Airplane + Hydrofoil - Good or Bad Idea?

this video is sponsored by established titles more on them later earlier this year I built an RC hydrofoil boat that uses sonar sensors and a drone flight controller to actively control its height above the surface of the water then more recently in the past few months I've made a whole series of videos exploring different ground effects vehicle designs and Concepts in that time I've gotten tons of comments and emails suggesting that I put hydrofoils on a ground effect vehicle my response has always been that I don't think the two would be a good combo for reasons that I'll get into later in this video but due to the large number of viewer requests I decided to give it a try now this actually won't be the first time that I've tried to combine the two in the first ever video of my flying sled ground effects vehicle from 2015 you can see here that I tried adding a little popsicle stick hydrofoil to the bottom of the aircraft now popsicle sticks with square leading and trailing edges don't make great hydrofoils so this test can be taken with a grain of salt I think my intention at the time was not to actually have it be submerged but instead just act like a planing surface that skimmed across the top of the water you can see that this sort of worked and the aircraft was able to use the foil as a planing surface but it didn't have enough surface area to support all the weight and when the foil would drop down below the surface the aircraft would slow down quite a bit and require a lot more throttle to take off again so this test probably is one of the reasons why my mental model is convinced that hydrofoils and ground effect vehicles are a bad combo fast forward seven years later and here we are with the version 2 of The Flying sled this one I made specifically for ground effect vehicle testing as you can see it does its job extremely well it's even the only RC ground effects vehicle that I know to exist in the whole wide world that can take off from the water but can't actually fly out of ground effect like a normal airplane if the thrust is limited to the exact right amount that is so this is such a fantastic and stable aircraft that I thought it would be a great platform to use for hydrofoil testing there are two types of hydrofoils that I want to try the first is a fully submerged foil these stay fully submerged under the surface of the water and usually require an actively stabilized control system for stability the second type is surface piercing foils these enter the water like a V and are thought to be more stable since they can have dihedral like an airplane wing so let's start building the biggest challenge here is going to be making the hydrofoils have as little drag as possible this is extremely important because water is about 800 times more dense than air so it's much more difficult to make something move fast through it to keep drag down I want to make the foils as thin as possible while still being strong enough not to bend this is why I chose to make them out of high carbon stainless steel the primary purpose of These Bars is for making knife blades so they're super stiff to form them into an airfoil I just went to town with an angle grinder slowly but surely abrading them down to the right shape then once the shape was close I switched to a finer grinding wheel and then eventually sandpaper to really smooth them out to make the T shape of the submerged foil I had Ethan here try MIG welding the two bars together but that really didn't work out super well I really probably needed to TIG weld these with stainless steel filler Rod but oh well the MIG welding wasn't pretty but it worked well enough after quite a bit more sanding and grinding I arrived at something that seemed like it would be pretty dang hydrodynamic I had basically just made a t-shaped knife the trailing Edge was super sharp to mount this foil on the flying sled I 3D printed a holder on my new creality Ender 3 S1 plus 3D printer the vertical bar slides right into the slot and is clamped in with some bolts on either sides I glued this onto the Leading Edge of the flying sled and set the angle of attack so that it matched to the Air Wing this is so that hopefully the transition from foiling to Flying was as seamless as possible with minimal change in Pitch it was then ready for the first test let's hope we don't hit a fish the water conditions were far from optimal on this day the surface was pretty bumpy but despite that I was still able to get a feel for how it worked the first thing I noticed is that having the foil so far forward really took a toll on yaw stability even with the flight control are using its Gyros and accelerometers constantly adjusting the rudder and differential thrust to stabilize the yaw axis it was still a bit squirrely the whole reason why I put the hydrofoil on so far forward in the first place was for pitch stability think about it like this the hydrofoil is underwater generating lift if it's pushing the nose upwards this is going to cause the plane to pitch up and as a result go up once the foil leaves the water there will no longer be a force pushing the nose up so the plane will pitch down and as a result go down then the cycle repeats this is a self-stabilizing feedback loop that should hold the airplane at just the right altitude for flight in the ground effect so that's one benefit of the forward hydrofoil placement but another thing to consider is that even though it's super smooth the hydrofoil in the water still has a lot of drag relative to the air this drag is effectively pushing backwards way below the CG which causes the plane to pitch down by moving the foil forwards having it push up on the nose will help to counteract this negative pitching moment caused by having the foil so far below the center of gravity despite the yaw stability issues the flying sled was still able to do its thing fairly well as you can see the foil just kind of Dives in and out of the water as the plane moves along here's what happens when you accidentally flip a switch the wrong direction and cause the motors to Pivot up instead of down oh the insta 360 is gone no so I opened the log on the computer and I'm going to see if I can find the spot where it crashed crash right about here so I need to get the coordinates of that point somehow I then plugged in the coordinates to my phone and fashioned myself a little plastic bin with an anchor and got ready to go swim around in the poop there's a lot of weeds I hate Lake Weeds now for a word about the sponsor of this video established titles one thing you probably didn't know about me is that I'm actually an official Scottish Lord just like Lord Farquaad I didn't get this title for my prestigious purebred Scottish Heritage nope instead I got it by purchasing one square foot of land in Scotland their rules say that if you own any amount of land you're officially a lord or a lady you could even officially change your name if you 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we can make our own little RC test flight Kingdom thanks again to established titles for sponsoring this video wow it's really shallow I tried to position my location on top of the pin as closely as possible and then dropped the anchor on my makeshift barge so that I would have a reference point to search around then I started rummaging around through the weeds in search of the little bean shaped camera after about 15 minutes of swimming around right as I was starting to get way too cold I finally found it foreign [Music] as it was sitting on the bottom it got visited by a little tiny fish and even got some shots of the flying sled passing by overhead looking at the onboard footage we can see that the foil is kind of just acting like a planing surface rather than a hydrofoil but testing it in such bumpy water is kind of unfair so we'll have to look at that more closely later another obvious thing is that the hydrofoil is catching quite a lot of Lake weeds I was having this problem on my last hydrofoil project too there were a bunch of comments saying I should make the Leading Edge of the foils sharp to cut through the lake weeds but I think that's a bad idea for two reasons one is that the craft isn't really going fast enough to actually cut them even if the Leading Edge was razor sharp another is that having a sharp Leading Edge on an airfoil or hydrofoil can actually create more drag than a rounded Leading Edge this is because the foil needs to be able to work at varying angles of attack in the front of an airfoil's Leading Edge there is actually a stagnation point right where the air splits this stagnation Point stays perfectly aligned with the oncoming air even when the angle of attack changes with a sharp Leading Edge there is no stagnation point that changes with angle of attack so if the wing is at any non-zero angle of attack there will be forced to make a sharp Direction change which causes drag at extreme angles of attack you also get flow Separation on the top of the Leading Edge this is all only true about subsonic flight at supersonic speeds air behaves very differently and sharp leading edges are actually good so that's why we can't have sharp hydrofoils the next day I set out to try and find some smoother water with less weeds I also increased the yaw stabilization gains to try and make it easier to control once again the foil seems to be more content writing on the surface of the water versus underneath the water it only really went underneath at lower speeds another issue is that unless the vertical portion of the hydrofoil was facing directly into the oncoming water the flow would separate away from the Leading Edge and cause ventilation which would even creep down to the lifting surface part of the foil ventilation is when air pockets get sucked down along the foil and kind of cling to it this is really bad because it significantly reduces the lift typically ventilation is only a big problem with surface piercing foils not submerged foils like this one but even when a submerged foil is constantly going in and out of the water ventilation becomes much more of an issue the only time it doesn't seem to be a big issue is when the aircraft starts from a low speed and then accelerates and immediately takes off but oh wait this never happens because so long as the hydrofoil is fully submerged the aircraft never has enough speed to take off it's only able to build up enough speed if the foil is planing on the surface by just holding this hydrofoil in the water we can get a really good feel for how it behaves there's very little drag at slow speeds we're going like two meters per second right now it makes a lot of lift when I increase the angle of attack but it's really interesting how the lift just gets killed when there's ventilation like that you see how there's air pockets trapped under there around the foil that just ruins all the lift it has a lot of turning power that's for sure when you change the yaw yeah wow look at that perfect example of ventilation there's really hardly any drag at all I I can just hold it by the tip wow look at that a plain cut out line right through the clouds that's crazy so today I moved to the hydrofoil back quite a ways it used to be right up here on the Leading Edge and now I moved it way back there so we'll see if that fixes some of the yacht instability issues by moving the hydrofoil backwards I really didn't notice a big Improvement in yaw stability which was interesting I did notice that it was slightly easier to keep the foils fully submerged or at least it would just stay submerged for longer when it was transitioning in between displacement mode and flight but this isn't necessarily a good thing because as I said earlier it seems the foils have less drag when they're planting on the surface so yeah the hydrofoil is proving out to not really be all that useful in order to actually have the foil stay underwater like they're supposed to you would definitely need a rangefinder with an altitude control Loop to automatically control the vehicle's height it's just not possible for a human to do at least not at this small scale and with such a small margin of error there's less lift than I thought just trying to plane on the surface it definitely lifts a little bit but less than I thought it would I can definitely feel more drag when it's just planing on the surface then once again I accidentally flipped the motor switch the wrong direction and ended up floating upside down look at that thing it's driving itself in circles it's probably trying to be in return to home mode okay this is going to be kind of sketchy to make it stop [Music] oh well the autopilot seems to still work okay I don't want to pop my kayak with this hydroboil because it's a knife wow they got a crane on their barge wow that is the cutest little Tugboat I've ever seen they've got the whole Fleet of construction vessels coming through look at that luckily none of the electronics on the flying sled were damaged so I was able to use it again for testing the surface piercing foils I decided to make these out of aluminum bar instead just because I ran out of steel after quite a bit of grinding I arrived at a nice flat bottom airfoil shape and then to smooth them out I sanded with finer and finer sandpaper up until 600 grit they ended up looking pretty nice and smooth these ones ended up being quite a bit thicker than the steel foils so hopefully that doesn't increase drag too much I then fired up my new snapmaker 3D printer and printed some brackets that the hydrofoils bolt onto and then these got glued onto the bottom of the flying sled I mounted these closer to the CG since they don't stick down as far they shouldn't have as much effect on the pitch it was pretty clear right away that these things had a lot of drag it took quite a bit more power to build up speed to take off and the yaw stability wasn't much better than the submerged foil from the onboard footage you can see that there was pretty much nothing but ventilation going on with these foils I think this is kind of inevitable with surface piercing foils at super high speeds relative to their cord length sure you could add some little fences to try and prevent air from traveling down the foil but that's just going to increase drag which it already has way too much of even with no ventilation going on so all in all hydrofoils on RC ground effect vehicles uh I'd give it a 2 out of 10. would not recommend to a friend to most people it probably does seem like the two would be a great combination because we've all been told that hydrofoils and ground effects vehicles are both very efficient if you've ever researched hydrofoil efficiency you've probably seen a graph like this that compares two Hull designs it shows total drag on the vertical axis and speed on the horizontal axis we can clearly see that foil-borne lift is more efficient than a planing Hull at medium speeds however all this changes a lot when we're talking about a ground effect vehicle that also gets lift from wings I couldn't find any real data on this but here's the general feel I got from my experiments as we start to accelerate faster and faster the hydrofoils lift off and become more efficient than a planing hole as we go even faster the ground effect vehicle's Wing starts to generate aerodynamic lift this reduces some induced drag from the hydrofoil but it's still fully submerged so you're stuck with a lot of parasitic drag the planing Hull on the other hand gets lifted out of the water so it's just barely skimming the surface this is more efficient than having to drag an entire hydrofoil through the water then once you do take off you're stuck with the added drag and weight of the hydrofoils as I've been working on this project the company Regent has been doing the initial test flights of a small scale prototype of their sea glider eventually they hope to sell full-scale versions of this craft to be used for Coastal Transportation purposes and what do you know it has hydrofoils before I comment on those I'll comment on the aircraft in general they are clearly marketing it as a ground effect vehicle and if you're making a ground effect vehicle why on Earth would you put the wings as far away from the ground as possible it doesn't make any sense to me look at every other ground effect vehicle ever they all have their wings as low as possible to take advantage of the ground effect as much as possible you really want your trailing Edge to basically be on the surface of the water so I would not call this thing a ground effects vehicle at all it's just a cool nice looking electric seaplane that flies low I don't necessarily think this graph is scientifically accurate but with Regent's High Wing design they're going to be operating way down here in the sub 10 efficiency gain region whereas something like the attk aqua glider or the Gunter yerg airfoil flare boat is probably operating way up here at like 50 efficiency increase or more but then again Regents High aspect ratio Wing might make up for the efficiency loss from not having the wing super low to the ground not sure and their design is not completely reliant on being super low to the water like the airfoil flare boat so this will allow them to operate over rougher water but anyways onto the hydrofoil part of it hydrofoil lift and stability start to rapidly degrade right around 60 miles an hour this is the speed at which they start to cavitate cavitation is when the pressure over the foil gets so low that the water actually starts to boil and turn into vapor vapor is much less dense than water so it causes the foil to lose a lot of its lift and unsteady cavity Dynamics can cause serious pitch and roll instability and also large increases in drag so the Regent C glider either needs to take off and land at under 60 miles an hour or just rely on its big wings and tail for stability and lift at speeds over 60 miles an hour and just power through the increased drag now one of the big Design Elements they talk about is a blown Wing they claim that having Motors all along the Leading Edge will maintain lift and control at slow speeds so if that works it could help them take off and land at under 60 miles an hour but 60 miles an hour is still super slow for a plane with a 65 foot wingspan which is what their full scale model will have so that's a big challenge another reason why hydrofoils can be challenging on an aircraft is that as they near the surface of the water they create less lift than they did when they were deeper so now you have to deal with this non-linearity in foil-borne lift as the aircraft takes off if that's not enough hydrofoils also have a very narrow speed range in which they operate effectively airplanes on the other hand operate over a very wide speed range this can vary greatly based on wind speed direction and payload weight so it's difficult to match AirSpeed at the plane with the optimal foiling speed so there you have it a bunch of reasons why not to use hydrofoils on aircraft but there's also a big reason why to use hydrofoils on aircraft and that is that there really isn't a better option one of the reasons why ground effects Vehicles haven't become popular is that they can't really handle choppy water at all hydrofoils could really help make takeoffs and landings much smoother but then I think the hydrofoils would need to be retracted so that the aircraft can fly lower to the surface to utilize ground effect more effectively for normal seaplanes one of the big downsides has always been that the hulls have had to have been made super strong to withstand the huge amounts of force from waves hitting them on takeoffs and and landings this adds a ton of weight to the airframe making the plane less efficient one interesting example of having to deal with this problem was the f2yc dart it was an amphibious fighter jet that needed water skis to take off and land this was to prevent the fuselage from hitting waves at high speed if implemented correctly hydrofoils could help sea plants with Rough Waters but aerodynamics and hydrodynamics don't always scale down very well so at RC scale the concept is really difficult to make work and kind of Impractical in my opinion so I strongly prefer my flying sled to have no hydrofoils it just works so much better that way that's all for this video thanks for watching bye in 2022 Daniel built something new Hydro hydrofoil slicing through the waves with these maximum efficiency Hydro hydrofoil [Music] come on R2 pilot do it turn that sonar into movement stabilize that little boat it doesn't even need to float foreign there was a single water bottle on the Drone for a buoyancy but uh don't know if that was enough we hear beeping somewhere it's somewhere we can just barely barely hear the esc's beeping you see it is that it oh yeah that's definitely it oh hell yeah dude it's the water bottle is vibrating that's how the beeps are coming through oh you're so what look at that oh that's so good hello oh it's so good amazing we got it yes you got a little corrosion there yeah just a little bit other than that looks great what the nice the the trusty old inflatable kayak does it again although my teeth is pretty wet we're slowly sinking this is what happens when you crash in the parking lot and lose your GoPro nut just gotta tape it on brushless motors and propellers so much swag this ship is Stellar screw your yacht it can't even fly and not like my hydrofoil [Music] ansel's flying the flying sled how does it feel feels great it's really easy to fly oh dog does it feel like an fpv drone no what does it feel like a napkin it does feel like a knock it gets blown around quite a bit it's kind of like a kite yeah I don't know if you can pull off this turn oh what do you think about it in ground effects when you're low to the water oh it's Epic it's weird though because like normally you have control and then you just like start pushing down and nothing eventually it stops going down and just hovers for you yeah just uh it's a positive feedback loop or a negative feedback loop come on flight controller do it removal unwanted movement made of sticks and foaming glue 3D printed plastic to get rid of all that friction satisfy your nerd addiction throttle Up full steam ahead hydrocoil yeah [Music]

Channel: rctestflight

Views: 638,121

Keywords: ground effect, ekranoplan, hydrofoil, hydrofoils, rctestflight

Id: 5bffh-WjE5w

Length: 21min 34sec (1294 seconds)

Published: Thu Nov 10 2022