Nenadović biplane

[cluttonfred]

As I mentioned over in the Name the plane. thread, Miroslav Nenadović (sometimes Nenadovitch or Nenadovich) was a Yugoslav aerospace engineer and longtime faculty member at the University of Belgrade. He did his graduate work in France in the 1930s notably in finding the most efficient combination of gap and stagger for a biplane wing. I am starting this thread to discuss the "Nenadović biplane" and share related reference materials. Here's the first one, more to come.

 

[Sockmonkey]

This brings to mind a question.
Does a wing setup like that give a better L/D ratio than a single larger wing of the same span and combined area of the two smaller wings?

 

[TFF]

It looks like a Cassutt or Sonerai with a second wing. The question that you ask the designer is why? Simple as because I thought it was cool to I needed to break a world record. That part of the story is missing.

 

[rotax618]

The aircraft with the Citroen engine in the second article, the props at the midspan of the upper wing would induce quite a large airflow over the lower wing, sort of like a “straight” channel wing. It would be interesting to see a photo of the aircraft.

 

[Martin R.]

It would be interesting to see a photo of the aircraft.

1:25 - 2.35

 

[cluttonfred]

 

[cluttonfred]

I am still looking for the original Nenadović thesis from his graduate work in France the 1930s. The contemporary reporting about it in France at that time suggested that the right combination of slotted wing was actually *more efficient than the equivalent monoplane* (presumably equivalent in overall span and area). If anyone has access to online technical research tools and can pull up a PDF of the original, I’d be very grateful.

“Recherches sur les cellules biplanes rigides d'envergure infinie”
Author: Miroslav Nenadović; Université de Paris
Publisher: Paris, E. Blondel la Rougery, 1936
Dissertation: doctoral Université de Paris 1936

 

[plncraze]

If you do a search for Robert Briand Addoms you will get a bit closer to the above mentioned thesis. Addoms replied to yet another paper about biplane efficiency in the AIAA Journal of Aircraft. Addoms himself did his doctoral thesis on an efficient biplane.

 

[cluttonfred]

I was able to pull up the attached article from French aviation review LES AILES back in 1935. I am not going to translate the whole thing, but here are some highlights and the key figures:

THE NEW SLOTTED BIPLANE HAS BEATEN THE PURE MONOPLANE
Recent experiments by M. Nenadovitch at the St. Cyr laboratory show that with a particular configuration of biplane one can see improvements over a monoplane of the same profile of 25% less drag, 15% more lift, and 51% better speed range.

• Nenadovitch's work builds on the work of Toussaint and Girerd, and was carried out under the supervision of Toussaint.
• To reduce the impact of a specific airfoil choice on the interaction between the two wings, a symmetrical Joukowski 14% airfoil was used for testing (here is a Joukowski 12%).
• The biplane configuration chosen for testing after much previous experimentation is: positive stagger of 1 chord, gap of 1/3 chord, and decalage of -6 degrees (lower/rear wing incidence 6 degrees greater than upper/front wing). [Note that décalage in French means stagger but in English that word has come to describe the relative incidence of the two biplane wings, which is called interinclinaison in French.]
• Note that the biplane wing polars outperform the monoplane both in terms of minimum drag Cx and maximum lift Cy and Cy/Cx (L/D). Pitching moment is given for each of the biplane wing individually but not together or the monoplane so no conclusions there.
• I believe that Cz in this context refers to a specific value of lift corresponding to the weight of the aircraft [please correct me if that is wrong] and the author goes on to point out that Cz max / Cx min (maximum load over minimum drag) is predictor of speed range, and here too this biplane beats a monoplane.
• The explanation for this surprising result, according to the author, is the slot effect between the two wings in which the low pressure above lower rear wing accelerates flow over the upper front wing and prevents turbulent flow, therefore reducing drag.
• Finally, the author makes a comparison between the wings of a dragonfly and a Mignet Pou-du-Ciel, suggesting that these examples also seem to benefit from the Nenadovitch effect to perform better than expected.

"At left, individual curves, drag and lift. At right, polars for the upper and lower wing and the whole Nenadovitch slotted biplane. It's worth noting that the biplane polar completely envelops that of the monoplane. At right, the moment curves for the two individual biplane wings."

Detail of configuration from graph above.

 

[karmarepair]

It's fascinating how the props migrated from the Trailing to the Leading Edge - I wonder which picture is more recent. I'm guessing the Camo version is newer. I also wonder if the props are being driven by belts, or ?

View attachment 111826
View attachment 111827

 

[Vigilant1]

• The biplane configuration chosen for testing after much previous experimentation is: positive stagger of 1 chord, gap of 1/3 chord, and decalage of -6 degrees (lower/rear wing incidence 6 degrees greater than upper/front wing). [Note that décalage in French means stagger but in English that word has come to describe the relative incidence of the two biplane wings, which is called interinclinaison in French.]

View attachment 111835

Thanks for the research and info, Matt.
If we disregard the slot effect, the lower rear wing would stall well before the front wing due to the 6 degree greater AoA it has. And if we assume the acft CG is somewhere between the two wings, that will tend to make the tail drop and the AoA of both wings to further increase (rear wing into a deeper stall, maybe front wing stalls soon). So, a positive feedback situation, at least until the front wing stalls or the nose falls far enough..
Maybe the slot fixes all that, or I'm missing something.

 

[Sockmonkey]

It's fascinating how the props migrated from the Trailing to the Leading Edge - I wonder which picture is more recent. I'm guessing the Camo version is newer. I also wonder if the props are being driven by belts, or ?

IIRC in the original it was belts.

Thanks for the research and info, Matt.
If we disregard the slot effect, the lower rear wing would stall well before the front wing due to the 6 degree greater AoA it has. And of we assume the acft CG is somewhere between the two wings, that will tend to make the tail drop and the AoA of both wings to further increase, so a negatfront wing to fur

The higher AOA on the rear wing seemed odd to me too until I imagined the front wing as being a really big slat for the aft one.

 

[cluttonfred]

The article in my first post mentions "Powergrip timing belts" as the mechanism used to drive the props. Those are fiberglass-reinforced toothed rubber belts still manufactured by Gates.

PowerGrip® Timing Belts | Gates Corporation

Today’s highly sophisticated car engines operate at higher temperatures, higher speeds, and produce more power requiring highly engineered materials to meet high mileage replacement intervals as demanded by OEM’s. Gates engineers automotive timing belts in partnership with dozens of top...

www.gates.com

And here is a cutaway (source unknown) showing the engine installation and prop drives of the AS 37 in pusher form. It's actually simpler than I expected and the pusher layout with props safely out of the way is starting to grow on me. The tractor version seems a little too likely to end up with someone putting a hand or worse in one of the props.

It's fascinating how the props migrated from the Trailing to the Leading Edge - I wonder which picture is more recent. I'm guessing the Camo version is newer. I also wonder if the props are being driven by belts, or ?

 

[cluttonfred]

Vigilant1, I don't think you can disregard the slot effect and still understand what's going on here. In this case the whole really is greater than the sum of the parts. That would be like measuring the performance of a slotted flap and its wing separately, which wouldn't adequately describe the behavior of the combination. As Sockmonkey said, it's better to imagine the two functioning as a rear wing with a big slat or a front wing with a big slotted flap.

If we disregard the slot effect, the lower rear wing would stall well before the front wing due to the 6 degree greater AoA it has. And if we assume the acft CG is somewhere between the two wings, that will tend to make the tail drop and the AoA of both wings to further increase (rear wing into a deeper stall, maybe front wing stalls soon). So, a positive feedback situation, at least until the front wing stalls or the nose falls far enough..
Maybe the slot fixes all that, or I'm missing something.

The higher AOA on the rear wing seemed odd to me too until I imagined the front wing as being a really big slat for the aft one.

 

[Vigilant1]

Vigilant1, I don't think you can disregard the slot effect and still understand what's going on here. In this case the whole really is greater than the sum of the parts. That would be like measuring the performance of a slotted flap and its wing separately, which wouldn't adequately describe the behavior of the combination. As Sockmonkey said, it's better to imagine2 the two functioning as a rear wing with a big slat or a front wing with a big slotted flap.

Understood. I'll have to look at those graphs on something bigger than my phone to see if the info is there.
Mark

 

[cluttonfred]

Here are two more pics of the AS-37 tractor variant, which to my mind illustrate why that combination of cockpit arrangement and prop location gives me the heebie-jeebies as an accident waiting to happen, though I suspect that the prop wash over both wings probably improved STOL performance. Yet another design that cries out for an affordable small turbofan to eliminate those pesky props. Somebody get on that, please. ;-p

 

[Vigilant1]

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And here is a cutaway (source unknown) showing the engine installation and prop drives of the AS 37 in pusher form. It's actually simpler than I expected and the pusher layout with props safely out of the way is starting to grow on me.
View attachment 111844

It's also handy that, due to the way the weight is distributed, the pusher props aren't behind the main gear. Especially on a STOL plane that can be expected to operate in the dirt, that could be tough on props.
But, it's definitely NOT a removable wing.
Has anybody seen published flight specifications for F-WYBQ in its pusher mode or its tractor mode? It would've interesting if the projected advantages of the split wing materialized:
"over a monoplane of the same profile of 25% less drag, 15% more lift, and 51% better speed range."

 

[Vigilant1]

Has anybody seen published flight specifications for F-WYBQ in its pusher mode or its tractor mode? It would've interesting if the projected advantages of the split wing materialized:
"over a monoplane of the same profile of 25% less drag, 15% more lift, and 51% better speed range."

FWIW, here's what Wikipedia shows for the AS-37A (split wing, twin prop pusher):
General characteristics

• Crew: 2
• Length: 6.00 m (19 ft 8 in)
• Wingspan: 6.30 m (20 ft 8 in)
• Height: 1.60 m (5 ft 3 in)
• Wing area: 13.60 m2 (146.4 sq ft)
• Empty weight: 400 kg (882 lb)
• Max takeoff weight: 620 kg (1,367 lb)
• Powerplant: 1 × Citroën GS 1220 4-cylinder air-cooled, 48 kW (65 hp)
• Propellers: 2-bladed

Performance

• Maximum speed: 185 km/h (115 mph, 100 kn) at sea level
• Cruise speed: 170 km/h (110 mph, 92 kn) at sea level
• Range: 1,500 km (930 mi, 810 nmi) with 90 L (24 US gal; 20 Imp gal) fuel
• Service ceiling: 4,500 m (14,800 ft)
• Rate of climb: 3.5 m/s (690 ft/min) at sea level
• Take-off run: 200 m (655 ft)
• Landing run: 140 m (460 ft)

_----------------------------
If it is to be believed, that's pretty darn good for a two seat plane running on just 65hp. The large disk are would have been a help for TO and climb, especially given the modest power available.

The Wiki article also indicates a simpler single prop pusher version with a single low tail boom was built (the "Knoepfli VSTOL")

 

[cluttonfred]

Well, it would be hard to measure those differences in real life without having two almost identical aircraft, or one aircraft with interchangeable wings, to limit the variables. I actually thought those numbers for the AS.37 were not terribly impressive as it was pretty heavy (400 kg/880 lb empty weight) for what is essentially a VW-powered two-seater, so it's hard to say from that example. The payload of 220 kg/485 lb for people, fuel, and baggage is not bad for such low power, but it sounds like the plane would have benefited from a serious diet to trim the empty weight.

I wonder if putting the props on airfoil-shaped masts projecting left and right from the fuselage above the rear wing might be more effective? You could have a removable, self-contained unit of engine, mount, and a T-shaped structure with the props on the ends of the T. The wing installation would then be much simpler and folding or removable wings could be an option. Better yet, put one large prop on a single vertical mast and give it a T-tail in the propeller stream rigged to counteract the nose-down tendency with power like many amphibians.

 

[Vigilant1]

. The numbers on the AS.37 are not terribly impressive and it was pretty heavy (400 kg/880 lb empty weight) for what is essentially a VW-powered two-seater, so it's hard to say from that example.

Thanks. No doubt it was heavy for the amount of power. The large fan area of the twin props came at a price in weight and complexity. Still, if a plane heavier than the max LSA weight really needed just 65 HP to climb at nearly 700 fpm and could cruise at over 100 mph, that's impressive. But, that's a big IF.
Hmmm-- One engine, two wings with belt-driven twin pusher props between them-- already done by some brothers in Dayton decades before.