# Razorback Freewing progress

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#### rtfm

##### Well-Known Member
Hi,
I have decided to create a dedicated thread to chronicle my design/build progress. First off, I have decided to revert to an earlier name for my project: The Razorback. And since it incorporates a freewing design, I've named this thread accordingly.

Some history:
I got my PPL three years ago, and decided to build my own plane, rather than pay out good money to rent one every time I wished to go flying.

At first, I thought I'd build a kit. I got the plans for a Sonex, and started that. Only to discover that I didn't much care for working with Aluminum. So I stored the plans, and got a set of KR2 plans. Wood might be more my style. I decided after a few months that I didn't much care for the design, so those plans also went into storage.

Now I was in a quandary: My knowledge of existing kit planes was fairly minimal (even though I spent a lot of time on the Web looking for them). I decided to have a go and design one of my own - after all, how difficult could it be? I was urged into this mindset in part (to be honest) by the fact that I was broke, and if I was busy at the design-board, I wasn't spending money, but I was working on my plane. Having more time than cash, I set to it. The other half of my motivation was that I had discovered gyros in a major way, and had to have one. Have you checked out gyros lately? They're nearly all of them pig-ugly flying things. I was sure I could do better...

The first attempts were so embarrassing I try to put them out of memory. At the time I was extraordinarily pleased with them. So I have drawings of them safely locked away in the workshop, but they are not for the faint-hearted...

Sketch 1:
The first sketch is my very first freehand back-of-an-envelope design, done over a few beers one rainy afternoon in the pub. These are the scrawlings which got me started.

Sketch 2:
The second followed soon after, when I discovered tractor gyros via Tervamaki's web site. I had also discovered SketchUp.

Sketch 3:
This is Don Shoebridge's rendering of my 2nd sketch into 3-D

By this time, I figured I was ready to start building. Note - I didn't really have a clue where I was going. I figured if it looked OK, it would fly OK.

Build attempt #1 was the tractor configuration above, glass over wooden superstructure (1st picture, 2nd row). The next picture along is me in the workshop shaping foam with the curve of a cross-cut saw - quite effective, really. This build attempt
lasted long enough for me to spot that the frame I was building it on had warped, and I could not unwarp it.

Build attempt #2 Was out of aluminum tube. A pusher design with an overhad tail. All was proceeding great (I even overcame some of my reluctance to working with aluminum) - until I discovered that I could not sufficiently triangulate the fuse without having aluminum tubes pass directly through the engine!

Duncan

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#### rtfm

##### Well-Known Member
More Razorback Freewing progress

Build attempt #3: Also a tractor, also an overhead tail boom. Actually, the design progressed through a number of phases, each one more brilliant and stunning than the one before. Looking back, only a father could have loved them, but it was fun at the time. I thought I was so clever.

The photos show various attempts to nail this configuration before committing to a prototype. I ended up trying to fabricate the last of these... However, I really battled to get both sides symmetrical. I chopped it up and consigned it to the dumpster.

The final graphic in this set is my good mate Bob Kelly's 3-D rendition of a 2-place version of the design. I thought it looked terribly cool. I still do. It's a beauty. Pity I couldn't get the construction technique right.

Design #4: I tried this design again, but I was making too many design changes, and the wooden prototype couldn't keep up with the changes. Bugger. Chop chop chop.

Duncan

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#### rtfm

##### Well-Known Member
More Razorback Freewing progress

Build attemp #5: I managed to buy a really nice engine really cheap. It is a Suzuki K10A (1000cc, 103hp, turbo, FI - about 190lbs all up. No more design errors because I didn't know how large the engine was going to be...

Then a number of things happened quite quickly after each other.

First, I discovered a CUTE little Russian tractor gyro so simple in design it was almost comical. It was based on a composite, one-piece "blade" or strongback, and it looked so simple and so elegant. I could see real potential in this approach. So I set about using the little Russian design as a base, and came up with the first of the current crop of designs

Second, somewhere along the way, I discovered Carter Copter and their Slowed Rotor/Composite design. Rotors AND fixed wings. I read all I could on the subject, and even corresponded with the guys at CC. I was convinced it could be done, and so set about designing a gyro with wings.

Finally, I stumbled upon the Flying Flea on the web. The Flying Flea is an old design which has wings which are free to rotate in pitch. They are attached to the airframe on bearings. I was fascinated by the concept, and rapidly consumed (I think) every last article about this aircraft design feature.

One of the major benefits I could see in the freewing was in overcoming a problem which was plaguing Carter. With wings fixed in pitch, they were having to cope with excessive drag at non-optimal flight regimes, since gyros fly at strange angles because of the rotor. I had been reading about the Flying Flea with its freewing design, and decided that this was an elegant and simple solution to the problem. Coupled with the freewing's inability to stall or enter a spin, I was hooked on the idea of mounting a freewing on my gyro. One of the reasons I found gyros so attractive was the fact that in the event of an engine out, you could land in someone's back yard if need be. Freewings won't give you that level of comfort, but they will remove the danger of spins and stalls. Not a bad compromise. And if you have both rotor AND freewing...

The photos:
Photo 1: The little Russin gyro landing. This was the first picture I saw of it. It is still on my wall in the workshop - for inspiration.
Photos 2 & 3: Next year's model. Slightly updated and looking very cool on the ground.
Sketch 1: My attempt at a design based on a Blade.
Sketch 2: What it evolved into
Sketch 3: I decide to add a freewing...

Duncan

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#### rtfm

##### Well-Known Member
More Razorback Freewing progress

Build attempt #6: Not to be put off by a few setbacks, and bouyed up by the sense I was getting that I was actually beginning to understand more about what makes an aircraft tick, I set to it again. A solid strongback, carefull stringing of longerons to get the hull shape, careful measuring and shaping of strategic bulkheads. I sat in the frame, and checked it for size. I measured the engine compartment. The engine would fit. I shared my free time between the workshop and the computer, making changes to the sketch, and then implementing them on the wooden prototype.

But again, I ran into a roadblock. My new strongback (built out of three layers of 1/2" chipboard) - while rigid enough, was unable to accommodate the subtle changes I was making on the drawing board. I would have to start again. In my eagerness to get building, I had jumped the gun. I was growing weary of this - besides, it was now almost three years since I had started, and I still had nothing to show for it except a pile of trashed build attempts.

Then I discovered this forum. THANK GOODNESS! The guys here are amazingly helpful, and soon suggested I invest in Raymer's books. I did so. And to my surprise and joy, I discovered that much of what had taken me three years to design was now actually pretty much on the ball. I had finally started getting it right.

Design #7: With Raymer's aid (and some sage advice from Topaz, Orion, Billski and others) I am continuing with the design evolution. I have a new tail, new canopy design, new way to mount the freewing. I've started getting the CG right, and am thinking about how to do the actual fuse construction. I have an empty workshop at the moment, because I'm only going to do this one more time. I can't afford to screw up any more.

Here are some of the design changes I have made recently...

Photo 1:
Design #6. Mounted on wheels so I can get it out of the workshop. The strongback is clearly visible, as are the wooden longerons.

Sketch 1:
This shows the freewing no longer attached to an overhead extension of the strongback. Now it is fixed on each side of the fuse by two struts, as in the Flying Flea designs.

Sketch 2:
Most strikingly, I've changed the tail. It is aesthetically pleasing, I think, and falls right in the middle of the optimum size estimation as calculated by Raymer

The second major change is in the way I'm attaching the freewing. I figure that if the carbon fibre spars can withstand (if Marske is to be believed) 19G's with the load at 90 degrees, then similarly constructed wing struts can stand at least that. No need to any triangulation. They are attached directly to the main bulkhead (as are the main gear legs - note the move from tricycle to tail dragger - done with some reluctance - but it is just so much easier to construct. Besides, I think it looks cool. Dunno about actually managing the aircraft on the ground though...

Whew - well, this is where I am now, and that's how I got here. The more I look at the last sketch, though, the more I think the tail is too small. Might enlarge it a bit... (And so it continues).

Currently investigating materials for the next build attempt. This time, I'm going to hang off till I have everything at hand - strongback (two 1/2" layers of chip board over a foam centre), the longerons (using 1/4" cheap aluminum pipe from the local merchant. Good uniformity of bend, light weight and reasonable strength, 1/4" foam and enough composite stuff to lay up both sides.

This will allow me to build the two sides of the fuse and find out exactly where the strongback needs reinforcement, adjustment etc. Then Ill dismantle it, use the prototype strongback as a template, and build a proper carbon fibre/foam/fibre version before mounting the waiting fuselage.

Well, that's the plan, anyway. More photos to follow as the work progresses.

Duncan

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#### rtfm

##### Well-Known Member
Hi,
I have been working hard to reconfigure the front of the Razorback in an attempt to fit in the Victor 2 Plus engine. This is an Italian aircraft engine (2-stroke) which seems like it might just be what I need. It has plenty of power, is light, and isn't too heavy on gas. I'm still a little worried about it being a 2-stroke, but hey - I've heard very good things about the engine reliability.

This is the engine info cut and pasted directly from the manufacturer's web site.
Power: 102Kw (102Hp) @ 6200 RPM
Weight of the engine complete with:
exhausts, air filters, radiator, propeller and liquid = 52kg (115lbs)
Twin Bing 36 carburettors
Aluminum cylinders with Nikasil ceramic coating
Exhausts
Type "C" reduction
Double Ducati electronic ignition with alternator for recharge battery in fly
Electric starter
Flying consumption at 5200 RPM (75hp), 8 liters/hour

However, trying to fit this engine under the cowling has proved a challenge. The main problem was that the redrive thrust line is MUCH lower than I had been assuming. I had to drop the prop almost 7". Which meant that I had to redesign the whole nose section. The only way I could see to make it look half decent was to "invert" the "normal" configuration of having the cooling intake below the spinner. I've put it above the spinner - and while it looks a bit odd at first, the look grows on you, I think.

I also shortened the nose section (the engine, shown drawn to scale, is very small) and it also looks OK. The aircraft is now 15'7" (down from 16'2").

The wings will be attached by two cantilevered struts connected directly to the strongback at their base, and bonded to one of the bulkheads. This has meant angling the bulkhead (and the strongback) back to the correct angle so that the wing pivot point is correctly positioned relative to the new CG.

One thing I haven't mentioned so far, is that the strongback is not only a "hidden" structural backbone - it extends rearwards, and actually forms the tail of the aircraft. In other words, the tail is part of the strongback. The strongback itself is constructed out of three layers of 1/2" foam core, each covered with a layer of carbon fibre and reinforced with carbon rods. Then all three layers are bonded together to form a three-ply laminate. This arrangement is actually way overkill strength-wise, but still weighs only 11kg.

Regards,
Duncan

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#### rtfm

##### Well-Known Member
Engines

I have been researching available engines for the Razorback. Following comments on this forum about the inadvisability of trying to produce an aircraft installation from an auto engine, I have bitten the bullet, and decided that I'll have to spend the money and get a "proper" aircraft engine. While the sort of money I'm considering is too fanciful for my wallet in the forseeable future, time has a way of making this work out. So I'm planning in faith.

Here is a list of the engines I have researched. This list is currently incomplete, so I'll return from time to time and add engines as I find them. Hopefully this will become a useful resource for others who are also looking for an engine.

At the moment, I am keen on the Simonini (great power, very light, good consumption - but it is a 2-stroke. Don't want to be attempting my trans-Tasman crossing behind a 2-stroke...) and on the Rotamax (I LIKE rotary engines - very simple, extremely reliable, small and light. But no flying examples...)

Engine: MZ301
Power: 85hp
Weight: 110lbs
Consumption:
Cost: $9130 USD Extras required: None Drawings: Supplied Comments: 3-pot 2-stroke. Forced air cooling, comes with C-type gearbox fitted, Generally positive web feedback Engine: Simonini Victor 2 plus Power: 120hp Weight: 115lbs Consumption: Cost: 5,800 Euro Extras required: None Drawings: Supplied Comments: 2-pot 2-stroke, comes with C-type gearbox fitted, consumption figures extremely good - check web site for figures, although some schepticism expressed on the Web ragarding this. Engine: GPAS VW conversion 2180cc engine Power: 75hp Weight: 148lbs Consumption: Cost:$4514 USD
Drawings: Not supplied. How to know if it will fit under cowl?
Extras required: Various add-on bits (starter, alternator, carb, exhaust... ) Weight with these extras? Cost with all required extras: approx $6300 USD Comments: A number of variants (Flywheel drive, front drive, reduction drive model) and displacements. Arrives as a self-assemble kit (fun). Good reports on Web. NO REDRIVE required, since it is direct drive. Air cooled, so no radiator. Engine: Aerovee (VW conversion by Sonex) Power: 75hp Weight: 161lbs Consumption: Cost:$6495 USD. Complete package.
Extras required: None.
Drawings: Not supplied. How to know if it will fit under cowl?
Comments: NO REDRIVE required, since it is direct drive. Air cooled, so no radiator. Good comments on Web.

Engine: Rotamax Single Rotor
Power: 100hp takeoff, 90hp continuous (Turbo version 120hp)
Weight: 100lbs (Autoflight gearbox extra 27lbs)
Consumption: 2.8g/hr (cruise) to 5.6g/hr (takeoff)
Cost:
Drawings: Supplied. Includes many angles (side, front, top, isometric)
Extras required: None? Have written to enquire
Comments: Possibly an excellent choice. Power is good, only two moving parts in engine has always been attractive, light, consumption good at cruise settings, integral gearbox good.

Engine: BMW conversion
Power: 70hp
Weight: (with centrifugal clutch and Rotax C type gearbox) = 76kg
Consumption: very airframe dependent, but the SFC is around 310g/kWh
Cost: $6000 USD Drawings: None Extras required: Would need to supply all the extras yourself Comments: A sexy looking engine (if exposed to admiration). 76kg is heavy for 70hp (cf twin rotor Rotamax - 76kg but 180hp) Engine: HKS 700e (680cc twin 4-stroke) Power: 60hp @ 6200 (max rpm) Weight: 126lbs ready to fly Consumption: 9 litre/hr (2 gal/hr) @ 4750rpm - cruise Cost:$7295 USD
Drawings: Yes.
Extras required: None. Complete package
Comments: Many flying examples. No bad reports, except possibly that it is a little under-powered

Engine: Hexadyne P60 (800cc twin 4-stroke)
Power: 60hp
Consumption: 3g/hr
Cost: $8800 USD Drawings: Yes Extras required: None. Complete package Comments: On paper, and excellent engine. Fuel injection, electronic ignition, good fuel economy, light. In reality, no flying examples, lots of disastrous reports on the Web. Do not touch! Engine: AeroTwin (972cc twin 4-stroke) Power: 65hp @ 4200rpm Weight: 100lbs (redrive?) Consumption: HIGH... Cost:$6900 bare, plus Oil tank, PSRU = $7800 USD Drawings: None provided Extras required: Clearly stated: oil tank, PSRU Comments: Sounds a real contender. Built/designed bu a Kiwi (yes!), modern and innovative. Some negative comments re: fule consumption, overheating of rear pot. Engine: B2 (Suzuki G-13 conversion) Power: 110hp Weight: 219lbs ready to fly Consumption: 3.5 - 4g/hr Cost: ??? Drawings: None provided Extras required: Not sure Comments: As of 2007, it was still under development. Fuel injection. Engine: Masschi (Italian) Power: Weight: Consumption: Cost: 25k Euro (?) You've got to be kidding... Drawings: Extras required: Comments: Lovely looking engine. Price is silly. Flying examples? Engine: Raven (Suzuki conversions) Various engines available. Range from 1000ULS (58hp, 112lbs,$5500)
1000 UL-T (62hp, Fuel injection, 125lbs, cost???)
1300 (90hp, 168lbs, 3 g/hr, $11,000 USD) Drawings: Some. Extras required: Seem to be complete, bundled with their redrive Comments: Seem to be nice engines. May be an option. Engine: Smart M160 Power: 100hp Weight: 143lbs complete ready-to-fly Consumption: 1.9g/hr (cruise @ 3900rpm, 3.3g/hr @ max speed (5,500rpm) Cost: 11,354 Euro (ouch!) Drawings: None available Extras required: Seems like there may be quite a few required Comments: LOVELY little engine. Quiet, small, strong and should be untra reliable. EFI, turbo, catalytic converter. Some flying examples. Engine: Power: Weight: Consumption: Cost: Drawings: Extras required: Comments: Engine: Power: Weight: Consumption: Cost: Drawings: Extras required: Comments: Last edited: #### Topaz ##### Super Moderator Staff member Log Member Well, Duncan, I don't have any kind of answer on engines for you, but it's great seeing your design develop and grow! Inspiring to the rest of us trying to do the same thing. That's turning into one neat little airplane! #### rtfm ##### Well-Known Member Topaz, Hi. Thanks for the kind words re: the little Razorback. As far as this search for engines goes - it's certainly a lot of fun. But I'm rapidly discovering that the world of aircraft engines is just like the world of software - more vaporware than anything else... I'm leaning towards the Rotamax simply because I have always liked rotary engines, and because it comes as a complete ready-to-put-gas-in-and-fly package. Neil Hintz (who makes the Autoflight gearboxes as featured on the Rotamax) lives just down the road from me, so I may get the engine without the gearbox and source one locally from Neil himself. He can fit the gearbox - and mount it all in the airframe also - he's great at that sort of thing. Maybe. My major quandry at the moment is "single" or "two-seater"? I don't want to have to build a second one, but I also don't want to bite off more than I can chew by building a two-seater. So I'll ponder this for a while and see how I feel a week or two down the track. In the meantime it is checking and doublechecking the design. If I use a ready-made aircraft engine, I can calculate the CG pretty accurately, which is a help. Cheers, Duncan #### Jeremy ##### Well-Known Member I can fill in some blanks on the BMW R100 engine, as I've converted one. Weight, complete with centrifugal clutch and Rotax C type gearbox is around 76kg. Power is around 70hp. With a 2.62:1 gearbox it swings a 70" 2 blade Powerfin very nicely. Fuel consumption is very airframe dependent, but the SFC is around 310g/kWh. Cost was around £3000 (about$6000 US) complete, including a new gearbox, reverse profile cam (allows the heads to be turned around so the exhaust port point forwards on a tractor installtion) adapter kit, dual ignition conversion, carbon fibre exhaust cans and completely rebuilt, zero-houred second-hand bike engine.

You could build one of these engines for a lot less, perhaps £2000 (about \$4000 US) if the engine was just refurbished without the fancy stuff.

Jeremy

#### rtfm

##### Well-Known Member
Jeremy,
Excellent. I'll update my list with your details.

Cheers,
Duncan

#### rtfm

##### Well-Known Member
Razorback Freewing 2-seater thoughts

Hi,
I've been pondering the relative merits of both the single-seat and 2-seat versions of the Razorback. While I certainly like the simplicity and cost effectiveness of building a single-seater, I can't escape the fact that once it is finished I'm "doomed" to fly alone. I can never take my wife on a joyride, or fly with a mate. This is a weighty consideration.

However, designing a 2-seater is SO much more difficult. So I've been throwing a few ideas into the air and watching what happens (figuratively, of course). Here's one design idea which has my interest piqued, but one which I'd appreciate some input on.

Essentially the idea is to retain the profile dimensions of the Razorback single, but instead of having a single strongback running down the spine, slice it down the centre and move the two halves apart. What we end up with is two strongbacks (one under each passenger), each one terminating in a tail (ie a twin-tailed design). Aft of the cockpit, the fuse would be a "ski slope". Also, each tail could be slightly smaller than a single tail need be.

This would provide me with:
• The same "sexy" profile
• Two strongbacks to support both the wing and the landing gear
• A very strong foundation for the horisontal stab
• A fineness ratio of about 4.8
• Ability to go flying with a mate
The Questair Venture is also a very "stubby" design, but it was one of the quickest homebuilts ever designed - largely due to its very small wetted area. I would expect the Razorback to perform similarly well.

One weightly (if you'll excuse the pun) consideration in building a 2-seater is the choice of engine. I can possibly still use the Rotamax single-rotor powerplant (120hp, 127lbs) - but the performance will be nowhere near as stellar as the same powerplant in the single. I could opt for the dual-rotor version of the Rotamax, however, pushing out 180hp, 180lbs). But I guess 120hp is plenty for such a small aircraft...
• The first sketch is of the Questair Venture (same length as the Razorback).
• Second sketch is a screenshot from an early 3-D rendering of this idea by my mate Bob Kelly - it clearly illustrates the twin-tail design
• Third sketch is a plan view of the Razorback dual-core.

Regards,
Duncan

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#### Dana

##### Super Moderator
Staff member
That looks kinda cool. With the two strongbacks you can truss them together and make a very strong fuselage, so the strongbacks themselves don't have to be as heavy... but at some point you end up with a more conventional monocoque fuselage structure.

If the fins/rudders are too close together you'll lose some efficiency due to interference... hmmm... what you've got now might lend itself to a V-tal with ruddervators, with extensions of the tail spars being part of the fuselage diagonal bracing, just a thought.

-Dana

If at first you don't succeed, skydiving is not for you.

#### rtfm

##### Well-Known Member
Dana,
Hi. Interesting thought. However, (strictly speaking) a single tail would be sufficient for this design. Having two will be overkill, so losing some efficiency won't be a major. On the other hand, having the airflow tunnel down the ski slope between the tails might produce some interesting effects... Wonder if this will be an issue?

Regards,
Duncan

#### Topaz

##### Super Moderator
Staff member
Log Member
I think you'll see a lot of drag if you use two tails, particularly at high angles of attack. Any separation from the flow coming diagonally up and across the aft fuselage will impinge upon the tails and flow around to the space between them. That flow direction might even 'stall' the vertical tails, which would not only create a lot of drag, but also reduce your rudder effectiveness.

I would think there will be interference drag, too, as the tails are quite close together.

If you can get away with just one tail, I think it would be better aerodynamically. Structurally more messy if you're using twin 'strongbacks', but probably worth it in terms of performance, especially during climb.

I'm also pondering the niceties of having one, stronger, 'strongback' between the pilots, rather than two separate ones. You could have a single tail more easily, and it makes a nice place for control runs and mounting a center stick between the pilot and passenger (so they can share the flying duties). Since you were already comfortable with having twin struts to hold your wing, you've lost nothing if you continue with that plan.

Your aircraft will become quite a bit heavier when configured for two people versus one, and so the wing and tail areas will also get larger, further growing the weight. Still, for the fixed-wing version, 120hp seems like it ought to be sufficient. Don't know how that would work on the gyro version, though. My gut says you'd need more power for that to overcome the drag of the rotor.

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#### orion

##### Well-Known Member
One quick note - two tails that close together may give a bit more tail "power" but not stability. You'll need to use only the net projected area so don't think that having two will allow you to make them smaller.

#### rtfm

##### Well-Known Member
Hi,
Good point. Thank you. The way I've calculated it, a single tail volume is sufficient. Having two is a bonus, and really appear at the rear of the plane as an "accident" since they are actually part of the strongbacks.

What do you think of the basic concept? Do you think it worth pursuing?

Duncan

#### orion

##### Well-Known Member
I think your evolution is coming along well and morphing into an attractive product. Personally though, I'd probably dump the free-wing concept and go with a more conventional layout that can take advantage of a fixed wing's capabilities, especially in the use of flaps. I do like the idea of two products, one with a wing and the other with a rotor.

On the latter, I like one of your earlier ideas where you had part of the fuse structure extending forward for mounting the rotor. But as an aside, I think the rotor needed to be higher to make sure it avoided the prop and tail during turbulence and hard landings.

#### rtfm

##### Well-Known Member
Orion,
You always come to the party. Thank you. It is so hard getting intelligent input into an aircraft design. My wife, for example, finds it difficult to grunt with feigned interest. Others tell me to stick to the plans (whatever that's supposed to mean...).

Let me take a while to consider your suggestions...

Cheers,
Duncan

#### rtfm

##### Well-Known Member
Hi,
I;ve spent the entire day at the computer working on the Razorback design. First I tried reverting to the overhead boom type of wing attachment (as mentioned by Orion above) but realised that I needed to know the CG of the finished aircraft reasonably accurately if I was going to be able to sketch the "rafter" half-way sensibly.

So the rest of the day was spent working out the CG.

First I entered all the components in a spreadsheet, with columns for weight and distance from the datum. Then I had to go onto the web to research the probable weights of each of the components. This is harder than it sounds...

Then I got Mr Gates to calculate the moment arm of each component, and with Mr Raymer's help, calculated the CG of the whole package.

Then I realised that I needed to examine the extremes. ie No gas, plus passenger, and full gas, but no passenger. The CG for each of these scenarios worked out as follows:

No GAS, + PAX = -24cm
Full GAS, no PAX = +6.8cm

I've indicated these positions in the attached sketch. This is a range of over 30cm (nearly a foot), which is (I think) very big. Question: What controls the "allowable" limits of CG? Is it just the power of the H-Stab to compensate? Reasoning that this was possibly the case, I calculated that at takeoff/landing speed (45kts) the H-Stab can produce sufficient lift to effectively bring the rearward CG well forward - into a "neutral" position.

Am I on the right track here?

And one other thing - Raymer (p 261 - Conceptual Approach) states: "Ideally, the spinner should cover the propeller out to about 25% of the radius, although most spinners are not that large". So I added a 25% spinner to the Razorback to see what it would look like. At first it looked comically large - but the more I looked at it, the more I liked it. And it makes the cowling really slick. What do you think?

Regards,
Duncan

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#### orion

##### Well-Known Member
Forward CG limit - the ability to come to full stall in ground effect. Ground effect reduces or eliminates wing down-wash over the tail and it can significantly reduce the effectiveness of the horizontal.

Aft CG limit - must be set forward of the neutral point, preferably the stick-free neutral point. Purely a function of stability requirements.

And yes, 12" does sound like too much for this small airplane. For something like this I'd expect your overall CG range should be in the two to three inch realm.