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Old Aug 01, 2013, 10:58 AM
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XF5U flying pancake

Hiya all,
I hope someone will find this interesting. I've just finished (as far as I ever finish something!) my most recent scratch build - an XF5U.

xf5u (7 min 2 sec)


I first saw pictures of the pancake in a book on UFO's as a kid and dismissed it completely in my ignorance as an unsuccessful attempt to build a flying saucer. Recently though I saw a documentary with a section about the V-173 and XF5U and realised at least some of why it was such a clever design. Interest was piqued. One thing I should say is that the original's were apparently very strong. No doubt this was in part due to their metalite construction but also down to the shape. Having no slender wing's or fuse lends itself to rugged construction and considering I built mine from lightweight pink foam, hollowed out, and it's had a few bad crashes with relatively minor damage I'd say that aspect transfers well to our scale!

Aerodynamically it's been more successful for me than I expected but I've had a world of trouble with other aspects - mainly trying to use the power train from a dead quad copter.
I was hoping to shell the whole thing in rigid fibre glass/epoxy but the resin I've used for a few projects now ate the pink foam brutally. Fortunately I discovered this on a motor boom, not on the main disk!

First attempted flight got off the ground okay but was hugely unstable, in roll primarily. CoG was pretty much exactly where the full size was indicated. So for attempt two I moved it forward slightly, changed the 'toe' of the vertical's (the original was toed out so I went with that against my better judgement) to toe'd in and, maybe crucially added the Hobbyking KK board running Openaero I'd ordered specifically for this project.
Second flight was much better. Again took off okay but now was stable and predictable. Didn't try anything to outrageous but was starting to enjoy it. I have to say the first time it flew directly overhead - that silhouette had me hooked. I've flown a fair variety of Rc aircraft but nothing that looks as unusual in the air! Then, power dropped suddenly as I was turning and she came down out of sight. No monstrous damage but still not great. I think I'd made some very poor assumptions about the power set up and had over stressed the original quadcopter ESC's. Worst part was I'd bought some quite nice sprung U/c legs. They'd been ripped off and I searched but could only find one. It looks like one got dragged to the banks of the river Mersey and was then deposited there! Repairs were done, new Esc's bought and new U/c rigged up.
Third flight again started well. Was flying around nicely, gaining confidence when there was a 'pop' sound resulting in a sudden roll and then another pop. Both motors had gotten too hot and detached from their poorly designed mounts! Legs ripped off again some minor damage to the motor booms etc.
Fourth flight was awesome. Bit of a breeze and it took off in about six feet! Flew well. Landed steeply and close to me (yes the right way up and undamaged!) Checked the motors and they were only very slightly warm so like an idiot thought 'One more quick flight'. Turning into the wind at low speed she suddenly rolled over and came in vertically. I was berating myself for being stupid enough to stall when I realised that not only was the original design supposed to be highly stall resistant but another plane I've got running Openaero can be relied upon not to drop a wing, instead able to descend steeply but under control. Turns out one of my ailavator servo's had come unplugged from it's extension! The servo leads were all trapped under the rubbery foam of the control board mounting and there was no pressure whatsoever on them. Apparently I had glued the other side together but not this one.
I've only had one flight since then and it was great. I did try switching off stabilisation but wasn't happy and so switched it back on. It certainly wasn't like the first flight attempt but it didn't fel quite settled yet either. She's been painted now, at least largely. The motor mounts and half the booms completely rethought, more secure and better for cooling. I've replaced the last couple iof inches of foam/fibreglass tube with plastic piping of the same diameter. The Esc's are finally into a little airflow at least and I'm down to eight by four three blade props. I wish it had a little more speed/authority but without changing the motors (which I'm loath to do as this was supposed to be a quick and cheap project using spare parts!) I don't think I can up prop without risking overheating again. I have edged the CoG forward a touch more and suspect this may help a little but to be honest it flies pretty well anyway. (If you've watched the video you can see that even with the small props she's slightly faster than an inquisitive sea gull )
You can see pretty much all you'll want to see of the construction in the video but I'd say it's a 40% 40% 40% split, pink foam, filler and TLAR composite.

I do have some electric retracts sitting doing nothing but every plane I've had with retracts has seemed to be more trouble than it's worth so I reluctantly took the decision to go with fixed UC. I normally don't like that, preferring to belly land but the design didn't really lend itself to that and I'm glad I got past my prejudices now. The STOL aspect is something I'm pleased with and looking forward to exploring more and I hardly ever notice the legs dangling down in flight anyway.

Thanks Mr Zimmerman!

(Someone help me! This project has given me a new taste for building/designing again and for the slightly exotic and so I've found myself reading all about coanda and magnus effects! )

Kev

Oh, edited to add - I'm sorry about the quality of camera work but I was flying alone and trying to test fly a new plane whilst point a camera velcro'd to my Tx!
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Old Aug 01, 2013, 12:54 PM
Smells like SCIENCE!!!
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Canada, ON, Toronto
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I like it, but then I'm a sucker for odd-looking planes. Nice job!

Question: which way are the props spinning? On the downward swing, are they going away from the fuselage or toward it? (I ask because I have a vague recollection of this being very important... to counteract wingtip vortex? Something like that.)
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Old Aug 01, 2013, 01:15 PM
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Thanks very much!

Yeah the rotation of the props is the clever bit (of the design not my model!) They actively cancel the wingtip vortices and so reduce/remove the disadvantage of such a low aspect ratio wing. Only problem is that power off glide is not going to be great but I haven't risked that yet. So they rotate outward on the upper arc - if that makes sense.

Kev
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Old Aug 01, 2013, 04:28 PM
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Yes it was believed that the props could counter wingtip vorticies, but it was just BS. The fact is whether you "counter" it or not the energy is still lost (The vorticies still form even if they are then "countered" by the porp wash). Low aspect ratio wings have a lot of parasitic drag no matter how you slice it. There is no magic cure.
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Old Aug 01, 2013, 11:53 PM
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Originally Posted by acvar View Post
Yes it was believed that the props could counter wingtip vorticies, but it was just BS. The fact is whether you "counter" it or not the energy is still lost (The vorticies still form even if they are then "countered" by the porp wash). Low aspect ratio wings have a lot of parasitic drag no matter how you slice it. There is no magic cure.
Hi Acvar,

Hmm, if you've read somewhere that Zimmerman's idea's were wrong could you post where because I've been reading a lot about this recently and have never found a single person or site claiming that, on the contrary several places state that the design DID counter wingtip vortices successfully, rather than, was simply intended to.
My aerodynamics knowledge is rudimentary so please bear with me. Drag induced at wingtips of an airfoil creating lift is due to the higher pressure air beneath the wing rotating around the tip to the lower pressure region above, swirling, if you like. If the swirl of a large prop centred almost at the tip is rotating in opposition to this, the vortices won't start and then be cancelled, they just won't start. The prop wash will simply induce a tip to root spanwise force.
Of course it's not a magical cure - Rpm of the props, AoA of the wing etc all vary the effects but the accepted wisdom seems to be that it worked, well enough to seduce first Vought and then the Navy.
Of course wingtip vortices aren't just an issue for extremely low aspect ratio wings. Would you consider an airliner wingtip winglet to be magic cure? Perhaps not but certainly worthwhile.
Also I assume your last note was referring to induced rather than parasitic drag?

Kev
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Old Aug 02, 2013, 09:20 AM
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Lets look at it anohter way. Lets say there is a rock in the middle of the table. Lets say I push on that rock with all my might in a westwardly direction. Lets say at the same time you push on the rock in a eastwardly direction. Does the rock move? No. Despite this I am still expending lots of energy trying to move it as are you. Things don't change if the rock is air molecules and you and I are the wings and the propeller. Sure the tip vorticies are reduced, but the loss of energy persists so basically you have accomplished nothing of value. The vorticies are just the effect. Adding energy to the system to obfuscate the effect does not remove the cause.
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Old Aug 02, 2013, 09:53 AM
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Originally Posted by acvar View Post
Lets look at it anohter way. Lets say there is a rock in the middle of the table. Lets say I push on that rock with all my might in a westwardly direction. Lets say at the same time you push on the rock in a eastwardly direction. Does the rock move? No. Despite this I am still expending lots of energy trying to move it as are you. Things don't change if the rock is air molecules and you and I are the wings and the propeller. Sure the tip vorticies are reduced, but the loss of energy persists so basically you have accomplished nothing of value. The vorticies are just the effect. Adding energy to the system to obfuscate the effect does not remove the cause.
That argument seems very over-simplified, or maybe I just don't see how it explains the situation. Using that same model, how would you explain the effects of increasing wing aspect ratio, changing wingtip shape, adding wingtip devices, or eliminating wingtips altogether? Also, can that model tell me if there's any difference between rotating the props one way or the other?

Edit: Also, see the following document... Wind tunnel testing and analysis that shows lift increase and drag reduction due to wingtip mounted propellers, with prop rotation counter to wingtip vortex direction being the more efficient configuration.



(warning: 21 MB pdf) http://www.lr.tudelft.nl/fileadmin/F...imchev__2_.pdf
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Old Aug 02, 2013, 10:24 AM
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Hi Acvar,

Absolutely, I take your point. However, the swirl from the prop is happening anyway whether we want it to or not. It's not a new load on the system so it's certainly not trying to create 'free energy'. All Zimmerman advocated (at least in this regard of the design) was to utilize that effect to reduce/remove the disadvantages of the planform. Maybe he just tilted the table enough so that all your might just held the rock in place?

Hi Ibillwilson, Cheers for that. I was looking for something similar but was in work last night and it was a competition as to which was least effective - the decrepit work network or my befuddled head! Needs a good read, so thanks again.

Kev
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Old Aug 02, 2013, 02:08 PM
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Hi Acvar,

Absolutely, I take your point. However, the swirl from the prop is happening anyway whether we want it to or not. It's not a new load on the system so it's certainly not trying to create 'free energy'. All Zimmerman advocated (at least in this regard of the design) was to utilize that effect to reduce/remove the disadvantages of the planform. Maybe he just tilted the table enough so that all your might just held the rock in place?

Kev
I understand this, but lets forget about the prop wash for a second and asume for the sake of argument that we have a magic wand that when we wave it totally eliminates tip vorticies on the wing. When we wave that wand would the amount of drag instantly decrease? No. The drag has allready been induced. The energy was allready used to create the bound vortex and the presure difference between the top and the bottom of the wing. The only thing that would change is that the energy that is wasted as a tip vortex could now be used as lift. Could you then design the plane with a smaller wing? Sure if the advantages was seen over a wide flight envolope, but somehow I doubt that was the case. If it was I am sure we would see quite a few modern planes employing this design. The fact that we don't seems suspect. I admit I did not study this particular design in school, but then again that leaves me to question why not? Could it be because it ultimatley did not pan out?


Quote:
That argument seems very over-simplified, or maybe I just don't see how it explains the situation. Using that same model, how would you explain the effects of increasing wing aspect ratio, changing wingtip shape, adding wingtip devices, or eliminating wingtips altogether? Also, can that model tell me if there's any difference between rotating the props one way or the other?
My oppinion on all of that is forget about it. You don't have the tools to make any of it profitable. Without a wind tunnel, a bunce of sensors, and a computer to run computations the likelyhood of you getting any of it right is nill. Even if you did somehow by some freak of chance get it right it would not amount to enough for you to even notice the difference. Keep the weight down, keep the structure sound, make your plane apeal to you visually, and throw a motor on it and enjoy.
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Old Aug 02, 2013, 06:16 PM
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"I understand this, but lets forget about the prop wash for a second and asume for the sake of argument that we have a magic wand that when we wave it totally eliminates tip vorticies on the wing. When we wave that wand would the amount of drag instantly decrease?"

Yes!! It is the vortex that is creating the additional drag not the pressure differential itself. If your magic wand prevented the vortex from forming there would be no vortex to create the drag.

"No. The drag has allready been induced. The energy was allready used to create the bound vortex and the presure difference between the top and the bottom of the wing. The only thing that would change is that the energy that is wasted as a tip vortex could now be used as lift. Could you then design the plane with a smaller wing?"

If I understand your explanation correctly then it seems we were almost in agreement all along. If using prop wash to counter wing tip vortices 'converts' the induced drag to lift,... great! Success.

"Sure if the advantages was seen over a wide flight envolope, but somehow I doubt that was the case. If it was I am sure we would see quite a few modern planes employing this design. The fact that we don't seems suspect. I admit I did not study this particular design in school, but then again that leaves me to question why not? Could it be because it ultimatley did not pan out?"

Not at all. The XF5U project ran far too long due to problems with the complex power train not due to any aerodynamic deficiency. It was abandoned because by 1947 the USN felt that the future lay solely with jets. Interestingly the paper ibillwilson linked to is looking at exactly this approach with regard to extremely low aspect ratio UAV's where the advantages of the planform could be worth it's disadvantages. The paper's own conclusions and references to historical data both conclude that this effect is significant and very much worthwhile with such a planform.

"My oppinion on all of that is forget about it. You don't have the tools to make any of it profitable. Without a wind tunnel, a bunce of sensors, and a computer to run computations the likelyhood of you getting any of it right is nill. Even if you did somehow by some freak of chance get it right it would not amount to enough for you to even notice the difference. Keep the weight down, keep the structure sound, make your plane apeal to you visually, and throw a motor on it and enjoy."

With regard to the difficulties of modelling this effect I must disagree. As with most things it would of course benefit from detailed optimisation but ultimately any prop swirl in opposition to wing tip vortices will deliver an advantage and when the planform, ie very low aspect ratio and twin counter rotating wing tip props is desirable for other reasons, eg being a scale model of the type, it garners this effect for no disadvantage!

Kev
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Old Aug 02, 2013, 08:53 PM
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Originally Posted by acvar View Post
...
My oppinion on all of that is forget about it. You don't have the tools to make any of it profitable. Without a wind tunnel, a bunce of sensors, and a computer to run computations the likelyhood of you getting any of it right is nill. Even if you did somehow by some freak of chance get it right it would not amount to enough for you to even notice the difference. Keep the weight down, keep the structure sound, make your plane apeal to you visually, and throw a motor on it and enjoy.
You totally avoided answering the question, but no matter... The thesis that I linked to basically proves the concept. On a wing of low aspect ratio, putting a tractor prop at the wingtip significantly increases lift and reduces drag, with the outward swing on top being the more efficient configuration. Even better, it looks like this effect is fairly robust and easy to implement correctly.
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Old Aug 02, 2013, 08:58 PM
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Kev... Are you interested in FPV flying at all? Your plane would make a great camera platform. One question: where is the CG?

Bill
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Old Aug 03, 2013, 04:13 AM
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Hi Bill, (seems the reply I typed in in work has disappeared!)

Funny you should ask about Fpv - yes I am and I had half an eye on that when 'designing' the Xf5U. I've Fpv'd most of my planes, a couple of wings, a kit bash 1.4m flying boat, an elderly Picojet and even my little foamy Dr1 (which was more fun than you might imagine! If you get really bored
Red baron versus the Red bull (4 min 21 sec)
) I was originally planning on having two interchangeable nose/cockpit sections, one scale(ish) and one for the Fpv cam but have ended up properly fixing the current one due to the need to have the battery inserted a long way into it. So, the current plan is to slice the nose, roughly along the lines of the front 'glass' section so that that will be removable, probably velcro'd on to switch to cam when I want it.
There's plenty of space in the foam voids for my gear but here are a couple of issues. Firstly take off and landing at least are a little nose high, normal flight seems okay from the ground though. I may have to experiment with a wingcam to find a satisfactory angle. Second, obviously it's great that there needn't be a prop in the cam view but it does have two esc's and two decent lengths of motor wires. Hoping they won't reduce the range too greatly. Finally at the moment my Fpv gear is dead! I'm pretty sure it's the Vtx that's gone (odd no crash or anything to blame ) so ordered a new Vtx and Rx from Hobbyking a few weeks ago, even though they were OOS. I got an email a couple of days ago to say they'd been dispatched so they're on their way now from the global warehouse

Had another good flight this morning - again alone so very poor video - and I've decided that as I'm off work for two weeks I can justify a tiny bit more expense and have ordered two new motors. The current one's are a little suspect. Specs say they should be okay with a 10x6 eProp but mine started to smoke with those fitted. With the (3x)8x4's it flies fine but not fighter like at all, more like I'd have hoped a V-173 would have turned out, light and floaty if you know what I mean.

CoG is slightly ahead of the full size drawings indicated spot, but only slightly. Mine is at 14cm from the LE immediately adjacent the cockpit/nose and that's on a 60Cm maximum chord. To be honest it's not the easiest shape to measure accurately so it'd be safest to say a short distance ahead of the 1/4 root chord point!

Kev
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Old Aug 22, 2013, 05:40 PM
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Well I finally got to meet my brother at the beach today and got some footage of the Flapjack in it's current form. Re-motored since the first video's she's much more assertive in the air and consequently confidence inspiring. Did a loop and roll at long last but neither were very impressive, the roll especially nearly ended in disaster but I suspect that's more to do with the current set-up than any aspect of the design.
STOL and vertical performance is great. She's definitely the sweetest plane to land I've ever made and maybe even ever just owned. Still, on the heavily ridged sand I did manage to flip her over on landing . Damage was very minor but it loosened a motor mount which caused all the vibration you can see in the aft facing clips. Some of the shots are from my brothers Durafly F4U Corsair (seemed an obvious pair ) and we reckon they're about the same speed. The Xf could handle a higher pitch prop fine now but I didn't build her as a speed demon and with the legs hanging in the breeze it hardly seems likely or appropriate so I think I'll stick with the 2 blade 10x6's for now.
I also finally got my new Fpv gear and had it installed. Worked fine on the bench but was useless out at the beach. I'm thinking it needs a filter and then it's time to experiment with the installation location. WiP as ever.
All in all still really happy with her. Anyway, enjoy.

Kev

XF5Ucomp1 (2 min 54 sec)
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Old Aug 22, 2013, 10:45 PM
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Beautiful! It looks so good in the air and while landing. You did a really great job with this.
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