High aspect ratio design for long range, optimized for cruise efficiency

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karoliina.t.salminen

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So I thought that Jay's thread gets hijacked if this thing is continued to be discussed there, so I opened a new thread.

So consider this (if the initial requirements were) and we leave out all hybrid propulsions from this speculation at this time:
- Typical cruise altitude 25000 ft. Service ceiling much higher, but because we have pretty much concluded that it is hard to get permission to fly higher than this, so be it, this is then maybe the real life cruise altitude.
- Indicated airspeed at that altitude at 55% cruise power in the realm of 135 kts IAS (200 kts TAS). Low power is assumed because otherwise the fuel tank size needs to become enormously big.
- 2500 nm range at that speed means 12.5 hour endurance requirement.
- "easy trip" with this kind of plane is 1000 nm (half fuel). Tanks sectioned with individual fuel caps allowing partial fueling when full fuel is not needed.
- Assume turbonormalized diesel cycle engine.
- Payload requirement: 3 * 90 kg (270 kg) + full fuel for 2500 nm range. The extra 90 kg is for carrying rescue equipment and luggage. Some of this weight can be compromised (third person can be diminished to 70 kg), but that's about the lowest limit it can go because life raft and the stuff like that just tends to be not coming without weight.
- Assume higher aspect ratio than current certified planes like DA40 and considerably lower than Jay's motorglider or Stemme.
- Assume airfoil which has relatively high profile L/D at low alpha and assume shiny surface finish resulting from molded composite construction (with CNC made tooling).
- Assume carbon construction instead of glass. "Reference plane" - Diamond DA40NG, with similar engine, is made of glass fiber. I have seen a destroyed wing of a DA40 and another wing under inspection, and I could find no carbon from this plane, all glass, so the plane does not have optimally light structure, but a compromise for manufacturing cost.
- Initial stall speed requirement, same as DA40, 49 kts sea lever. With full fuel for extreme range, the stall speed can be compromised a little from this. 55 kts is about as far as I consider it being acceptable for considering engine failure at start.
- Assume slotted trailing edge high lift devices with 75% span - deviation from a typical sailplane.
- Assume pusher propulsion with constant speed propeller(s)
- Space for two side by side in the cockpit + space for all kinds of pilot accessories needed for a 12.5 hour IFR leg.
- Assume tailed conventional configuration, like Stemme S10, but with shorter wings and lower aspect ratio than Stemme, but not as low as Diamond or Cirrus or Lancair. Not a motorglider, but not a low aspect ratio GA plane either, something in between.

As this has been discussed before on Jay's thread, lets continue in here instead.
So comments on these? Now the wing size, plane size, aspect ratio, engine power required, structure weight, feasibility, achievable mpg?
I will do a new excel for this concept, and will be calculating these things along the way, but it would be your turn for comments, after the initial requirement list. What requirement exactly of the above you would see needing compromising the most?
 

Denis

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Karoliina, one question reamins unanswered. Where will we get this turbonormalized Diesel, and will this technology enable us to reach the goal?
 

autoreply

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Pressurization?

If we start from what works, you could look at a few aircraft already flying, even if their structural and aerodynamic configuration is wildly different from yours, it still gives an idea about realistic performance, weights and so on. The (your) DA40 Diesel and the Lancair IVP come to mind. The Socata TB21 is also an interesting design, very close to the performance you want. Aside the "normal" problems with slender wings, your fuel volume puts a limit on how slender your wings can be and if we would design an aircraft for this mission I'd be surprised if it would be very different then those aircraft.

@ Denis, wasn't both the Austro and the Thielert turbocharged? If I recall correctly, both can maintain sea level pressure up to the high teens.
 

Denis

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Austro and especialy Thielert, to my opinion, are just examples of engineering voluntarism, not the real aero engines. Maybe Continental will develop the resently purchased from SMA design into a real workhorse, but it is obscure as well as any benefit from diesel technology in aircraft.
 

Himat

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Austro and especialy Thielert, to my opinion, are just examples of engineering voluntarism, not the real aero engines. Maybe Continental will develop the resently purchased from SMA design into a real workhorse, but it is obscure as well as any benefit from diesel technology in aircraft.
I do see a few pluss sides with a diesel aircraft engine.
-A diesel engine have a slightly better fuel consumtion.
-JET A1 is the most used used aircraft fuel. On some airfields there is no 100LL.
 

karoliina.t.salminen

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From Volkswagen, a custom install, because this is experimental plane for myself rather than being a product for others. To make it product, it would possibly then be equipped with engine like Austro or Continental diesel.

This technology is not the main enabler or even driver on this, but happens to be one of the choices that will work when 100LL is absent or prohibitively expensive. Puts the operating costs to reasonable levels. Mogas is not ok for classic engines even with STC or type certificate that allows using it in the future because even if for example Shell V-power works ok if there is no ethanol. But the ethanol free V-power is scarce and requires sampling from multiple gas stations before finding a place where there is no ethanol on it. I would estimate that the availability of ethanol free V-power will be zero in the future because the ethanol content is in some EU-legistlation which overlooks the chance that someone might want to use it in aircraft. Classic engines can't handle ethanol because ethanol can break the mechanical fuel pump and that's very inconvenient circumstance if that would happen.

Using diesel will also lead to about 50-60% savings in fuel cost per liter and more savings when it is counted that less liters are needed per travelled distance. As I have explained before it is not best idea to design a plane today that might not be built for many many years to be limited to 100LL when it is already getting problematic in Northern Europe. Savings are less when using Jet-A, especially in USA or Canada, but that stuff is at least available and because of jet engines it will be available also in the future.

I am already in a little trouble with the gasoline Diamond as there is no source of reasonable priced fuel for it. The very high fuel cost ends up being very few flying hours per year. 3 euros per liter 100LL cost is not very small if you count that the engine takes 30 liters per hour on average when takeoffs and landings are counted (even if I can fly at around 25 liters per hour by leaning aggressively). 30 liters per hour * 3 euros = 90 euros per hour (~120 dollars per hour). Not so long ago one could be able to rent a whole airplane with 90 euros per hour. Not anymore.

You can do the math as homework how much it costs if:
- you have a 180 hp diesel with SFC 0.36 and very high efficiency low drag airframe (fuel cost 1 euros per liter, and drinks 20 liters per hour, that's something like 20 euros per hour fuel cost alone)
- or if you have a 300 hp Continental TSIO-550 and have a more normal airframe and fly the same speed (fuel cost 3 euros per liter, drinks 50 liters per hour, and that's something like 150 euros per hour (200 dollars per hour fuel cost alone))
Then you compare 3 euros per liter vs. 1 euros per liter fuel cost for example for 20000 km trip. If you would consider flying the latter one at this cost level, you would happily go to suffer to the economy class of airline and forget about personal flying for transportation.

This is basically the justification for diesel and for airframe that is not like you find from every plane from airport.

- diesel fuel is available in the future
- diesel fuel is less expensive
- with this kind of concept, fuel cost savings are up to 750% compared to gasoline powered 300 hp plane like Cirrus with similar specs with today's prices. I can't completely estimate tomorrow's prices, but these most certainly are going to be higher than today's prices.

The reason for increasing prices is obviously politics, taxing, legistlation, ignorance etc. But who would win fight against windmills if it is easier to go around them from left and right? 100LL is like film in cinema production. The remaining days are numbered. It is just matter of time.

Actually because cost is becoming a driving factor for personal air transportation, we should forget about miles per gallon. Instead a more descriptive would be dollars per mile or dollars per 1000 miles.
 
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N15KS

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It's interesting to add up the fuel savings over the life-time of the airplane. My RV-10 burns around 12 gallons per hour. Over 5,000 hours, that adds up to 60,000 gallons or $360,000 USD in todays fuel prices. I hate to even think about the cost in tomorrows fuel prices! In any event, a 10% or 20% reduction in fuel consumption would save $36,000 to $72,000 USD.

Another major cost of aircraft ownership is storage. At my airport, hangar space for a single costs around $300 USD per month or $3,600/yr. Again, over the life of the airplane, it really adds up. Assuming a life of 30 years, that's $108,000 USD spent on hangar rent.

At some point, increasing the span will make it difficult to store the aircraft in a conventional hangar and the storage costs will go up. For this reason, I believe that a high aspect ratio aircraft is an excellent candidate for a folding wing. For example, a 60ft span that folds to 10ft would save money, both on the ground and in the air. A while back, I did some trade studies on this. By reducing fuel consumption and hangar costs, it's possible to save more than $100,000 USD over the life of the aircraft.
 

Himat

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Some very basic calculations.

You want 180hp for 12,5 hour, estimated at 20l/h wich gives 250l fuel. Jet A1 is 0,775 to 0,84kg/l.
250l at 0,84kg/l, total fuel=210kg
You want 3 pax, each 90kg, total pax=270kg
Useful load is then, Wu=480kg

I did chose to use 0,84kg/l as energy densinity of fuel is measured in MJ/kg and thous i did chose max range. At least my diesel car have the best mileage with "heavy" fuel.

With a 0,4 load fraction (Wu/Gross Weight) the plane end up at 1200kg ready to go where 720kg is the empty weight.
With a 0,5 Wu/Wg the plane is 960kg ready to go, half useful load, half structure.

If you take Wg heavy/Wg light =0,5/0,4=1,25 you have got the factor the "heavy" airplane L/D must be better than the light airplane L/D to have the same cruise efficiency.

As I see it, structure efficiency is as important as aerodynamic efficiency.
Ain't easy to make an efficient airplane.
 

autoreply

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Good points N15KS. If you want to lower the cost (a lot) many people overlook the (substantial) other costs, apart from fuel and writing off your engine+maintenance. I've given that a lot of thought for my own design and concluded:

  1. You need folding wings, eventually fitting to a trailer so you can store it on a trailer tied-down on the outside. Needs automatic connections and practical (1-minute) rigging) Here (NL), that'll save you something like 4000 euro/year in hangar rent.
  2. Fuel. Ethanol-based mogas or diesel/jetfuel doesn't matter that much to me. Karoliina is dead-on about the troubles with other fuels. Here, avgas is already 3 times what you pay for it...
  3. Insurance. You need fixed (nosedragger?) gear to keep insurance acceptable, apart from the substantial extra cost and effort for retractable gear.
  4. No turbochargers, given their much higher initial and maintenance cost. I would pick a IO-580 over a T(S)IO-360/90 anytime for comparable high-altitude performance.
  5. Proven (aviation) engine. This might be very hard for this (diesel) design, but insurance, building cost and building time will likely be much lower, while reliability would probably be much higher.
  6. Proven concept to keep design efforts limited and give a reasonable chance of a successful design. Not necessarily implying one should pick a low-wing tractor, but I would prefer to stay with a configuration that has successfully flown in the past and learn from the specific issues of that configuration. (Cirrus VK30, Mini-Imp and so on for this design)
 
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karoliina.t.salminen

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Well the situation is that we have no storage space (hangars) much available anyway. So it does not matter how long is the span. I think for winter storage, the wing tips can be removed even if it is not exactly 1 minute operation.

What it comes to insurance cost, I don't know how much experimental aircraft have insurance value. So if you spend for the parts 100000 and say that comparable new aircraft would cost 1000000, would your plane be insured for 1000000 or 100000? I think many here fly experimentals with only the mandatory insurance and no hull coverage to keep the costs shoestring level.
 

Denis

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An idea of using an oversized naturally aspirated engine instead of a turbonormalized one is very promising for such plane. The pressurization system should be driven electrically, together with the possible electrical de-icing system, which will be also needed for such mission. Adedicated high-power alternator can be installed on the engine without a problem.Particularly, the lycoming layout with starter and alternator mounted near the prop flange is very convenient for such purpose. A largegenerator with is rotor dirctly mounted at the prop flanege will possibly act alsoa s a starter, while the stock belt-driven alternator will be retained for redundancy. This advanced welectric system can be retrofitted after successfull flight tests and verification of the plane, what will save considerable amount of finances during the project development.

The etanol in fuel is absolutely no problem with right approach to the problem. Maybe it is the GA future.

Aviation Grade Ethanol

The electrically driven pressurization system can be very economic in operation and superior against a bleed air system. It is especially interesting for a plane with relatively small cabin volume an only 2 or 3 persons aboard.

The next important advantage of the oversized NA engine is the possibility to use a highly efficient fixed pitch prop. Besides of the cheaper cost, it is a major simplification of the maintenance.

The fixed gear option, to my opinion, can be tolerated here only in a taildragger configuration. The nose wheel is very draggy and adds more weight. The streamlined taildragger fixed gear may compete with retractables on weight against drag basis.
The retractable taildragger configuration with fixed tailwheel is also much simpler and lighter than a retaractable tricycle gear. The retractable gear wtih Wittman style spring legs similar to one used in retractable Cessnas looks here an optimal solution. This gear retracts into the fusealge and is dedicated to high wing layout, which offers the ultimate aerodynamic and weight culture.
I think that all pusher configurations configurations should be ruled out, because of the serious design complications and performance penalties, which will accumulate with range.
 

Rick McWilliams

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Tri gear configuration and conventional gear configuration seem to have the same drag. The tripacer and pacer have identical performance. The nose gear on the tripacer is ugly but ther seems to have no drag penalty. I flew many trips in my Ercoupe (tricycle gear) with a friend in his Cessna 120 (tail dragger). We would trade airplanes on different legs of the trip. The Ercoupe would always burn less fuel. He upgraded to an O-200 to keep up with the Ercoupe C-85, and burned even more fuel. These are both high drag airplanes with a flat plate are of about 4.4 ft2.

The taildrager / nose dragger discussion is so goofy. All of the performance claims are BS. Strangely you will never hear the story about the Luscombe that was passed by an Ercoupe, or a R22 helicopter from the Luscombe pilot. I was flying the other aircraft. When a good crosswind shows up the real tail dragger pilots will show their stuff. When it gets above 20knots crosswind component only nose draggers are still flying.

Tricycle gear handles much bettter and affords better visibility on the ground. The ground loop is embarassing and hard on wing tips. The landing gear can be lighter with oleo-pneumatic shock absorbtion. The superior damping reduces the required drop test height, and maximum loads. Good landing gear works great on rough taxiways. I do not think that weight is affected by the tricycle / conventional choice. Gear doors and gear wells can generate a bunch of drag. Well faired fixed gear can reduce the drag of wheels by a factor of 1/3, maybe more. It looks so nice to retract the wheels behind well fitted gear doors.
 
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Denis

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The nose leg typicaly provides as much drag as both main legs in tricycle gear. The close comparisons of Tripacer and Pacer (at otherwise equal conditions) reveal thet the latter is by 5-10 knots faster than the former. The main factor here is the interference between the vortice flow from the nose leg and the fuselage bottom. The careful fairing of the nose leg may minimize this factor, but finally the total wetted area of the tricycle gear components is still considerably larger than that of nthe conventional gear. The nose wheel must be always oversized in respect to its static load, and often the small single engined planes require the use of all three wheels of the same size for satisfactory ground handling. The weight of the tricycle gear also cannot be made the same as that of the taildragger one and the difference can comprise several per cent of the MTOW. These circumstances may explain the fact that the most efficient touring motorgliders use either the inline gear of sailplane type, or the taildragger gear. Also there are examples of tri-gear palnes (Vary-eze, Long-eze), where only the nose leg is retractable. There is the NASA report on the wind tunnel evaluation of the Vary-eze model, showing how much the nose leg contributes to the equvalent drag area.
 

bmcj

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I've seen claims of a 10 knot cruise increase for the Texas Taildragger (in this case, a tailwheel conversion of a Cessna 150 with all other factors unchanged). Granted, a taildragger usually needs a slightly longer main gear for proper landing attitude (stall angle) and sometimes even for prop clearance with the tail lifted, but the overall drag and weight still seem to be lower by all accounts I've read. I've flown tricycle versions of the Aeronca Champ, and they are a truly horrible flying plane compared to the taildragger version. Cruise, climb, fuel economy, and glide all suffered.
 

autoreply

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Comparing both the claims and the pireps from identical designs or even identical aircraft (RV series with nose/taildragger variant, Glasair Sportsman), they typically claim/measure 5-10kts more speed with a taildragger. They also claim (and measure) a higher weight for a nosedragger, though I don't have any solid numbers about the differences there.
 

karoliina.t.salminen

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Denis & others with ideas of direct drive aviation engine. Aircooled direct drive aviation engine quite unlikely, I have been thinking this over and over, I have considered this during the last few years, and I think I have descoped it already, no matter what the weight penalties, aviation engine is already ruled out for the one-off prototype which is intended for my personal use.

Also what it comes to development cost of a DIY auto engine for one-off experimental project, well, my friend who made the Varttimarkka to my understanding was on pretty much shoestring budget. It was new airframe and new engine. His engine costed few thousands, not tens of thousands or hundreds of thousands, with conversion probably costed less than a used overhauled Rotax 912. What it comes to maintenance cost; maintenance of experimental aircraft is done by builder. Well in fact I do maintain also the Diamond myself as a helper of our mechanic, but with homebuilt plane there is not even this need to pay for the mechanic except for annuals. I don't think the maintenance cost is any prohibitive because auto engines have generally less expensive parts and less expensive accessories than aviation engines and they are also more reliable than those in the aviation engines (such as the story of the Diamond's electric boost fuel pump which is flawed by design and fails every 200 hours).

I have also considered this deicing system and I have also pretty much concluded that the complexity of installing such system and also the penalties associated with it (ruins laminar flow) exceed my interests at least at this time. Flying long distances over Atlantic has a rule as instructed by Ed Carlson: Never fly in bad weather. Flying in bad weather over North Atlantic is moron factor, darwinism and population control (by borrowing his exact words). The stories about some ferry pilots flying to headwind belong to this "moron factor" according to him. Ed never flies in severe headwind, with low ceiling and known icing. I was teached by Ed and if I learned anything from him, I think this was it and it sticks. We have flown few times to icing accidentally (elsewhere than North Atlantic) with the DA40, but the DA40 flies much lower than what I was intending for and flying inside cloud is more likely, therefore with higher altitude flying machine this icing could have been possibly avoided (except on climb and descent), I am not really intending this machine for continuous flight inside the cloud on known icing conditions. I have only less than this lifetime to do this, I don't think I have time to develop and debug a deicing system.

Trigear is most likely configuration (not taildragger). There is of course this option to retract only nosewheel like on canards, at least that is quite simple to implement. However, retracting the mains too with gear doors would be a target if I can find a simple way to do it. I am not that far that I would be thinking this kind of mechanics.

What it comes to ethanol in fuel; the current Lycomings are built from so poor materials that they can't withstand ethanol properly. I have seen a report that exactly it is the mechanical fuel pump that has such materials on it and with the ethanol in the fuel it wears out fast. There is no difference on what the ethanol is certified to if the engine is so antiquated that it will not simply work with it. The antiquated engine is the problem, not the fuel itself.

I was considering a flexifuel engine running RE85 (ethanol fuel), because RE85 was even less expensive than diesel for a short while, but now RE85 costs more than diesel again. The pricing did not last. The price of the diesel has gone up too, so it is not a holy grail of any kind, except for of course biodiesel - there are few small manufacturers making this and this Varttimarkka aircraft already successfully flew with biodiesel. That could be a cost saving to put 300 liters of biodiesel in before departure and fly the first leg with less expensive fuel.
Anyway, filling the tank with heavily taxed diesel fuel will cost around 600 dollars with today's prices and it directly points out how much it would cost to fly from Finland to Kangerlussuaq non-stop. That's about half way if the destination is Oshkosh, so it would be 2400 euros to destination and back (also a price of my favorite 27 inch iMac with core i7 and 16 gigs of ram). That would make 1200 euros per person, which is only a little more expensive than in economy class of airline and the price gets about the same when it is counted that when arriving on airline, a car needs to be rented from Chicago instead of arriving directly on Oshkosh. I find these numbers highly inspirational, if found feasible.

What it comes to wing volume on slender wings, I think the configuration becomes multi-pod if there is not enough volume inside the wing for fuel. I still haven't concluded the gross weight and therefore not the wing area, and therefore I don't know the volume or the fraction of the wing volume that can be used for fuel tank - for example DA40 has a aluminum tank inside the wing which is removable for maintenance if the wing is removed and the full volume is therefore not fillable with fuel.

There is example plane out there which has very large fuel capcity: Lancair evolution:
Fuel Capacity 168 gal.

That's 638 liters, enormous even by my standards. The span is less than 12 meters on Evolution. It climbs well because the engine is powerful. In my case engine would be less powerful and lower span loading is needed.

About the issue of flying at high altitude (RVSM limitations) - today is my lucky day because I looked at Wikipedia:
Reduced Vertical Separation Minima or Minimum (RVSM) is an aviation term used to describe the reduction of the standard vertical separation required between aircraft flying at levels between FL290 (29,000 ft.) and FL410 (41,000 ft.)

Only aircraft with specially certified altimeters and autopilots may fly in RVSM airspace, otherwise the aircraft must fly lower or higher than the airspace, or seek special exemption from the requirements. Additionally, aircraft operators (airlines or corporate operators) must receive specific approval from the aircraft's state of registry in order to conduct operations in RVSM airspace. Non RVSM approved aircraft may transit through RVSM airspace provided they are given continuous climb throughout the designated airspace, and 2,000 ft vertical separation is provided at all times between the non-RVSM flight, and all others for the duration of the climb/descent.


So if you fly over 41000 ft and transit through RVSM airspace by continuous climb and descent, then you can have non-RVSM aircraft flying at higher than 28000 ft.
 

Tom Nalevanko

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"There is example plane out there which has very large fuel capcity: Lancair evolution: Fuel Capacity 168 gal."

My Stallion holds 232 gallons; and I walked up and down a ladder many, many times calibrating the tanks, 2.5 gallons at a time. And I had to repeat this twice! A once in a lifetime experience. It is roughly the size of a Cessna 182.
 

bmcj

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"There is example plane out there which has very large fuel capcity: Lancair evolution: Fuel Capacity 168 gal."

My Stallion holds 232 gallons; and I walked up and down a ladder many, many times calibrating the tanks, 2.5 gallons at a time. And I had to repeat this twice! A once in a lifetime experience. It is roughly the size of a Cessna 182.
Wow, 232 gallons. What kind of range do you have? Do you measure it in hours or days?

I carried 150 gallons in my DGA-15, but that was a pre-WW2 plane and carried a 450 hp radial (thirsty... 22-25 gph).

Bruce :)
 

Tom Nalevanko

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Stretching the range: 232 gallons x 1hr/13 gallons x 180 knots = 3,200 nautical miles = 3,700 statute miles and only about 3/4 day having fun flying! Hawaii is only 2,100 NM from California.
 
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