My Flycycle - barely more than a sketch

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Tiger Tim

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Have you ever had a song stuck in your head and trying to ignore it only makes things worse? I had that the other night lying in bed except it was an airplane design on my mind, a sort of simplified semi-scale thing that was inspired by a bunch of threads and brainstorming elsewhere on this forum for the last few years. The outlines and general just-for-fun nature are based loosely on the Piper Skycycle, specifically this fairly imperfect model of it:


So anyways, I was lying in bed picturing different small details and how they might possibly integrate together in some elegant way when I realized I’d be spending the weekend away from home in a hotel in a city essentially on pandemic lockdown. I was going to have plenty of time on my hands so why not fumble through some actual design and maybe even a little amateur engineering if I make it that far. For the trip I’ve printed off AC 103-7’s appendices as well as Beaujon’s book and I’ve bought myself a note book to scribble in.

In my mind this little plane is centred around a single large aluminum tube with a rollover/style bar over the top and the pilot sitting totally exposed. Flying surfaces are all aluminum tube at this point and the wings either strut or wire braced, TBD. I was thinking of a modern paramotor power plant. Anyways, the whole point of the exercise is both to kill time and to better understand the process and what compromises have to be made. I figure anyone can draw pictures and project fantasy numbers based on what they want; I’m interested here in what actually can be done, at least so far as I can get from the limited resources I’m using.

Anyways, without further ado here’s the first sketch of the Flycycle 103:
FD4BAEFF-A269-4458-949C-4872BB67F696.jpeg
 

Tiger Tim

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I’m about half an hour in and may have made a mistake here. Appendix 1 of AC 103-7 is to find maximum level flight speed based on horsepower and a very basic breakdown of airframe drag which I imagine can be worked backwards using speed and drag factor to determine max horsepower. Here’s my build-up of what they call Drag Factor:
A34012AA-870D-4039-9F51-51F001D07DE8.jpeg

The most pessimistic total is 15.1, which I then take to this graph:
EDD53764-8EE3-44C7-B3C1-917F007839E6.jpeg

Am I seriously seeing that the Flycycle needs seventy horsepower just to do sixty three knots in level flight? In my mind this was going to be a pleasant 25hp evening floater. Surely I’ve gone wrong somewhere, any thoughts?
 

Tiger Tim

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A quick re-calc to see if I could make a more sensible HP calculation:
6C3C503B-8734-49D1-A17E-C430138B5EB8.jpeg

I changed seating position to supine after re-reading the definition, which I think I’m comfortable with. I also reduced the estimated size of the tailfeathers since my original figure was based on what you’d want on a beginner’s rubber powered free flight model. I’m now down to a maximum of 56hp to keep it under the speed limit though that still sounds quite high.
 

Tiger Tim

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Very cool. I could swear that I have seen a design very much like that but I can't seem to find it.
Yeah there’s an odd familiarity to it, isn’t there? That’s a good thing though, maybe we’ll end up figuring out what it is and I can see how my numbers stack up.
 

Victor Bravo

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'Wrencher beat me to it, you had the speed off by 15%. Besides, there is no rule that you must achieve the maximum speed allowable. So if you want to go loafing around at 45 knots, or 45 miles an hour, there's nothing wrong with that.
 

Tiger Tim

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Part 103 is 55 knots, or 63 mph, I believe. Perhaps that will put the hp down in a more reasonable range.
Hm, so it is. That looks to put me just under 40hp for max speed at my optimistic drag factor which is a lot more reasonable. Thanks!

there is no rule that you must achieve the maximum speed allowable.
You’re right, of course, but I look at power as ability to climb. Realistically an ultralight probably should cruise at around 40-45mph but it also shouldn’t need full power in cruise, IMO.
 

Tiger Tim

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Update: before I head out for work I went through the chart again for 55kts and my HP limit is now between 38 and 48 (depending on drag factor) which still sounds high to me but is more reasonable either way.

Next up will be to sort out minimum wing area based on max stall speed.
 

Aesquire

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There's a lot of Pt 103 craft that don't go "too fast" on 40 hp. And at least a few that added a strut or cable to slow them down to the regulatory speed limit. Imho, no one cares about that number unless you pass Piper Cubs and annoy someone.

Stall speed, otoh, is a safety issue as well as a regulation.

Sure, you may be comfortable landing at 727 speeds, but the structure/crush space/etc. on an ultralight, isn't.

Landing gear that's a wheelbarrow wheel & tire on a fiberglass rod works fine... Within it's limits.
 

Tiger Tim

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Pressing ahead, I knocked out AC 103-7 Appendices 2 and 3 which basically show whether a certain wing area and construction is theoretically compliant.
C802CF09-63B3-48E9-8993-8DAE1BA671CD.jpeg

What I got is that 130 square feet of wing area will be sufficient to make the 24 knot stall speed with a 170lb pilot at full fuel and assuming the empty weight is the max allowable at 254lb. This requires a single surface wing of at least 7% camber (or flaps but I prefer simplicity).

So far all numbers seem reasonable. Now that I have some idea of maximum power and minimum wing area as a sanity check, I’ll move on to Beaujon’s book and see what I come up with. Seems to me like this process is going to be more of a circle than a straight line where I’ll keep looping back to the start as different areas of design are honed until the gains become small enough to not matter.
 

challenger_II

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When running your hp/speed calculations, do not forget to factor in the prop.
From a historical aspect, the original Goldwing would do 80+. Catto cheated, and went to a finer pitch, greater diameter prop, to get the bird into the speed range.
On low horsepower, wing area is your friend. Always be aware your Child may be used in areas of greater
Density Altitude, and may, just be flown by those of us that are "Low Aspect Ratio" pilots.
"Design for Success, Anticipate Failure", and aim for the middle ground. :)

As a side note: Look at the Ultracruiser. Designed empty weight, power loading, wing loading. This may give you a target region to work with.
In designing the Mini Max, Wayne Ison had an excellent platform, but, as originally designed, was too fast in stall, and top speed. He blunted the leading edge of the wing, and cured both problems with one stroke.

Again, just my 2 cents worth. And, I like your concept! :)
 

cluttonfred

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It's a little bit like an aluminum tube version of the Czech ZJ-Viera, but I could swear I saw something very much like that exact concept back in the 90s or so complete with the sort of roll tube over the pilot.

1624145170149.png
 

Tiger Tim

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It's a little bit like an aluminum tube version of the Czech ZJ-Viera
Even more like the Viera’s proof of concept, only they were a lot smarter than me.
 

Tiger Tim

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Learn from my fail tonight:

I thought I was being clever by keeping the leading and trailing edge spars short, around ten feet or so but it led to an awfully low aspect ratio wing in order to get the area I needed. The result was waaaaay too much chord so back to the drawing board.
C4C651A8-74C1-4180-88ED-0B5864EC8539.jpeg

Lesson learned and tomorrow I’ll come at it from a different angle, starting with the fuselage or specifically the pilot, fuel tank, engine, and landing gear to determine a wing root chord. We’ll see how that goes.
 
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