Three-Lifting Surface 3

[Sparweb]

A lot of discussion ensues when somebody brings up the topic of "canards", which invariably involves lousy stall characteristics, Mr. Rutan's designs, and landing speeds about 20 knots below Vne.
The solution seems to exist in one very unique Italian aircraft called the Piaggio Avanti which has 3-count-em-three lifting surfaces. The main wing has flaps. The forward wing has flaps, too. The tail wing has some kind of elevator-stabilator mechanism that I don't quite understand that controls pitch with and without flaps.
As far as I can tell, this is the only 3LS aircraft certified by the FAA.
Are there any others? Whenever I search the internet, all I get are wannabe designers' pipe dreams and scaled-down knock-off's.
Apart from its aerodynamic complexity, what hidden drawbacks might be in the design?

 

[dan100]

Hi.
My interpretation of the Piaggio design is that it is a clever way of achieving a cabin area free from the spar carry-thru, without resorting to putting the same on the outside of the fuselage cross section. As the spar is located behind the cabin, and the nose of the aircraft is long and the tail is short. All cargo is located in front of the wing. To offset this the engines are put on "backwards". To avoid large trim drag an low authority when flaps are extended from the short-coupled tail the front lifting surface is added. The drawback could be questionable post stall behaviour. No doubt this can be solved as the design is certified.

 

[Sparweb]

Cabin space is an issue I didn't think of before. The Israel Aircraft Westwind has its mid-wing spar carry-through defining its aft pressure bulkhead, too, but this consumes about 1/3 of the potentially available fuselage area. The ability to move the wing back by many feet vastly expands the usable space. BTW, there is a small baggage compartment behind the wing, too - definitely not accessible when the engines are running!

STF

 

[macoovacany]

Eagle-X, orginally designed in Australia and now owned by the Malaysians is also a three surface configuration. The Eagle-X is certified to FAR-23 standards.

I will say no more.

Timbo

 

[bfuchs]

Hi,
There are quite a few pout there ... check out

http://www.velocityaircraft.com/

http://www.eracer.org/

http://www.siinc-sources.com/Dragonfly/

etc...

All you need in the canard, to ensure stability under stalling is to get a profile for the forward wing to stall before the rear wing. That ensures your nose down pitching moment that keeps the plane controllable.

 

[Sparweb]

bfuchs,

Don't confuse simple kit canard aircraft and with the more elaborate 3 lifting surface design.

The Velocity is heavier than the Cessna 172, and stalls at 65 knots. That's more than the LANDING speed of the 172. Canard aircraft are not as safe to operate as conventional aircraft. Are any canard aircraft certified? I don't think so; not many at least.

The three lifting surface design keeps the landing speeds down to levels safe enough for certification of the Piaggio and the Eagle-X.

macoovancancy,

Do you happen to know offhand if the Eagle-X has flaps on its main and forward wings?


STF

 

[Earnwald]

“…Canard aircraft are not as safe to operate as conventional aircraft…..”
Burt Rutan originally started designing canards in the experimental (homebuilt) industry for exactly the opposite reason. The leading cause of light plane fatal accidents is stall-spin during the landing pattern. A canard is supposed to be stall-proof since the canard surface stalls at lower angle of attack than the wing’s stall angle of attack. The canard then no longer has enough control authority to keep the nose up so the aircraft never reaches the wing’s critical angel of attack. Do I have that right?

Another certified canard was the Beech Starship, certified in 1988 it went into production in 1992, and out of production in 1994 after only 53 were built. It had great performance but cost $5 million, as much as some jets.

 

[kuae99]

Canard: Boeing's unfortunately abandoned Sonic Cruiser would most likely have been a canard. The problem was not controllability: it was keeping the canard from hitting the jetway. Further, the instability of canards can easily be overcome with modern digital control systems.

3-surface: Check to see if the Piaggio P-180 Avanti is in Jan Roskam's most recent book Airplane War Stories. It's on the cover.

 

[macoovacany]

Offhand, the Eagle-X has flaps on both the fore plane and the aft plane. The pilot has only one flap control. Also the aft plane has a wing fence and vortex generators immediately ahead of the ailerons. The wing section outboard of the wing fence has a drooped nose. The FAR certification requirement requires aileron control down to , and into, the stall of the aircraft. The aft plane (the larger one) has the aileron section in the UPWASH of the fore plane. Offhand.

Timbo

 

[Sparweb]

I recall seeing the same drooped leading edge halfway outboard the main wing on the Piaggio, but no fences.
I also noticed that the avionics technicians having difficulty getting around the front wing to work on the electronics bay in the nose. In compensation, it makes a handy work table (perhaps not such a good thing).

Thanks for all the ideas. So far the score is:

Certified 3 lifting-surface aircraft=2 (Piaggio & Eagle)
Certified canard aircraft=1 (Starship, but we won't forget the Wright Flyer)
Certified conventional aft-tail aircraft=everything else.



STF

 

[Miper]

SparWeb
Going back to your original question, one drawback of the 3-surface design on aft-engined configurations is that it can make it difficult to access the nose of the airplane, where avionics and other systems service and maintenance are done. Also makes it hard to put a convenient baggage compartment there. The Eagle-X, which has its engine in the nose, has a swing-away engine mount to provide access to the engine and firewall.

Another possible drawback is that the forward wing may block a large portion of the pilot's forward vision, which can make navigation difficult and affect safety in the airport pattern. The Avanti 3rd surface is too small to be much of an issue here, but the Eagle's forward wing is quite large.

In a way, the Eagle-X almost seems to be more of a biplane with large negative stagger than a 3-surface airplane like the Avanti. The Eagle-X seems to be nicely detailed and Dave Higdon's flight test of it is very positive.

http://www.avweb.com/news/newacft/182746-1.html

Is the company still alive? It's website isn't.

http://www.eagleair.com.au/

Was the Eagle certified to FAR PART 23 or to JAR-VLA?

 

[Miper]

The fences and drooped leading edges on the main wing of an airplane with a forward wing are usually there to counteract the tip vortex of the front wing. The vortex can significantly increase the angle of attack of the outboard main wing. Drooping the outboard leading edge is like twisting it nose down.

The fences help to keep the vortex disturbance and the wing profile discontinuity from affecting the inboard wing too much.

For additional comments on canard and 3-surface configurations, see faq2-760.

 

[JohnFortier]

As I see it, at lower speeds, i.e. landing, takeoff and low speed manouvers, with lift shared between the foreplane and the mainplane, and the stabilator providing vertical control, the aircraft will remain stable in the vertical plane, as long as the foreplane stalls before the main plane.

Unlike a pure cannard, a three surface setup does not rely on a stalled surface for control once the foreplane stalls. The control is provided by the stabilator, which is not stalled, since to provide meaningful control it must remain in a near neutral lift position. This removes the main danger of the cannard configuration. As I said before, trying to maintain control with a stalled airfoil isn't something I really want to experience.

Since both the fore and aft planes on the Avanti have flaps, it follows that during landing and takeoff, the lift from both is enhanced and this mutual enhancement should be such that the centre of lift remains constant. However, a thought occured which I will try to check out: With the flaps on the main wing lowered, the turning moment of the flaps on the aircraft will be resisted by the foreplane. If the foreplane flaps deploy further, pro rata, than the mainplane flaps, this will increase the lift of the foreplane and counteract the turning moment. This will also ensure that the foreplane will stall before the mainplane, which is a pre-requisite for stall stability.

In the cruise, it should be possible to trim the aircraft such that the stabilators also contribute to lift. Whether this will increase of lower drag is a moot point. (Don't point that moot at me!) It could be achieved by increasing the angle of attack of the foreplane slightly and compensating for the nose up moment this would cause by placing a nose down trim on the stabilator. It would increase lift at the expense of a, probably, slight increase in drag and may explane the Avanti's high cruise altitude ability.

For general information, here some performance figures for the Avanti.

PERFORMANCE
Maximum Cruise Speed
395 KTAS/ 732 km/hr at 28,000 ft/8538 m
Ceiling
41,000 ft/12,500 m
Maximum Range
1,750 nm/3,241 km (6 PAX, VFR Reserve)
Engines
Pratt & Whitney, Canada PT6A-66

EXTERNAL DIMENSIONS
Wing Span
46.03 ft/14.03 m
Length
47.28 ft/14.41 m
Height
13.06 ft/3.98 m

CABIN DIMENSIONS
Height
5.74 ft/1.75 m
Width
6.07 ft/1.85 m
Length
14.93 ft/4.55 m

Weights
Maximum Take-Off Weight
11,550 lbs/5,239 Kg
Standard Empty Weight
7,500 lbs/3,402 Kg

Take-off run - 2,850ft
Climb - 2,950ft./min
Climb - 1,106ft./nm

Useful Load (Incl. Crew)
4,100 lbs/1,860 Kg
Maximum Payload
2,000 lbs/907 Kg

Seating Capacity
Maximum Seating Capacity
9 plus 2 crew
Typical Executive Payload
6 plus 1 crew

I find the climb rate of 1000 fpm on one engine particularly impressive. Whether the three lifting surface design contributes to this I don't know, but I would suspect that it does. Take off run of just over a half mile is also quite useful for an aircraft with this performance.

I think there may be two reasons why this design meets with resistance:

1 - It gets confused with pure cannards such as the velocity,

2 - It's different. don't forget that aviation is a one hundred year old industry and in any industry of such an age there tends to develop a desire to stick with the tried and tested - and familiar. Sometimes this is a good thing; no-ones come up with a real replacement for the C-130, for example. sometimes it holds the industry back and prevents people from judging products on their merits, such as performance and comfort.

Actually, the more you look at the three surface design, the better it looks. I wouldn't be surprised to see an airliner designed on the same principles before long, just don't expect it from Boing or Airbus!

John

 

[i278]

Miper:

The FAA Type Certificate lists the following as the cert basis (TCDS #A00005LA):

14 CFR part 21, § 21.17(b) using Joint Aviation Requirements - Very Light
Aeroplanes (JAR-VLA) at Amendment 0 dated 26 April 1990, through Amendment
VLA/92/1; § 21.29; and 14 CFR part 36 through amendment 36-21 effective
December 28, 1995.
Noise Control Act of 1972.
Eligible for day-VFR operations and normal category maneuvers only.

I assume that other authorities that have type approved the Eagle probably accepted JAR-VLA as the cert basis as well. Just guessing.

And on a completely different note, I was quite enjoying the technical discussion in this thread. Not that it matters.

Regards

 

[JohnFortier]

Rumours of a stretched and/or jet version of the P-180: anybody heard anything conclusive?

John

 

[Miper]

i278,
Thank you for the information about the Eagle. Day VFR only would probably make the airplane a hard sell in this country, especially at $120,000 or so for a 2-place.


-----

John,
I have liked the Avanti from the first time I saw a rendering of it.

There is no doubt some truth in your comment that the Avanti meets with resistance because it is different. I have heard a few people mention that it's fuselage has an unattractive, almost mouse-like look because there's no "notch" at the windshield.

Some little-understood fallout from the Starship is that, because it failed to live up to its high technology hype, new technologies lost their credibility among many pilots, customers, and business airplane builders for about 15 years. After Raytheon had spent over $1Billion on the Starship (about 5 times the cost of developing an equivalent, garden variety jet) and then failed to find a market, no one seemed to care that Piaggio succeeded in its implementation of new technology, because the Avanti didn't sell well either.

And there are other facets to Piaggio's sales problems as well:

It takes a lot of time and money to establish a toehold in the business aircraft market and Piaggio has not yet invested enough there to fully establish its credibility to potential customers. In particular, a turbine airplane customer who might fly anywhere in the world wants assurances that he can get overnight turnaround on minor maintenance and servicing almost anywhere in the world. Piaggio can't do that yet.

Quite literally, turbine airplanes are sold for luxury and status to the richest .00003% (roughly 1 in 3 million!) of the world's population. With turboprop airplane prices generally equivalent to a same-size business jet, the turbine market usually prefers the quieter cabin and higher speed of a jet. (It may seem counter-intuitive, but it actually costs a bit more to design and produce a turboprop than a jet, because engine integration and control for the turboprop is more difficult.) The jet is not as efficient, but it is usually quieter (inside), faster, and more prestigious; and those who can afford them are not much concerned about fuel costs. In general, the fraction of sales going to turboprops has slipped over the past 20 years compared to jets.

I also wonder how quiet the Avanti cabin is. The props operate in the wing wake and must, therefore, generate considerable external and internal sound signatures. These can be controlled, but are they? and at what weight penalty for the soundproofing?

Finally, the airplane is so optimized for low drag that it appears to be difficult to maintain. This may not be the case, but access to systems appears to be more constrained than on competing airplanes. One example is the heated leading edge of the wing. Other manufacturers make this a removable assembly because it often requires maintenance or parts replacement (failed temperature sensors, bird strike, hangar rash). On the Avanti, there is no access provided. Also, access to avionics in the nose is more difficult because of the 3rd surface.

Unfortunately, while the major companies tend to appear hidebound by their entirely natural tendency to keep doing what has worked before, the new companies with new ideas often find those ideas much more expensive to implement; and then they don't have the capital to establish themselves in the marketplace. Avanti is slowly overcoming this

---------

Have not heard anything about a possible jet version of the Avanti, but it seems like a natural thing to do, except for one thing. Swapping out the props for jets might make a worse airplane - not because there would be anything wrong with it, but the turboprop is such a magnificently optimized design. It would take a very careful look to be sure that a jet has anything to offer.

I know this seems to conflict with my comment above about a jet being preferable, but when you have a baseline for the market to compare, you have to show a real difference. You might remember that when Mooney put a Porsche engine in its airframe, the new airplane cost more, had lower payload, and was slower. Of course, it didn’t sell. They could have succeeded if they’d done a new airplane for the engine. A new design would have allowed them to hide the negative aspects of the engine and fully exploit the positives. Piaggio may have the same kind of problem with simply putting a jet into the Avanti.

About 50% more fuel volume would be needed in a jet version of the Avanti, and the airplane's gross weight would increase; so it would probably need a new or modified wing, associated changes in the other two lifting surfaces, and maybe beefed up gear and fuselage components. That could be a $60 Million program, depending on the scope of the changes.

 

[JohnFortier]

Miper,

Good thoughts on the "jetifying" of the Avanti. One of the things which attracted me to the aircraft in the first place was the fact that it was a turbo-prop. A turbo-prop with jet performance but prop economics, which is a hard combination to beat.

Apart from the sheer looks of the thing, one of the Avanti's biggest selling points, to me, is the combination of economy and performance. I'm not among those who will ever become careless about economy, unlike, as you said, many corporate jet owners. I have investors and it's not, in reality, my money to waste.

Intersting point about noise. I remember, back when I was young and remarkably stupid, stepping out of the door of a perfectly serviceable Hercules at 2000 ft and, before my 'chute damn near castrated me, being relieved to get out of the noise in the aircraft. It's said that the Hercules is so quiet on the outside because it concentrates all the noise inside.

Now I don't have proof of this, but I suspect that the Herc' fuselage is resonant at or near the frequency generated as the compression wave from each prop blade strikes the fuselage. I know that the noise actually varied as the RPM varied, with quiet and loud points occuring, which would tend to confirm this assumption. The flat side of the fuselage would simply act as a sounding board for the compression waves and the lack of any real damping material inside (apart from the bodies of myself and the other would be escapees)just made the situation worse. This is probably close to a worst case as far as turbo-props are concerned.

The Avanti props, on the other hand, will produce compression waves which strike the fuselage at the narrowest point as far as elevation area is concerned, that is, just before the swept back vertical stabilizer. This won't eliminate resonance effects, but the rounded fuselage and smaller area of compression wave impact will certainly reduce it.

Another factor is the fact that the Avaint is pressurised to sea level up to 24000 feet. (Which seems a bit excessive, but I'm not going to complain). This means that the fuselage, apart from its intrinsic stiffness, will increase in stiffness as the aircraft climbs. This will reduce fuselage resonance and therefore noise. Which reminds me - I must check to see if the bagage compartment is pressurised.

One other point: Piaggio themselves don't claim that the stabilator ever provides lift, so my thoughts about trimming for three airfoil lift in the cruise were wrong. The tailplane is there to ensure that the aircraft gets the best of both worlds, lack of trim drag and continuance of control if the foreplane stalls.

There have been some remarks about the possibility of the foreplane wash impinging on the mainplane. Actually, looking at the front elevation of the aircraft it can be seen that the foreplane is actually much lower than the mainplane. The foreplane actually has anhedral which, if I remember correctly, will reduce tip vortices. The downwash from the foreplane will pass safely under the mainplane even in landing configuration, unless the aircraft is in a ludicrous attitude. The stabilator is mounted at the top of the vertical stabilizer, which means that it is well out of the way of either fore or mainplane turbulance, so generally, there is a complete lack of interference between horizontal surfaces.

I had at first thought that the swept back vertical stabilizer was a bit of Italian design! About as pointless as the equivalent on the Cessna. However, if the stabilators are mounted at the top of the vertical stabilizer the sweep back increases the distance between the stabilators and the mainplane, increasing the moment of rotation on the aircraft when the stabilator is operated. So - cool and practical - very Italian!

Your points about maintenance are well taken. It may be that Piaggio have gone too far with the smooth skin effect. It needs looking into. However, I would assume (and like all assumptions this may very well be wrong) that Piaggio have got the maintenance details right as well. For example, were I to design a heated leading edge which was sealed, I wouldn't place any fail prone elements, such as temperature sensors, actually in the leading edge itself. the skin of the fuselage may not, due to compression effects, be at the exact same temperature as the wing leading edge, but a sensor placed on the fuselage skin and connected to the heater control via a very simple processor will compensate for temperature differences and still turn the heating elements on when required. The heating elements themselves would be very robust and, obviously, powered via circuit breakers. As far as the foreplane interfering with avionics maintenance, I'm pretty sure that this is something the techs will get used to. As long as they don't dent the foreplane with their heads too frequently!

The more I investigate this aircraft, the better it seems. If you throw out your preconceptions about what an aircraft "ought" to look like, its a very clever, advanced and practical design. It's gotten to the point where I'm actually looking for faults - for example, that landing gear is going to take up fuselage space, or, two thousand miles isn't THAT far!

Regards

John

 

[Sparweb]

A few answers that I can provide:

The Avanti is pressurized to a maximum of 9 psi differential, which is 3 psi greater than the typical turboprop. The consequence is that the fuselage skins are thicker than usual, too.

There are many things that change when a leading edge is drooped, any one of which could have been the reason for Piaggio doing so on the main wing of the Avanti.

The baggage compartment is behind the aft pressure bulkhead, therefore, not pressurized. It's not terribly big, either. The pilot I talked to would have preferred to have an auxiliary fuel tank there.

The foreplane is definitely in the way of avionics work, but in exchange, you get a handy work table at waist height(the anhedral isn't so steep that your screwdriver won't roll off).



STF

 

[Miper]

John,

The economy of turboprops appeals to many buyers who share your view. Perhaps the most notable was Sam Walton, who was famous among jet sales teams for refusing to have a jet because he considered them to be wasteful and projected the wrong image to his investors, employees, and customers.

The science of interior noise propagation and suppression is still incomplete, but your observations are correct. Turbopropeller noise is mostly concentrated in the plane of the prop, so it is important to keep the prop as far as possible from the fuselage. Piaggio did a better job of this than Rutan on the Starship configuration he sold to Beech. The props on the Starship are only about a foot away from the fuselage, which is a very stiff carbon-epoxy composite sandwich that has scant natural damping. The sound energy levels in the cabin without noise treatment are said to be painful.

Raytheon ended up with 4 separate soundproofing treatments on the Starship. 1. Small mechanical absorbers that were mounted on the fuselage wall between the windows at ear height. They vibrated out of phase with the blade passage frequency at cruise. 2. Two layers of heavy, leaded vinyl on the aft pressure bulkhead, on the floor, and on the entire fuselage shell from the aft bulkhead to the cockpit. 3. A fiberglass layer to absorb higher frequency noise. 4. Unusually heavy interior trim panels that may have included more lead for dampening low frequency noise.

Detailed analyses of canard configurations by Ilan Kroo and others show that, from a minimum interference drag standpoint, the best vertical location for a canard is well above the main wing. This is similar to the result from biplane theory, that shows positive stagger to be better than negative stagger. Canard test results confirm this conclusion, even if it isn’t intuitive. Like you, I had thought a low canard was better before I saw these results. In any case, few configuration lend themselves to a high canard, and the proposed Boeing Sonic Cruiser is one of the few I’ve seen.

The front view of the Avanti shows that the canard tips are approximately in line with the inboard edges of the nacelles. This arrangement certainly minimizes the effect of the canard tip vortex on the wing.

I have learned not to be too sure of where the canard and wing wakes will go. We rely on water tunnel and wind tunnel tests for that, and even then we get surprised. The aft fuselage strakes on the Avanti are probably not part of the original design. They are, no doubt, a late fix added in flight test to pass some extreme attitude certification point. Was the problem due to impingement of one wake on another surface? Probably yes, but only the people involved know for sure.

Again, I consider the Avanti configuration to be one of the most elegant and sophisticated aerodynamic designs ever done. Please understand that I only pointed out some possible drawbacks because the original question in this thread asked for them. Whether these things are real or important drawbacks is another thing entirely.

 

[Miper]

SparWeb,

My comments about outboard leading edge droop were made with the Eagle-X in mind. Does the Avanti have droop? I can’t see any in the photos I looked at on the web. The usual use of drooped leading edges on the outboard wing is for improved spin entry / recovery on light planes . Jim Patton and Paul Stough had excellent success with this solution in a famous NASA G.A. spin program (late 1970s – early 1980s). The Cirrus airplanes and several others use the outboard drooped leading edge. Anyway, as you say, many things change with droop, so there can be several reasons for doing it.

Fuselage tanks are sometimes necessary, but designers and operators don’t like them much because they add significant complexity to an otherwise simple fuel system. Also, in-flight fuel and CG management is often required, which pilots complain about. FAA & JAR rules require that the tank not be placed in the rotor burst plane of any turbine engine wheels. That’s taken as a (plus & minus)5 degree disk about the engine centerline at each wheel. This limits possible locations for a tank. Right now, the P-180's aft baggage is in line with the turbine wheels, so Piaggio would probably have to move almost everything in the tailcone to accommodate a fuselage tank.