    Next, only if you are willing to vacuum bag with pre-pregs and autoclave cure will you make much weight progress in composites over aluminum. The basic thing in little airplanes is fiberglass lets you build aerodynamic and aluminum lets you build light. Pick one.

    Reducing frontal area may help out a little, but your drag comes from a bunch of places: frontal area, total wetted area, induced drag, interference and excreseance drag, and cooling. The mods you have discussed will only make modest reduction in frontal area, may increase prop wash velocities over the fuselage, and will be neutral on everything else.

    If I were looking to clean up that bird, it would be to cowl the engine(s) and configure fuselage/wing joints and vertical/horizontal tail joints for minimum inteference drag.

    Billski

  3. #3
    Registered User BBerson's Avatar
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    Re: Small fast 'fun plane'

    In his book, Alex Strojnik says the CriCri gets some drag reduction from less propwash on the fuselage.
    I think the engine drag is worse.
    BB

  4. #4
    Registered User bmcj's Avatar
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    Re: Small fast 'fun plane'

    Most aerodynamics textbooks will tell you that the best fineness ratio (length divided by width) for a free body ranges between 2.0 and 3.0 (depending on which book you reference). As such, a fuselage does not have to be as "thin" as you might think. Of course, interference of the surfaces and propwash add their own effect, but once you go below the optimum fineness ratio for the fuselage, you start adding skin drag faster than you reduce frontal area drag.

    Also, as Billski said, even with a small change to increase speed, you are talking about a different design. You need to revisit the structure to see if it will hold up against the increased loadings during higher speed maneuvering. In essence, you may increase the top speed it is capable of, but the max maneuvering speed will remain roughly unchanged. Flutter also becomes a big issue. The top speed of the original design may have been dictated by flutter limits; push those limits and you may find yourself minus a few flying surfaces (like a wing).

    Lastly, do a search on this forum for "Cri-Cri", "Cri Cri", or Columban. The Cri Cri has been discussed here before and one of the members is building one (with lots of good photos in his posts).

    Bruce

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    Registered User mikemill757's Avatar
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    Re: Small fast 'fun plane'

    You might want to look at the SD-1 @ SPACEK s.r.o.. It's a small, single seat wood/composite low-wing with a small (25-40 hp) engine. At least one is flying with a B&S v-twin.
    Mike
    You can only tie the record for flying low.

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    Re: Small fast 'fun plane'

    Thanks for the fast replies.

    You all make excellent points, indeed I would be starting a new design. But I particularly like the spirit of the cri-cri for it's simplicity.

    The point bmcj made about flutter is something I've thought about but really have no idea how to avoid. From what I've read it seems to be more of an art than science.

    Other than ease of construction what is the rational for a straight wing? With the desire for elliptical lift distribution would a tapered wing provide any benefits.

    Imagining that the flutter issue could be delayed (speed wise) it increasing top speed simply a matter of adding more power?

    I'm going to work up a simple 3d model to show what I'm thinking of doing. Perhaps toward the weekend, when I've got some more time.

    Again thanks for the attention.

    Reuben.

  7. #7
    Registered User Dan Thomas's Avatar
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    Re: Small fast 'fun plane'

    Quote Originally Posted by Microflight View Post
    Hi everyone,

    I've been reading here as a guest for quite a while and seeing all the wisdom here I thought I'd ask a few questions.

    I'm sure most of you will be aware of the little french contraption the cri-cri. Tiny little thing

    Wing area of 3.1m2
    Weight 75kg
    Crusing speed 185kmh

    All the pertinent information is here.Cri-Cri Homebuilt | Technical description of the Cri-Cri aircraft

    What I'm wanting to know is, if the seating position was improved so the pilot was more reclined, and the canopy much more aero-dynamic. The twin engines replaced by a simple single in the front with a similar amount of power 30hp-ish.

    Would the top speed be significantly improved?

    After these steps, which seem to me relatively common sense. What further could be done to improve the cruise and top speeds without adding more power, sweeping wings, changing profiles etc?

    Perhaps i should have mentioned earlier. I'm wanting to build something along the same lines as this. in terms of size and weight. It will be scratch built.

    Also as a side note for those with the knowledge, the whole thing dry ends up at only 75kg in Aluminium, it there any chance it could be significantly lighter in composite? Would it even be worth the additional work?

    Sorry it turned into so many questions.

    Thanks,

    Reuben.
    Better to look at little aluminum airplanes that are already single-engined and better streamlined. Google the BK-1, the Hummel Bird (sometimes spelled Hummelbird), the Midget Mustang and the Teenie Two (not really a good choice).

    The Midget Mustang goes like a scared cat and takes engines from 85 to 150 hp, IIRC. Better be a good pilot for that one. The Hummelbird is a 1/2-VW powered airplane that cruises at around 100 mph on 30 hp. Some of them use Rotax two-strokes and at least one has a McCulloch drone engine or about 70 hp. The BK-1 is a redesigned Hummelbird and uses the whole VW engine. Bruce King is the guy to see. The Teenie Two is an old adaptation of the Jeanie's Teenie, a VW-powered thing that had some serious shortcomings.

    There's also the Sonerai. And Leeon Davis designed a couple of tiny metal airplanes, the DA-5 and the DA-11. The -11 was powered by an 18-hp Briggs & Stratton and cruised at 125 mph. It had an empty weight of something like 175 lbs (with a full electrical system yet!), something that would be hard to beat with any other form of construction. He never sold plans for it but I think a grandson is in the process of getting them published. He'll sell a bunch, I'll bet. See
    Leeon Davis' DA-11

    Dan

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    Re: Small fast 'fun plane'

    Thanks Dan,

    some great suggestions there. I had a quick look on Google. Nothing quite as small the the columban but still good looking birds.

  9. #9
    Moderator Topaz's Avatar
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    Re: Small fast 'fun plane'

    Quote Originally Posted by Microflight View Post
    ...The point bmcj made about flutter is something I've thought about but really have no idea how to avoid. From what I've read it seems to be more of an art than science.
    Run a search on "Flutter" on the forum here and you'll get a lot more information. It's still largely empirical, but there are established methods to reduce the possibility.

    Quote Originally Posted by Microflight View Post
    Other than ease of construction what is the rational for a straight wing?
    That's usually it, but a straight wing also generally has good post-stall characteristics.

    Quote Originally Posted by Microflight View Post
    With the desire for elliptical lift distribution would a tapered wing provide any benefits.
    Mostly in climb. You'd need to have a very high wing loading to see a significant benefit in cruise, much more than you're remotely likely to have in an aircraft such as you posit here. Taper will somewhat reduce the weight of the wing and increase the roll rate. Only you can decide if those are worth the extra time/cost of construction. With Rutan-style foam/glass wings, the extra time/cost is relatively minimal, however.

    Quote Originally Posted by Microflight View Post
    Imagining that the flutter issue could be delayed (speed wise) it increasing top speed simply a matter of adding more power?
    A little, but you'll do far better on the top end through drag reduction than by adding power. As for drag reduction, look at sailplanes. There's more drag-reducing tricks going on there than you'll see with any sportplane going. A modern competition sailplane has less overall parasite drag than the nosewheel on a Cessna, even if that latter has the 'low drag' wheel pant on it. Heck, an ASW-24 or something probably has less drag than a wing strut on the Cessna. It's that low.

    Adding power is most effective at increasing climb rate, while drag reduction is most effective at increasing speed. However, realize that "top speed" is virtually useless except for racers and record-breakers. For an every-day sportplane, the cruise speed at ~75% power is the number that matters, second only in importance to stall speed. Also, don't sacrifice everything for speed. You'd be amazed how much difference a few knots don't make in the types of relatively short-range trips you'd make in an airplane such as this.

    If you're young and just want the "go fast" feeling, build a HummelBird and go buy a sportbike. You'll feel "faster" on the ground in a machine like that than you ever would in the air. And the Hummel is a neat little airplane; right up the alley of what you're describing. I've been seriously tempted to build one myself. http://www.flyhummel.com/hbphotosdickson.htm
    "If you have built castles in the air, your work need not be lost; that is where they should be. Now put the foundations under them." - Henry James Thoreau
    Member of the Lake Elsinore Soaring Club.

  10. #10
    Registered User PTAirco's Avatar
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    Re: Small fast 'fun plane'

    Quote Originally Posted by Microflight View Post
    Thanks for the fast replies.

    But I particularly like the spirit of the cri-cri for it's simplicity.

    .

    The Cri-Cri is really not a simple airplane to build. It may be small, but it has a high parts count and complex parts at that. Bonding, welding and machining involved. Take a look at one of the threads showing pictures of a Cri-Cri being built.

    There is no shortage of simple single seat airplanes, especially for VW power. The Hummel Birdfor example, seems a well proven design that is about as simple as you can make a sheet metal airplane and seems to suit your purpose.
    "Aeronautical engineering is highly educated guessing, worked out to five decimal places. Fred Lindsley, Airspeed."

  11. #11
    Registered User bmcj's Avatar
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    Re: Small fast 'fun plane'

    Quote Originally Posted by Microflight View Post
    Other than ease of construction what is the rational for a straight wing? With the desire for elliptical lift distribution would a tapered wing provide any benefits.
    The straight "Hershey bar" wings get surprisingly close to an elliptical lift distribution without any special treatment. If you want to modify them more, then they can be further tuned with washout or aerodynamic tayloring. Search the forum and you will find other trhreads where this was discussed.

    Regarding speed increasing with horsepower, Topaz is correct. A bigger engine will increase your climb rate (unless the weight of the engine cancels the gain). Since drag increases with the velocity squared, a gain in top speed is best accomplished with drag reduction, not engine enlargement.

    Bruce

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    Registered User vortilon's Avatar
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    Re: Small fast 'fun plane'

    I think the Cri Cri is amazing but to small for my body weight. I tried to purchase plans from the designer once and he said he would not sell anything here in the USA because he was sued. Has he had a change of heart?
    We the unwilling led by the unqualified have been doing the impossible now for so long with so little we now feel it's possible to do anything with nothing.

    http://www.azairframe.com/index.html

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    Re: Small fast 'fun plane'

    Have a careful look at Leon Davis's DA11 in comparison to the Cri Cri - its does almost the same in terms of cruise speed at almost half the horsepower . The design could be tweaked to increase the cockpit size and maybe more power . Simple flaps could reduce the stall speed . The fuselage design is well thought out and simple to build . Ask anyone who has built an aeroplane they will tell you its last 0.2 mm ( 10 thou ) of surface that takes all of the time . So building with composites means lots of finishing . Alloy surfaces reduce that finishing time and machine milling ( see Cri Cri build ) helps a lot . The big thing Colomban got right with the Cri Cri is the feel . I have spoken to a 747 pilot who built and flys a Cri Cri and many other pilots say it just one of the sweetest planes to fly - i guess thats part of the design challenge
    Flyoz

  14. #14
    Registered User bmcj's Avatar
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    Re: Small fast 'fun plane'

    Quote Originally Posted by flyoz View Post
    Have a careful look at Leon Davis's DA11 in comparison to the Cri Cri - its does almost the same in terms of cruise speed at almost half the horsepower . The design could be tweaked to increase the cockpit size and maybe more power . Simple flaps could reduce the stall speed . The fuselage design is well thought out and simple to build.
    The problem with the DA11 is I don't believe there are any plans available.

    Bruce

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    Re: Small fast 'fun plane'

    If you're young and just want the "go fast" feeling, build a HummelBird and go buy a sportbike.
    Doing it, feels great! But it aint flying.

    I had originally thought that I would build in composite. I'm a professional 3d designer so I thought with my ability to model the who craft in 3d I would then have a foam plug cnc milled and glass that to take a mold from etc etc.

    Which began to seem like far to much work for a one off. I have worked a little with sheet metal but must admit the 'sexy-ness' of composite curves is something I'm still impressed by. Replicating that sort of thing in sheet metal requires more skills than I'll ever have.

    Ultimatly I've got a few ideas to try but I figured I should build something 'simple' to start with. The hard part is finding something simple that still warrants pouring major time into and is going to perform addequetly in the end.

    I've spent a good deal of today looking through all the suggestions you have all mentioned.

    Earlier in the thread wsimpso1 said


    If I were looking to clean up that bird, it would be to cowl the engine(s) and configure fuselage/wing joints and vertical/horizontal tail joints for minimum inteference drag.
    Why do we not see any metal planes with composite 'fillets' to blend them together? Would that not improve the inteference drag etc?

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