V-173 / XF5U - Flying Flapjack

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sanman

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Another concept that interested me was the Vought V-173 / XF5U Flying Flapjack





My understanding is that this VSTOL aircraft concept was also capable of very high speed, and I was curious as to whether it could be built in a more updated form as an ultralight.

The original aircraft had some complicated gearing with a common shaft linking its 2 propellers. Each of those wooden propellers was so large that their blades were hinged to allow some flapping.

I was thinking that the prop blades might not have to be so large, and could also be thinner, and made of composites capable of more flexion than wooden props.
This might avoid the need for the blade-flap hinges, and could allow fixed-pitch propellers.

I was also thinking that appropriate control electronics would avoid the need for the gearing with the shaft linking the 2 props.
So you could have 2 small gasoline-powered engines, or possibly just go with 2 electric motors and have a gasoline-powered generator to extend range.

But this idea of a larger wing-body for VSTOL seems quite interesting.

Any comments or feedback?
 
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Victor Bravo

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I spent a lot of time thinking about the V-173 configuration, as I'm sure a lot of others here have done.

My personal ideas, based on an amateur-level, single-digit IQ assessment, were:

Having two powerplants would create a huge risk, because the propellers are out at the tips. One side powerplant failure would make it impossible to control, except for completely cutting the good engine and having a bad glider. There would likely be no "single engine minimum control speed".

There would not be a lot of room for engines out near the tips anyway. There is room of course for electric motors out there, but the cost of the motors and batteries would be pretty high, and make the thing awfully heavy. When the "magic battery" arrives, we can revisit the electric option. But for now, a Flapjack would be better off with gasoline.

The safest and perhaps easiest way would be to have the two propellers connected physically. A composite spanwise cross-shaft, two 90 degree gearboxes, and two shorter propeller shafts. When one propeller moves, the other one moves an equal amount, regardless of the power level, engine failure, etc. One of the main shafts would have to fail catastrophically before you need to cut the power and glide.

With the spanwise cross-shaft, you can drive it from anywhere, which allows you to siimplify and save money, and use ONE engine amidships instead of the two engines in the V-173. Any reliable flat four or flat six aircraft engine could be mounted in the middle of the wing, behind the main spar, with the crankshaft axis running spanwise, and a multi-groove belt or cogbelt running back to a wheel on the cross-shaft.

You'd need a helicopter style cooling shroud and possibly a cooling fan for ground operation, but you'd have very good cooling in flight because of strong differential pressure from the ambient air directly below the engine and directly above it.

With the cross-shafting you can have counter-rotating propellers opposing the tip vortex flow, just like Zimmerman's original experiments, and it is likely you could get the tremendous low-speed capability of the original Flapjack because of this.

Also, please understand the V-173 was probably not a "high speed" airplane with that landing gear sticking out like that. Now the XF5U woulda-coulda-shoulda been a high speed airplane, but that airplane either never flew or flew once and was never verified to have flown, and not ever measured.
 

berridos

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What i like is that those engines at the tip rotate each one i guess to the outside, avoiding the creation of a tip vortex. Possibly the effect is that the plane has in reality a much larger AR than based on the actual planform.
 

sanman

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That UFO design looks interesting too. What if you could have that UFO with coaxial contra-rotating props driven by one engine?

Is UFO perfectly round? Maybe a slightly elongated ellipsoid shape would be best.


But back to V-173 / XF5U , if you made it electric then you could call it the e-STOL

Just because you have electric motors driving each prop, doesn't mean that it can't all be powered by gasoline as well.
It could sort of be hybrid powered, with battery+gasoline.

The electric motors would eliminate the need for the gearing and the common shaft.
You could have a small low-hp gasoline motor that would be continually charging up a capacitor bank that stores a 10-minute reserve of electric power.

The gasoline motor would be optimized for a very narrow power band, since it's just acting as a generator.

If you see the 2nd video I posted, it shows the RC model of V-173 folding up its landing gear after takeoff, and flying that way.

However, I wonder if an aircraft with folding landing gear can be a decent bush plane?
 

fly2kads

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I have had the pleasure of walking around the restored V-173 several times now, and it impresses me every time. I love how it combines low-tech (structure) and and high-tech (power train) together into a quick and dirty prototype.

I think the concept could prove interesting for a new, personal-size version. The deck angle on the ground would take some getting used to!
 

sanman

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Imagine having a single gasoline engine drive both propellers, but it's just that instead of using gears and shafts to transfer power from the engine to the props. you'd use wires to transfer that gasoline engine's power to the electric motors attached to those props (with a supercapacitor bank in between as a buffer)

As you say, the original structure was low-tech, and can still be done low-tech. You don't need carbon composites for this thing.

It seems to me that this round (elliptical?) wing body shape should be capable of both STOL and high speed flight while being very resistant to turbulence and flipover by cross-winds. I'm sure we've all seen the video of what happened to Mike Patey in his Draco:


The elliptical wing-body reduces the overall size dimensions of the aircraft, and its low aspect-ratio means less vulnerability to cross-winds.
Those cross-winds have less leverage over the aircraft, even if the overall wing-loading is low. This improves the safety of the ultralight aircraft.
 

sanman

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...
But for now, a Flapjack would be better off with gasoline.

The safest and perhaps easiest way would be to have the two propellers connected physically. A composite spanwise cross-shaft, two 90 degree gearboxes, and two shorter propeller shafts. When one propeller moves, the other one moves an equal amount, regardless of the power level, engine failure, etc. One of the main shafts would have to fail catastrophically before you need to cut the power and glide.

With the spanwise cross-shaft, you can drive it from anywhere, which allows you to siimplify and save money, and use ONE engine amidships instead of the two engines in the V-173. Any reliable flat four or flat six aircraft engine could be mounted in the middle of the wing, behind the main spar, with the crankshaft axis running spanwise, and a multi-groove belt or cogbelt running back to a wheel on the cross-shaft.
I just had another idea. What if each gasoline engine has a dynamo/generator on its propeller shaft, to generate electrical power when required -- and that same dynamo can be turned into an electric motor to spin the propeller in the event that the gasoline engine quits? That electric motor would be fed electric power from the other remaining operational engine, which would function in a dynamo/generator mode, thus transferring power electrically to run the propeller whose engine has quit. Then you don't need the common shafts or complicated gearing - all you need is a clutch to disengage your bad engine from the propeller shaft when required, so that the electric motor is able to take over.

What's wrong with that idea? Are there any issues or drawbacks to it?
 
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Doran Jaffas

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I spent a lot of time thinking about the V-173 configuration, as I'm sure a lot of others here have done.

My personal ideas, based on an amateur-level, single-digit IQ assessment, were:

Having two powerplants would create a huge risk, because the propellers are out at the tips. One side powerplant failure would make it impossible to control, except for completely cutting the good engine and having a bad glider. There would likely be no "single engine minimum control speed".

There would not be a lot of room for engines out near the tips anyway. There is room of course for electric motors out there, but the cost of the motors and batteries would be pretty high, and make the thing awfully heavy. When the "magic battery" arrives, we can revisit the electric option. But for now, a Flapjack would be better off with gasoline.

The safest and perhaps easiest way would be to have the two propellers connected physically. A composite spanwise cross-shaft, two 90 degree gearboxes, and two shorter propeller shafts. When one propeller moves, the other one moves an equal amount, regardless of the power level, engine failure, etc. One of the main shafts would have to fail catastrophically before you need to cut the power and glide.

With the spanwise cross-shaft, you can drive it from anywhere, which allows you to siimplify and save money, and use ONE engine amidships instead of the two engines in the V-173. Any reliable flat four or flat six aircraft engine could be mounted in the middle of the wing, behind the main spar, with the crankshaft axis running spanwise, and a multi-groove belt or cogbelt running back to a wheel on the cross-shaft.

You'd need a helicopter style cooling shroud and possibly a cooling fan for ground operation, but you'd have very good cooling in flight because of strong differential pressure from the ambient air directly below the engine and directly above it.

With the cross-shafting you can have counter-rotating propellers opposing the tip vortex flow, just like Zimmerman's original experiments, and it is likely you could get the tremendous low-speed capability of the original Flapjack because of this.

Also, please understand the V-173 was probably not a "high speed" airplane with that landing gear sticking out like that. Now the XF5U woulda-coulda-shoulda been a high speed airplane, but that airplane either never flew or flew once and was never verified to have flown, and not ever measured.
Keep the center line thrust. Make it a 2 place side by side. Sell me plans. I'll build one.
 

Hephaestus

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In the 80+ years since zimmerman and arup - we've gotten a whole new understanding of tip vortices control.

Strongly suggest looking into the options other than throwing a motor there.
 

wsimpso1

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I do find every suggestion of electric and gasoline adjacent and then running one or the other to be challenge. 23 years doing automotive trannies.

Connecting them together in such a way as to allow normal ops plus all of the failure modes requires some fancy footwork, to put it mildly. There are several levels of sensing, computer intervention and hardware (clutches, OWC, etc) that you can select from to do it, but it will be pretty big engineering effort. Then there is all of the torsional vibration management.

The single simplest scheme is like a helo. OWC on each prime mover to a gearbox allows each prime mover to come off line. Then you sense prime mover torque and rpm, looking to set alarms when one is lost, but the rest of the system operates. Lose a shaft or any gearbox and it is a glider.

After that is electric drive with an engine driven generator electric motors. Very little mechanical stuff, but lots of power electronics and software.

We already have an active thread on the topic.


Even those should be enough to make most of us just stick a nice light prop on the end of the engine, and go.

But if you just have to do it, have at it.

Billski
 
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sanman

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Here's some other aircraft with the similar round wing idea that I came across while surfing around.

The Nemeth Parasol:


The Arup S-2:



This is a model plane based on the Arup, but it's done as a high wing:



It's that last one that I find most interesting. It has that high Angle-of-Attack for STOL like the V-173 / XF5U.
But because it's a high-wing design, the undercarriage legs don't have to be so darn long.

To me, this last design has the makings of a good bush plane with good STOL characteristics.

The one thing I would correct is the cockpit & fuselage have to be aligned with the wing and the engine thrustline.
That would allow the cockpit to be level during cruise, just like the wing itself should be, which is what the V-173 / XF5U does.

So, high AoA upward-tilt of wing/thrustline/cockpit during taxi/takeoff & landing, and low AoA level-tilt of wing/thrustline/cockpit during cruise flight.

Here's an example of an aircraft with the cockpit suitably aligned with the STOL-thrustline:

 
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rotax618

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These low aspect circular/ellipsoid should be pretty simple to build, if you look at David Rowe’s UFO there are no ribs only stringers curved over a few very lightly built spars, the spars can be very light because of their depth and short span.


629C44EA-9443-4395-A6AC-1EB4BDE46C64.jpeg5F85E3CE-ADBD-42B5-98A3-3A7D968A4FBF.jpeg
 

sanman

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I could imagine a 2-place version as a tandem.

Here's another vid of the high-wing version:



But regarding UFO itself, he's mentioned that he enters and exits from a hatch on the underside of the aircraft.
Watch him sticking his legs out of the bottom of the aircraft at the end of this video, while taxiing to a stop after landing.



He'd improved the UFO-2 with retractable landing gear, which I unfortunately can't find a video of.



I'm not sure how you could do a side-by-side 2-seater, since that might require a smoother, more conformal design.
 
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billyvray

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Here I am to the rescue! Two vids of Rowe's UFO with taildragger retractable gear. However, later on he went back to a fixed tricycle gear - I've seen more recent pics that you can clearly see the patched in panels. Enjoy!


 

drive330

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Several years ago, I became interested in developing an RPV based on the V-173 and it’s more powerful fighter version the XF5U-1. The allure of very high speed coupled with STOL and hovering capability was intriguing. I spent literally hundreds of hours searching for any and all information related to Charles Zimmerman and his low aspect ratio experiments. Surprisingly to this day there are some V-173 and XF5U-1 reports not available to the public at Langley field.

I set about building a radio-controlled version to explore the feasibility of such an RPV. A major issue confronting both the V-173 and the XF5U-1 was vibration (never solved) from the propellers and complicated drive systems. An RPV powered by electric motors would eliminate some but not all of the vibration issues.

The model I built and flew had on board an “Eagle Tree” data logger with an airspeed module. At the end of each test flight I evaluated the propulsive efficiency. It became readily apparent that like any very low aspect ratio airplane the L/D proved to be no better than a helicopter in powered flight of @ 5:1 to 6:1. Further tests were conducted in which the propellers turned opposite to the full-scale aircraft, that is in the same direction as the wingtip vortices. These tests showed no difference in performance other than the change of trim required in the “ailevators”. Overall, the model was pleasant to fly and I was able to achieve hovering flight as was also a never met goal of the full-scale aircraft.

There is a wealth of information available in the form of wind tunnel testing conducted by the NACA on the V-173 and scale models of the XF5U-1. One report in particular titled “NACA Full Scale Wind Tunnel Tests of Vought – Sikorsky V-173 Airplane” confirms what I experienced with my test model. Amongst the conclusions in the report was “A very slight improvement” (emphasis on “Very Slight”) in propulsive efficiency regardless of the direction of propeller rotation. More importantly, with the propellers turning counter to the vortices rotation resulted “Large decreases in longitudinal stability”. The conclusions further note that “Rotation of the propellers opposite to the present arrangement so that they turn in the same direction as the tip vortices would result in greatly increased stability with only slight decreases in the propulsive efficiency”.

In essence the NACA testing revealed that Zimmerman’s premise of increasing the effective aspect ratio with propellers rotating opposite the direction of the vortices failed to work in practice. I read that Zimmerman was presented with the findings of the report but discarded them.

The projected performance numbers for the XF5U-1 put it at a disadvantage in several areas of performance in comparison to contemporary Naval fighters such as the Corsair and Hellcat. Real world flight test reporting of the quirks of the V-173 can be found in the book “Whistling Death” by Boone T. Guyton. Much of the desirable low speed characteristics such as short landing and takeoff distances and high alpha flight of the V-173 can mostly be attributed to a very thick airfoil (NACA 0016) and very low wing loading of @ 5lbs. per square foot and not the propeller system. Additionally, the NACA tests revealed that the very large diameter propellers themselves contributed to lift at positive angles of attack much like a helicopter rotor. Another disadvantage of the Zimmerman layout is the inability to employ high lift devices such as flaps.

There is a lot of “Urban Myth” related to the superiority of Zimmerman’s aircraft and the cancellation of the XF5U-1 program. There is as usual the conspiracy theory followers who chose to neither believe or ignore the very sound basis for the Navy losing interest and ultimately cancelling these programs. Subsequently the jet age came along and any propeller driven aircraft was viewed as being obsolete.

Present day designers wishing to explore the Zimmerman layout would be well advised to adopt a more conventional layout. Keep in mind the KISS principle. Or better yet just purchase any of the fine aircraft available on the market to suit their needs.

In closing, Charles Zimmerman was a very creative and gifted designer. He generally thought outside the box. Witness his further experiments in manned flying platforms of the 1950’s that drew military interest. Zimmerman became the head of the “A” for aeronautics of the later day NASA successor to the NACA.

Chris Parkyn
 

sanman

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The allure of very high speed coupled with STOL and hovering capability was intriguing.
And that's what it comes down to - for me, this is the Holy Grail.

STOL + HighSpeed Flight + Range


What's then the best way to achieve this combination, in a cost-effective way suitable for personal aircraft?


Much of the desirable low speed characteristics such as short landing and takeoff distances and high alpha flight of the V-173 can mostly be attributed to a very thick airfoil (NACA 0016) and very low wing loading of @ 5lbs. per square foot and not the propeller system.
That's good news to me, since it means an improved design could skip those overly complex propellers and rely on the wing-body for the performance benefits.

Additionally, the NACA tests revealed that the very large diameter propellers themselves contributed to lift at positive angles of attack much like a helicopter rotor.
What's wrong with that? While a helicopter is unfortunately in that same mode for its entire flight, thus limiting its speed & range performance, the V-173 is only in that mode during the Takeoff & Landing (STOL) portions, but meanwhile can fly like a regular aircraft in between, thus achieving the higher speed & range. And it does this without resorting to the complicated mechanisms of the V-22 Osprey. It's main shortcoming was its other complicated mechanisms of the gears and shafts - which is a more solvable problem, in my opinion.

Another disadvantage of the Zimmerman layout is the inability to employ high lift devices such as flaps.
But given the ability of the vehicle to change the orientation of its entire body, doesn't that amount to the whole wing-body becoming a "flap"?
To me, that seems like the most "flap" you can get (other than maybe deploying a parafoil drag chute out the back)
 
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sanman

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Going back to the UFO, I do wonder about the safety of an aircraft where you have to enter from below:




What if your undercarriage collapsed, say on a hard landing? You'd be trapped inside your vehicle.
And yet I don't see any other way to get out. Did David Rowe include any alternate entry/exit on UFO?
How was entry & egress achieved on the V-173 / XF5U?


This high-wing design from an old thread on this forum looks appealing too, although likely heavier and less efficient than UFO:




Given that the UFO is so lightweight, perhaps it would be less likely to collapse a rigid undercarriage in a hard landing.

A retractable undercarriage would have advantages in speed & efficiency, though.

It would be great if there could be a UFO with retractable landing gear & tundra tires, which could be used as a STOL bush plane.
But I hear those fat bush plane tires can kick up rocks, which would probably hit the underside of the UFO and damage it.
I wonder if there's some way to make tundra tires that would kick rocks out to the side, instead of rearwards towards the wing-body.
Or else, you could have the nosewheel undercarriage like in the most recent video I'd posted above, and not be a tail-dragger.
But it seems to me that tail-dragger would be the more efficient landing with least amount of ground roll.


Here's the January 2019 edition of Australian Sport Pilot magazine, and if you go somewhere in the middle, you'll see their full article on the UFO:

 
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