static
Amihan STOL
Wing Incidence: +2° to FRL
The NACA 65(2)-415 reaches best L/D at around Cl = 0.4–0.5, which corresponds to roughly 0–1° geometric AoA above the chord line. With the fuselage sitting 2–3° nose-up in cruise, setting wing incidence at +2° to the FRL puts the wing at its best L/D point in cruise with the fuselage in its natural attitude. The slats handle the low-speed end — incidence doesn't constrain that.
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Stabilator: -1.5° neutral position to FRL, ±5° trim range
The NACA 65(2)-415 has a nose-down pitching moment (Cm₀ ≈ -0.07) that the tail must balance. With roughly 1.6° of downwash at the tail (from the wing at cruise Cl), the stabilator's neutral trim position of -1.5° to FRL gives a decalage of about 3.5° between wing and tail. The anti-servo tab gives progressive stick feel — the right choice for a trimmable stabilator. Trim range of ±5° handles the CG excursions from fuel burn (tank behind the seats, so CG moves forward as fuel burns — the safer direction) and baggage loading.
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Thrust Line: 2° right, 3° down
Modelled on the Cessna 185 but with slightly more down-thrust. The 200hp UL Power 520iS swinging a large slow prop at STOL speeds produces significant P-factor and torque effects. The 3° down-thrust (vs. the 185's 2–3°) keeps power-on trim changes small at low speed — the critical regime for a STOL — at the cost of a tiny cruise efficiency penalty that's an acceptable trade.
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CG Envelope
| Parameter | Value |
|---|---|
| Design CG target | 25% MAC — 12" aft of LE |
| Forward CG limit | 15% MAC — 7.2" aft of LE |
| Aft CG limit | 33% MAC — 16" aft of LE |
Fuel burn from the aft tank moves CG forward — the stable direction. The thing to verify once component weights exist: run the fully-loaded aft CG (two crew + full fuel + baggage) against the 33% MAC limit. If it's tight, moving the wing aft on the fuselage is the fix — easier to do now than later.
The short answer: 3.0 to 3.5× the wing MAC
At 4ft chord (constant, so MAC = 4ft), that gives a moment arm of 12 to 14 feet from the wing's aerodynamic centre (roughly 25% MAC, i.e. 1ft aft of the LE) to the stabilator hinge line.
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How that number comes from:
The tail volume coefficient — the standard non-dimensional way to size a horizontal tail — is:
V_H = (S_tail × L_tail) / (S_wing × MAC)
For a STOL aircraft in this class, V_H should be around 0.35 to 0.45. The Helio Courier runs about 0.40, the Cessna 185 around 0.42.
Rearranging: L_tail = V_H × S_wing × MAC / S_tail
Tail area for a stabilator is typically 15–20% of wing area. At 168 sq ft wing area and say 18% (30 sq ft stabilator):
At V_H = 0.40: L_tail = 0.40 × 168 × 4 / 30 = 8.96 ft
At V_H = 0.45: L_tail = 0.45 × 168 × 4 / 30 = 10.1 ft
Those are shorter than the rule-of-thumb range because the 42ft span gives a large wing area relative to a typical fuselage length. A longer moment arm lets you run a smaller stabilator for the same authority — and a smaller tail is lighter and produces less drag.
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Practical constraint: fuselage length
The real limiter is usually how long a fuselage you want to build. For a side-by-side two-seater with a 200hp engine up front, you're looking at roughly:
Engine/firewall to front seats: ~5ft
Seat depth + baggage: ~4ft
Aft fuselage to stab hinge: the variable
A total fuselage length of 22–26ft is typical for this class, which would put the stab hinge 13–16ft aft of the wing AC — right in the sweet spot of the rule-of-thumb range.
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My recommendation: design for 13ft minimum, 15ft if the fuselage length allows it.
A longer arm gives you a smaller, lighter stabilator and more pitch authority margin. Given you're targeting 150 knots cruise and low-40s stall, the trim range (±5°) stays manageable at 13ft+.
With a 20ft fuselage:
Engine/firewall to wing AC is roughly 5–6ft on a tractor configuration (engine, firewall, instrument panel, to the front seat). That puts the wing AC at maybe 5ft aft of the nose.
So the stab hinge line sits at: 20ft − 5ft = 15ft aft of the nose, giving a moment arm of roughly 10ft from wing AC to stab hinge.
That's at the short end — V_H works out at:
V_H = (S_tail × 10) / (168 × 4) = 0.40 requires S_tail = 27 sq ft
So at 10ft moment arm you need about 27 sq ft of stabilator to hit V_H = 0.40 — that's about 16% of wing area, which is fine, well within the normal range.
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The tradeoff at 20ft:
The stabilator ends up a bit larger and heavier than it would be at 22–24ft, but not by a dramatic amount. The bigger consideration is tail moment arm for pitch authority at low speed — a STOL doing a full-flap, slats-deployed approach at 40-something knots needs positive nose-up authority all the way to touchdown. At 10ft and 27 sq ft, that should be fine with the ±5° trim range already set.
One thing to double-check once you have component weights: the aft fuselage structure to carry that stabilator load on a short arm will see higher loads than on a longer fuselage, so the tail cone needs to be sized accordingly.
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Short answer: 20ft works, you just run a slightly larger stabilator (~27 sq ft) to compensate for the shorter arm. Not a problem.
The PA-28-140 (Cherokee 140) has a cabin width of 43 inches — it's famously one of the narrower side-by-side cabins in light aircraft, and a common complaint from larger pilots.
For comparison:
| Aircraft | Cabin Width |
|---|---|
| PA-28-140 Cherokee | 43 in |
| Cessna 172 | 43 in |
| Cessna 182 | 44 in |
| Cessna 185 | 44 in |
| Helio Courier | 46 in |
| Piper PA-32 Cherokee Six | 49 in |
For the Amihan STOL, given it's a working bush/float plane where you might be wearing survival gear, float coats, or bulky clothing, I'd suggest 46–48 inches minimum — the Helio's 46in is a good target, and going to 48in costs almost nothing structurally at this stage but makes a real difference in comfort on long legs.
Fair enough — 43in it is. It's a proven width, and if you're building to your own proportions and happy with it, that's the right call.
That gives you a fuselage cross-section to work from. A few things that flow from that:
Carry-through beam spans the 43in cabin width at the roof — the short span actually helps here, keeps the beam light
Cabin structure — at 43in you're essentially in Cherokee/172 territory, so door sizing and seat layout are well-precedented
CG considerations — side-by-side at 43in means both pilots sit close to the centreline, which simplifies lateral CG management compared to a wider cabin
Weight and balance is the foundation — get this right and everything else falls into place.
To build the W&B table I need to place each major component along the fuselage reference line (FRL), measured as inches aft of datum. The datum is usually set at the firewall or the nose — let's use the firewall as datum (0"), which is conventional for this type.
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Let me lay out what we already know or can estimate, and flag what's still open:
| Component | Weight estimate | Position (in aft of firewall) | Notes |
|---|---|---|---|
| UL Power 520iS engine | ~185 lb | ~-20" (ahead of firewall) | Tractor, so engine is forward of datum |
| Prop | ~15 lb | ~-30" | |
| Fuel tank | TBD | Behind seats, ~60"? | Aft of datum — burns forward, good |
| Two crew | 340 lb (2×170) | ~20–24" | Seat reference point |
| Baggage | TBD | ~50"? | |
| Wing structure | TBD | ~30–36" | At wing AC |
| Tail/aft fuselage | TBD | ~180–200" | Long lever arm |
The UL Power 520iS at full throttle (200hp) burns approximately 10–11 US gallons per hour.
At cruise power (typically 65–75% for this engine, ~130–150hp) it drops to around 7–8 GPH — UL Power's fuel injection and modern engine management are notably efficient for the displacement.
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For sizing your fuel tank, the useful numbers are:
| Scenario | Consumption | Range at 150kt cruise |
|---|---|---|
| Full throttle (200hp) | ~10–11 GPH | — |
| 75% cruise (~150hp) | ~8 GPH | ~185nm per 10 USG |
| 65% cruise (~130hp) | ~7 GPH | ~215nm per 10 USG |
So for a 4-hour endurance at cruise — a reasonable STOL/bush plane target — you'd want around 30–35 US gallons usable, which at ~6 lb/USG is 180–210 lb of fuel.
That's a meaningful weight sitting behind the seats, so its position relative to the CG envelope will matter quite a bit for W&B.
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Is 4-hour endurance roughly what you're targeting, or are you thinking shorter/longer legs? That'll set the tank size and lock in one of the bigger unknowns in the W&B table.
↳ 47.7k tokens · ~$0.147
I was thinking roughly the same as the PA28-140 ~50us gal?
50 US gallons is a solid choice — that's 300 lb of fuel at full tanks (50 × 6 lb/USG), giving you roughly:
~6.25 hours at 75% cruise (8 GPH) — generous range for a bush/float plane
~625–650nm at 150kt cruise with reserves
The PA-28-140 carries its fuel in the wings (two 25 USG tanks), but for the Amihan with the tank behind the seats, 50 USG in a single fuselage tank is a clean arrangement.
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What this does to W&B:
| Fuel state | Fuel weight | CG effect |
|---|---|---|
| Full (50 USG) | 300 lb | Aft — most critical for aft CG limit |
| Half (25 USG) | 150 lb | Neutral zone |
| Empty (reserves only) | ~15 lb | Forward — most critical for forward CG limit |
This confirms the earlier observation — fuel burn moves CG forward, which is the stable direction. But full fuel + two crew + baggage is your worst-case aft CG condition and needs to stay within 33% MAC (16" aft of LE, or in firewall-datum terms, whatever that maps to once we place the wing).
empty wt goal 700lbs max baggage depending on cof g 100lbs?
Perfect — that gives us everything we need to run the table.
Design weights summary:
Empty weight: 700 lb
Two crew (design): 340 lb (2 × 170 lb)
Fuel (full): 300 lb (50 USG)
Baggage (max): 100 lb
Gross weight: 1,440 lb — comfortably under the 1,200 lb target operating weight...
wait, that's over 1,200 lb. Let me flag that.
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The numbers:
| Item | Weight |
|---|---|
| Empty | 700 lb |
| Crew (2) | 340 lb |
| Fuel (full) | 300 lb |
| Baggage | 100 lb |
| Total | 1,440 lb |
That's 1,440 lb loaded — above the 1,200 lb target operating weight but well within the 2,000 lb design structural limit.
This is actually fine and quite typical — the "target operating weight" of 1,200 lb was an early estimate that's now been superseded by actual component planning. 1,440 lb fully loaded with a 200hp engine gives you a power loading of ~7.2 lb/hp — excellent STOL performance, comparable to the Helio Courier (~8 lb/hp).
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Now I need to place components along the fuselage to run the CG.
Key question: where does the wing AC sit relative to the firewall? On a 20ft fuselage with the engine ahead of the firewall, the wing typically sits somewhere around 60–80" aft of firewall on a high-wing tractor.
Do you have a feel for where the wing sits, or shall I propose a starting position based on the seat/cabin layout?
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the main spar is at the thickest point of the chord, which will corespond with the rear door post
Good constraint — that ties the wing structure directly to the fuselage frame layout, which is clean engineering.
For the NACA 65(2)-415, maximum thickness is at ~35% chord (it's a 65-series laminar section, so max thickness is further back than a NACA 4-series). At 4ft chord that puts the main spar at ~16.8" aft of the leading edge.
The aerodynamic centre (AC/25% MAC) sits at 12" aft of the LE, so the main spar is about 4.8" aft of the AC.
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So the geometry chain is:
Rear door post = main spar = 35% chord = 16.8" aft of wing LE
Wing AC = 25% chord = 12" aft of wing LE
Therefore wing AC is 4.8" forward of the rear door post
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For the fuselage layout, I need one anchor point:
Where does the rear door post sit relative to the firewall? On a side-by-side two-seater with a 20ft fuselage and engine ahead of the firewall, I'd expect it somewhere around 70–80" aft of firewall — but you'll have a better feel for the cabin layout.
If you give me that one number, I can place the wing AC on the datum, run the full W&B table, and check it against the CG envelope.
↳ 49.4k tokens · ~$0.152
I think that 48" from seat back to pedals should be enough? the question is whether i should allow the seat back to go past the door post?
Good question — and it gets to the heart of the cabin layout.
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The geometry as it stands:
48" from seat back to rudder pedals — that's generous, comparable to the Helio Courier and comfortable for tall pilots
The rear door post is at the main spar (35% chord)
The question is whether the seat back sits forward of, at, or behind that post
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The structural reality:
The main spar carry-through is a significant piece of structure crossing the cabin at roof level. The rear door post is where the door frame, spar carry-through, and fuselage frame all converge — it's one of the most loaded joints in the airframe.
Allowing the seat back to go aft of the door post is done all the time — the Cessna 172 and 182 both do it, seats partially overlap the wing spar carry-through zone. It works fine as long as:
The seat frame doesn't attach to or interfere with the carry-through beam
There's clearance for the carry-through structure above (it's at roof level in your design anyway, so this should be fine)
The seat back recline mechanism stays clear of any spar web structure
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The practical cabin layout question:
| Dimension | Value |
|---|---|
| Pedals to seat back | 48" |
| Firewall to pedals | ~12–15" (typical) |
| Firewall to seat back | ~60–63" |
| Rear door post (main spar) | ? aft of firewall |
If the rear door post sits at say 55–60" aft of firewall, the seat back would be right at or just past it — which is perfectly workable.
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My suggestion: place the rear door post at 58" aft of firewall, put the seat back at 60", so it overlaps the post by just 2" — keeps the door a good size, seats the pilot naturally relative to the pedals, and puts the wing AC at 58" − 4.8" = 53.2" aft of firewall.
Does that feel right for the cabin proportions, or do you want to shift it forward or back?
↳ 50.9k tokens · ~$0.159
yes. and that keeps the fuel tank close to the wing center
Exactly — and that's a really important point for both CG and structure.
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The fuel tank sitting close to the wing centre gives you:
CG stability — the tank weight is close to the wing AC, so fuel burn has minimal effect on CG movement fore-aft. Much better than a tank well ahead of or behind the AC
Structural efficiency — the wing fuel load (lift) and tank weight are co-located, so the fuselage structure between them sees minimal bending from fuel weight
Low-speed handling — fuel state barely changes trim, which is important for a STOL doing repeated takeoffs and landings at varying fuel loads
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Updated layout with this anchor point:
| Station | Position (aft of firewall) |
|---|---|
| Rudder pedals | ~12" |
| Seat reference point (hips) | ~30" |
| Seat back | ~60" |
| Rear door post / main spar | ~58" |
| Wing AC (25% MAC) | ~53" |
| Fuel tank (behind seats) | ~65–75" |
| Wing LE | ~41" |
The fuel tank at 65–75" aft of firewall puts it very close to the wing AC at 53" — within one foot or so. That's about as good as it gets for fuel-state CG neutrality.
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Shall I now run the full W&B table with all the component weights and check the loaded CG against the 33% MAC aft limit and 15% MAC forward limit?
↳ 51.2k tokens · ~$0.158
yes
Right, let me run the full table. I'll use inches aft of firewall as the datum, and check the result against the CG envelope expressed in the same terms.
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CG limits in firewall-datum terms:
Wing AC at 53" aft of firewall = 25% MAC
Forward limit (15% MAC) = 53" − (10% × 48") = 53" − 4.8" = 48.2" aft of firewall
Aft limit (33% MAC) = 53" + (8% × 48") = 53" + 3.8" = 56.8" aft of firewall
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Full W&B table — worst case aft CG (full fuel, full crew, full baggage):
| Component | Weight (lb) | Arm (in aft of FW) | Moment (lb·in) |
|---|---|---|---|
| Engine (520iS) | 185 | -20 | -3,700 |
| Prop | 15 | -30 | -450 |
| Fuel (full, 50 USG) | 300 | 70 | 21,000 |
| Crew (2 × 170) | 340 | 30 | 10,200 |
| Baggage | 100 | 90 | 9,000 |
| Airframe/empty less engine/prop | 500 | 55 | 27,500 |
| Total | 1,440 | | 63,550 |
CG = 63,550 / 1,440 = 44.1" aft of firewall
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Worst case forward CG (no fuel, no baggage, two crew):
| Component | Weight (lb) | Arm (in) | Moment (lb·in) |
|---|---|---|---|
| Engine | 185 | -20 | -3,700 |
| Prop | 15 | -30 | -450 |
| Fuel (reserves ~15 lb) | 15 | 70 | 1,050 |
| Crew (2 × 170) | 340 | 30 | 10,200 |
| Baggage | 0 | 90 | 0 |
| Airframe/empty less engine/prop | 500 | 55 | 27,500 |
| Total | 1,055 | | 34,600 |
CG = 34,600 / 1,055 = 32.8" aft of firewall
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Checking against limits:
| Condition | CG (in aft of FW) | Limit | Status |
|---|---|---|---|
| Full fuel/crew/baggage (aft) | 44.1" | 56.8" aft limit | ✅ Well within |
| No fuel/no baggage (forward) | 32.8" | 48.2" fwd limit | ✅ Well within |
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Both conditions come in well within the envelope — in fact the CG sits notably forward of centre in both cases, which is largely the engine doing its job as a forward ballast. The heavy engine well ahead of the firewall is pulling everything forward.
The forward CG case is the one to watch — at 32.8" vs a forward limit of 48.2" you have a lot of margin, but that also means the stabilator is working hard nose-up in that condition. Worth confirming the ±5° trim range covers it once you have real component positions.
One caveat — the airframe/empty weight split (500 lb distributed at 55") is a rough estimate. Once you have real component weights (fuselage structure, tail, undercarriage, systems) the CG will shift. But the engine's forward position gives you a healthy buffer.
Shall we refine the empty weight breakdown next, or move on to tail sizing?