Francis Donaldson's thoughts on Evans Volksplane VP-2

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cluttonfred

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We've had lots of Volksplane discussions over the years including detailed comments by FritzW on how to lighten and improve a VP-2 here -- Enclosing an Evans Volksplane VP-2 -- and later in the same thread.

Recently I wrote to the LAA (British EAA) where I am also a member, asking about a type sheet or any required modifications to the VP-2 as the design is still on their approved homebuilt list as "063 EVANS VP-2, VP-2 (MODIFIED) Group A." I got this nice reply back from LAA Chief Engineer Francis Donaldson and his permission to share his thoughts here. I thought it would be good idea to record them in this forum for future reference.

Dear Matthew

We don’t have a data sheet for the VP2 because there are none being built in the UK at the moment.

It was not as successful a design as the VP-1, as most in the UK turned out either too underpowered or too heavy to be effective or ‘legal’ as a two seater, and if flown one up they don’t fly as nicely as the VP-1 – even more sluggish ailerons, in particular, and they need the much more expensive big-bore VW engines to perform whereas the single seater goes OK with a 1500 or 1600.

I write this as one who used to own a VP-2 and carried out a lot of flying on it, with an 1834 engine, I’ve also flown several other VP-2s as well as owning a VP-1 and flying that extensively, and other VP-1s too, so am in a good position to give a fair summary. In the UK, there are quite a few VP-2s layed up unused and unloved even though in good condition.

It seems as though the VP concept worked with the single seater but the two seater was very much compromised. The particular feature which causes poor aileron response (part from the very crude aileron design) is the lack of wing torsional stiffness which makes the wing flex when the ailerons are deflected, opposing the aileron action. It would benefit from the wing torsion boxes being extended in span, and shorter chord, longer span ailerons but that would all add more weight still.

The LAA does not require any mods to the VP-2 over and above the updates issued by Bud Evans, which come with the plans.

The design does have the ability to give a unique flying experience to a light passenger if left as open cockpit - my wife-to-be and I had a lot of fun with ours but she only weighed 55 Kg and I only weighed 77 Kg back then. The extreme exposure of the open cockpit makes it like flying a pioneering aeroplane of the Edwardian era, and one feels companionship with Louis Bleriot etc. All this is lost if you put an enclosed cockpit on it, which adds more weight and you end up with a very mediocre single seater much worse than a Jodel D9, say.

Kind regards,

Francis Donaldson
All this suggests to me (as others have said before) that "simplicate and add more lightness" should be the motto of a VP-2 build with the possible exception of some aileron torsion boxes to improve roll response. Oratex fabric, lightweight wheels and tires, and that little Verner 5VW or maybe a Suzuki G10 conversion are sounding better and better. ;-)
 

plncraze

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Thank you for sharing this! The VP-2 is weird in that the plans are online but there are not a lot flying. Budd Davisson has a pilot report on Airbum and it doesn't sound that bad. Francis explains it pretty well. Maybe some wing tips would help it too.
 

cluttonfred

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I am not discouraged by Donaldson's assessment and told him as much, saying I was well aware of the VP-2’s shortcomings but for whatever reason it’s a design that has captured my imagination for many years. In some ways, that’s as important to an amateur build as raw performance. That second conversation is what prompted that last paragraph about flying with his wife which was from a subsequent email and added at his request. The key here is *not* to change anything you don't have to and just make small tweaks to save weight wherever possible.

PS--Here's a question...what would be the lightest way to stiffen the existing wing structure in torsion to improve aileron effectiveness without major design changes?
 
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FritzW

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...what would be the lightest way to stiffen the existing wing structure in torsion
A simple torque box like the VP-1 would probably be the lightest and easiest way. I still don't understand why Evans put the TB in the middle of the wing instead out at the tip. It seems like it would be a lot more effective out at the tip.

Another way would be a Fisher style lattice. It would be a lot more work than a TB but would save you from spending a small fortune on turnbuckles.

Untitled.jpg

I think the only reason Evans used a bunch of full ribs close together, instead of fewer ribs spaced farther apart with nose ribs, was to get away from needing a TB. It didn't work out, the VP-2 still needs a TB so there's no reason to stay with full ribs every 10".

If you went with Stewart Headwind'ish ply ribs (truss instead of round holes), VP-1'ish rib spacing with nose ribs, put a torque box at the wing tip and fixed the aspect ratio of the ailerons, you'd have a lighter wing that flew much better.
 

fly2kads

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PS--Here's a question...what would be the lightest way to stiffen the existing wing structure in torsion to improve aileron effectiveness without major design changes?
Good options from Fritz! One more....

Changing the treatment of the leading edge may be of some benefit. The plans call for aluminum, but also note that 1/16 in. ply is an option. Regardless of the flavor, the leading edge is affixed to the ribs only, not the spar, making it more of an aerodynamic fairing than a typical D-cell structural element. (Unless I missed something, the wing walk is the only surface tied directly to the spar.) Adding wood strips to the top and bottom of the spar in between each rib, and attaching the leading edge material to that also, should stiffen things up. The challenge is that the spar isn't very deep, and such filler strips would need to be relatively tall.
 

cluttonfred

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Thanks, Fritz. I was also thinking that the torque box should go at the tip (expand the small plywood area to a full bay) but wondered if there was something I was missing.

How would a geodetic lattice work in practice? Would you need stick ribs or at least capstrips to avoid trying to glue to plywood edges? Would you reduce the rib contour by the thickness of lattice?

What about using Ercoupe-style, Warren-truss ribs between the spars to eliminate the torsion box, wire bracing, and compression struts?

fly2kads, that’s a neat idea but you’re right that it would add a lot of dead weight to the spars. I do think the ply vs. aluminum leading edge is preferable.
 

TFF

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The lattice could be implemented with the least change to the wing. It could be implemented to a finished wing. Look at Pop’s plane thread for some pictures.

Adding another torque box outward would not be that big of a deal if you plan for it.

Anymore than those two ideas would require real thinking to make right.
 

FritzW

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How would a geodetic lattice work in practice? Would you need stick ribs or at least capstrips to avoid trying to glue to plywood edges? Would you reduce the rib contour by the thickness of lattice?
The lattice would cross the ribs somewhere on the red lines. You'd just need to cut slots where they pass through (easier said than done).

45 rib layout lattice.jpg
 

cluttonfred

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Eeek, I thought the lattice went on top and bottom of the ribs, slotted through would be a real pain in the keister. I can live with $250 in turnbuckles.

The wingtip torque box seems easiest of the ideas to implement, with our without changing the aileron itself. Would the torque box be 1/8" ply top and bottom, and would you want to add blocking between the spars to make a continuous box that included the spars themselves?

Random thought...what about tapering the the stock aileron to about half it's chord at the tip to lighten up the feel?
 

mcrae0104

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I still don't understand why Evans put the TB in the middle of the wing instead out at the tip. It seems like it would be a lot more effective out at the tip.
Untitled.jpg
Remember, the torsion in the wing accumulates from the tip (just a tiny bit there) back to the root, where it is the largest. Each bay between ribs adds its own share of torsion (in proportion to the lift at that point on the span) and then passes it inboard to its neighbor.

Starting back at the root where the torsion is greatest, it's carried by the struts and drag/anti-drag cables (as a space frame).

Just outboard of that, where the torsion is slightly lower, it's carried by the green torsion box.

Outboard of that, I suspect that Mr. Evans decided that the front and rear spars were sufficiently stiff to carry the torsion as cantilevered beams.

A torsion box at the blue location would stiffen only the one bay at the tip--the one bay that doesn't need the added torsional stiffness because it's carrying so little lift and therefore has so little pitching moment--but it does nothing to help the bays inboard of that. The torsion has to be passed inboard, each bay to its neighbor. Deleting the green torque box would probably overstress the fwd/aft spars.

The D-tube shown on the "VP-2 MkII" does the work--no need for the skin out at the tip bay.
 

FritzW

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Remember, the torsion in the wing accumulates from the tip (just a tiny bit there) back to the root, where it is the largest. Each bay between ribs adds its own share of torsion (in proportion to the lift at that point on the span) and then passes it inboard to its neighbor.

Starting back at the root where the torsion is greatest, it's carried by the struts and drag/anti-drag cables (as a space frame).

Just outboard of that, where the torsion is slightly lower, it's carried by the green torsion box.

Outboard of that, I suspect that Mr. Evans decided that the front and rear spars were sufficiently stiff to carry the torsion as cantilevered beams.

A torsion box at the blue location would stiffen only the one bay at the tip--the one bay that doesn't need the added torsional stiffness because it's carrying so little lift and therefore has so little pitching moment--but it does nothing to help the bays inboard of that. The torsion has to be passed inboard, each bay to its neighbor. Deleting the green torque box would probably overstress the fwd/aft spars.
err... You've got the wrong torsion load. We're talking about the wing twisting when the ailerons are deflected, not drag/anti-drag.

The D-tube shown on the "VP-2 MkII" does the work--no need for the skin out at the tip bay.
The D tube doesn't do any work, it's purely aerodynamic. The drag/anti-drag loads are handled by conventional compression struts and cables and the twist is handled by the torque box.
 
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FritzW

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Just to clarify...

On a VP-2, you can stand at the wing tip and push down on the front spar and lift up on the rear spar and the wing will twist. When it does the cockpit twists also. If you twisted hard enough you'd probably break the cockpit gussets. The ailerons can't apply enough force to break anything but they do twist the wing enough to make for some pretty sloppy roll control.

Just like a crowbar, a little force at the wing tip translates to a lot of force at the root. Stopping the movement at the wing tip is easy, it gets harder and harder the closer you get to the root. A torque box at the root would have to be massive, it's on the wrong end of the crowbar.
 

plncraze

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What the world needs is a metal VP-2 with staggered seats to hold two "average" Americans.
Fritz, have you gotten to where you can think of something then sit at the computer do an illustration? You create some great pictures. Thanks!
 

mcrae0104

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err... You've got the wrong torsion load. We're talking about the wing twisting when the ailerons are deflected, not drag/anti-drag.
Like you, I am talking about torsion about the spanwise axis of the wing. (I did mention drag/anti-drag cables above, since they are a part of how the VP design reacts this torsion into the fuselage section, but I am not speaking of drag/anti-drag forces per se.)

The D tube doesn't do any work, it's purely aerodynamic. The drag/anti-drag loads are handled by conventional compression struts and cables.
If the D-tube doesn't take the torsion then something else will need to.

On a VP-2, you can stand at the wing tip and push down on the front spar and lift up on the rear spar and the wing will twist.
Right. What I'm saying is that putting a torsion box out at the tip doesn't help unless the structure inboard of that is torsionally stiff as well. The tip section has to pass the torsion on to the next bit of the wing, which contributes it own moment, and then passes that combined total to the next bit of the wing. If the next bit of the wing lacks the ability to resist the torsion, the wing just twists (torsion deflection).

Imagine a wing made up of three parts. The outboard part is a nice stiff torsion box. The middle panel is made of solid foam like pool noodles are made of. Finally the inboard panel at the root is very stiff just like the outboard tip. Now when you stand out at the wing tip and twist it, it's only as stiff as the weakest link. The middle section twists, while the outer and inner panels stay flat--but because the outer panel is attached to the middle panel, it is rotated into a different plane. This is what will happen when you remove the torsional stiffness from the green area and increase the torsional stiffness at the blue area. It makes the structure heavier and weaker.
 

cluttonfred

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mcrae0104, I think there is one point here that you misunderstood. The green area is a VP-1-style torsion box shown in a corresponding area on the VP-2 wing plans. The VP-2 has *no* torsion box as designed so any torsion box at all would be an increase in torsional stiffness compared to the stock design. On the VP-2, I have never heard of any structural issues, the impact is only on the feel and effectiveness of the ailerons.
 

FritzW

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what happened to that set of wings you were building with a friend?
The lumber yard DF spar stock he bought turned out to be pretty bad. It looked good in the rough but coming out of the planer it started to show some real problems. By the time the spars were CNC'd we could see some real no-go problems with the grain.

We finished up the ribs, a bunch of misc wood parts and all the aluminum parts. Last I heard he was looking for new spar stock.
 

mcrae0104

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mcrae0104, I think there is one point here that you misunderstood. The green area is a VP-1-style torsion box shown in a corresponding area on the VP-2 wing plans. The VP-2 has *no* torsion box as designed so any torsion box at all would be an increase in torsional stiffness compared to the stock design.
OK that's fine if the green part doesn't exist, but adding torsional stiffness at the tip does no good. When you have something less stiff inboard, you are just wasting weight stiffening up the outer portion for torsion.
 

FritzW

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If the D-tube doesn't take the torsion then something else will need to.
The torque box.

What I'm saying is that putting a torsion box out at the tip doesn't help unless the structure inboard of that is torsionally stiff as well
adding torsional stiffness at the tip does no good
It would be MUCH harder to stop the twist at the root than at the tip.

A little force at the wing tip creates lots of force at the root. Putting the torque box at the root would be like trying to loosen a nut with just socket and no ratchet handle.

Imagine the airplane with no fabric. There's a force trying to twist the wing. Would you rather try to stop the twist by holding the spars at the root or the tip?

twist.jpg
 
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