An Illustrative Example

[For those of you with access to x-Plane 10, there are two aircraft models that represent the following example and a seaplane (they include the Going Further extensions described in the next section: RVX.zip ]

Consider an aluminum 4-place kitplane weighing 1700 lbs empty and 2350 lbs with full 50 gal fuel, 2 crew and 50 lbs baggage.

(If this RV10 based example looks unrealistic to you, check out close realization, the now available four-place  APM-41 Simba .   This is not an isolated example; it applies to any almost any small plane using a heavy engine.  A somewhat lighter plane could transition to LSA if desired.  or, given the increase in strength, very high performance could be had going to a twin, or maintaining the same fuel capacity, very long range, ... 

Caveat: There's no design magic in this example.  It starts off as a light plane with a heavy engine.  But it does demonstrate that a light fuel efficient engine can adequately power a 4 seat plane.  Also, there are possible and likely downsides: if the reduced weight is maintained - is not lost to improvements the lightness enables - the plane is likely to be more sensitive to turbulence, roll and pitch periods will change possibly making giving it a less pleasant motion, and something will have to be moved or added to get back the weight distribution of the original design.)

On an initial pass how much could no this be lightened?  Using conservative figures:
  1. Engine IO540 505 lbs - Rotax 915T 185 = 320 lbs
  2. Fuel 300 lbs - 150+ =150 lbs (and 15 gph versus 3.5gph) [ref]
  3. Fasteners, battery, smaller tanks,engine mount, misc support components ... 75lbs [ref
  4. [Skin plate weight since plate could chosen (7075 & 2024 instead of 6061 or...) without any consideration of ease of riveting  100+ lbs]
  5. Nota bene: 7075's strength to weight is more than twice! that of 6061 and it's hardness e.g. on upper wing means less operational denting and compressive rippling compromising laminar flow. 
  6. [Progressive skin plate thinning from root outward 50+ lbs]
  7. At equal drag, expected lift increase from laminar flow ~10% [ref] * 2350 lbs = 235 lbs  [see note bottom of page on laminar flow difficulties]
  8. Subtotal 1015 lbs saved, flying weight < 1600., effective weight in flight 1335 (laminar flow advantage)
  9.  [With that overall weight, suspension components can be shrunk - perhaps 25lbs]
  10. [[aluminum/honeycomb sandwich wing skins: 35% wing skin weight saving with large increases in strength particularly compression. ]] 
  11. [And some underlying structure can be reduced, perhaps 50+lbs]
Ignoring the elements in [9-11] in order to be conservative, the power to weight comparison is:

IO540: 230hp/2350lbs = .1 hp/lb versus 915T: 135hp/1335 = .1 hp/lb EQUAL!, so performance and ceiling would not differ appreciably except that the 915 is closer aerobatic capable.  


Cost of new IO540: $65,000 (TIO540; $90,000);   of new 915T (projected $30,000):  (912 iS Sport $21,000). [see more complete engines table below].

Summary 2nd Example (approx RV-7)
  • Starting point:span 25' lb/hp= 1500/190 = 7.63; io360-190hp 330# fuel 252# tanks 35# wings 150# gross 1800# empty 1100# fuelcost/hour = $6*8= $48.
  • End Point: lb/hp =1025/135 = 7.6; Rotax 915T -145# fuel -126# tanks -15# prop+engine ancilliary+battery -25# wings 7075&2024 -50# gear&wheels&brakes -25# positive feedback on structure requirements 5% 90# ignored: lam flow advantage; fuelcost/hour = $3 * 4 =$12.
  • Empty weight advantage: 1100 - 426 ~ 675#  => Can now qualify if desired as LSA, payload 650#
  • Fuel costs/hour are down 75%: $48 down to $12. !!!
HP/lb unchanged - performance maintained.  (And with greater span (37.5', chord 3.23', with 200# pilot) but no increase in wing area or weight, could qualify as a glider; and again, all speed, altitude, day/night and medical cert regulatory limits are off.)

What's the point of an LSA with a potential for high performance? - Performance is a lot broader set of abilities than a limit of level cruise to CAS 138mph at sea level: climb, acrobatics, take-off, acceleration, ceiling, dive, TAS at higher altitudes ... and as an LSA motor-glider there are no speed or altitude restrictions.

The weight loss is so great that a 100hp 912 iS Sport (as used notably on Burt Rutan's new seaplane [tweaked to 135hp by Rotax] ) would still give reasonable performance, and extraordinarily low operating cost.


"Continuous Fasteners" give several other significant advantages: no vibration, greater intrinsic structural strength (individual fasteners reduce structural strength).  Clamping strength does not lessen,  so structural strength does not degrade.  Stronger, lighter alloys can be used.  Much easier to correct/redo construction errors.  Faster construction, vastly less boring.  Water-tight so that amphibious designs would be easier to construct.  Airtight so pressurized designs would be easier to construct.  No corrosion, regardless of environment.  L/D's high enough for amateurs to build competitive gliders in aluminum.  One of the continuous fasteners, VHB, has various families for joining very different materials: various plastics, all metals, composites.  If tanks go unchanged, enormous range becomes the norm - for example, if the original tanks are  retained and the engine is the 912 iS Sport, at about 100 knots, the range would be 50.gal/2.gph = 25 hours => 2500 nautical miles.  Ocean crossing range.  Loitering at about 60 knots, one could remain aloft for about 2 days. Rutan's new seaplane (greater drag than the example here)  will be able to hold station  for 35 hours. 

Going Further

By introducing an inverted V tail, further lightening and reduced drag can be achieved at the same time that the propellor is moved to provide more efficient flow.  It also facilitates the  possibility of a staged transition to hybrid, then electric, without any fundamental changes in the rest of the airplane, by protecting an electrically driven ring propellor forward of the tail.

[A usual objection to the V-tail is that it increases the danger of a spin reducing rudder power, so that a V-tail must be so much larger that any advantage in surface or interference drag is lost.  The real reason the V-tail is a weaker rudder is that, as with most other rudders, it is "right side up" which means that it fights the ailerons in rolling/banking the airplane.  An upside down rudder does not, the ailerons and rudder work together.]

With no propeller forward (and the engine free to move) the nose can now be redesigned without compromise, for low drag and best possible cooling.  No more blocky noses.  And in a seaplane the engine need not be on a high pylon to protect it, a pylon that puts a small seaplane at a disadvantage not only with respect to drag, but also with respect to rolling and pitching motion in a real sea.

 [[expand] Laminar Flow airfoils (that actually work)  present some serious problems, ....., stall without warning with very rapid pitch up , ..., ...., increased likelihood of groundloop ....  On the other the fact that the wing is now airtight wherever desired permits a solution to two of these problems and a new advantage in even greater lift (as far as Cl=2.06) , by simplifying the mechanism and operation of a suction system which ..., ]] 

[Move to Problems and TBD page:]  a possible problem to resolve is that the geometry of the landing gear is now a priori much less stable than the usual tri gear or the tail dragger; it is a long effectively narrow triangle with the point at the heavy end of the aircraft. This might call for some change in the usual position of the wing and cabin and/or a move aft of the nose wheel.

[A note on a claimed 30% loss for giving up on direct drive: ... TBD....this ignores the power efficiency advantages of putting the prop and the engine wherever best....if the prop is one can retain direct drive of desired... This 30% is an old figure... It ignores the compromise of trying to streamline the critical noise while trying to best  cool the engine... it ignores the advantage of being able to use an engine that need only run at a single speed when it needs to ruin at all...it ignores the pressure that's on to improve this efficiency.] 



For Reference


Fuel and Weight per HP Comparison

Model             Weight         gph           hp          hp/lb     hp/gph   aerobatic  overhaul    ceiling     price
 IO-540 505 15-20 230 .45 15.33 no 2000 hr $ 65,000
 TIO-540 600 13-22 300 .5  no 2000 27,000$ 90,000+
 IO-360 365 7-12 190 .52 27 no2000  19,000$ 50,000
 912 iS 150 1.25-4 100 .67 40 potentially 2000 $ 21,000
 915T 185 1.5-5 135 .73 ~40 potentially 2000 23,700 ~$30,000
 Jabiru 4 140 4 80 .57 20 no 2000  ~$15,000
 Jabiru 6 184 6.8 120 .65 18 no 2000  ~$20,000
 Jabiru 8 257 ~11. 180 .7 16 no 2000 ? $25000 
 AeroVee 161 3.5 80 .5 23 no ~1000  $8000
Volvo T6300  3 300 1.0 100 dry sump~3000( 25000)~$10,000 

The Volvo T6 is a turbo 4cyl auto engine w/multi-speed electric supercharger shown here to suggest how 
engines are  being developed as 2025 fuel standards approach with their fleet average requirements of 55mpg.

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