The biggest myth about split flaps

hey everyone Keegan here with dark Arrow this video is all about flaps and the flap design on the dark Arrow one this aircraft uses split flaps and we recently finished installing them I want to get into the details of this design and show how it compares to other flap configurations there also some myths floating around about split flaps that we hope to dispel so let's start by just looking at the flaps on the dark R1 prototype flaps are a removal control surface that increase the lift produced by the wing flaps are mainly used during takeoff and Landing they allow for slower Landing speeds steeper approaches and they also allow us to take off from shorter runways although they increase lift they also increase drag so for the majority of the flight they're in this retracted position which allows for greater efficiency during Cruise basically the flaps allow us to have a smaller wing with less drag than what we'd otherwise need if we didn't have flaps so as I mentioned this aircraft uses a split flap configuration where the flap hinges from the bottom Wing skin where the top Wing skin remains fixed we've briefly shown the flaps in other videos but it wasn't until recently that we had all the flap actuation Hardware installed that allows us to control them from the cockpit so this is done with a manual flap handle that rests between the pilot and co-pilot it's pulled up and down we went back and forth on whether we do a manual flap or have it electrically driven but we ended up deciding on a manual flap actuation just due to its Simplicity lightweight and the ability to quickly implement it for the Prototype I want to get into more of the details on how we arrived on this design and some of the compromises that we had to EV along the way but I'll hand it off to Riley to talk through all of that to explain why we picked a split flat for the dark AR 1 I thought it'd be good to show a couple different options of what other flap types are out there so we can see what different decisions we were looking at with our flap design I've got a couple different common flaps drawn out on the board here this isn't every single flap type out there but this is probably about 90% of what you'll see in general aviation we have the plane flap split flap slotted flap and the Fowler flap so we'll start by looking at the plane FL flap that's characterized by having both the top and bottom skin moving together deflecting down as a unified control surface in the flap and we can measure the performance of the plane flap by how much it increases our lift with the lift coefficient we can measure the lift coefficient by running a little test Wing through a wind tunnel and then measuring lift versus angle of attack we'll get a lift pull that looks something like this and when we drop the flaps it'll move our lift curve up into the left and it'll slightly modify the peak of the curve so dropping the flaps increases our lift coefficient and what we really care about is the maximum lift coefficient of the CL Max so for a plain flap that we might put on the dark a one we could see a CL Max of around 2.3 not bad the main appeal for the plane flap is just that it's simple and lightweight and so that's why you commonly see it on a lot of these aircraft like the Vans RVs it's found on a couple old warbirds and even modern fighter jets the maximum lift coefficient that we get with the plane flap is somewhat limited by the geometry at the h line so the air flowing over the top of the wing skin can stay attached and follow the Contour for small flap deflections anything less than about 15 or 20° but as we get into higher flap deflections past 20° the flow starts to separate and it can't follow that Contour on the upper surface of the flap so that separation somewhat limits the maximum lift coefficient that we can achieve with the plane flap that's where the split flap performs a little bit better the split flap only has the lower portion of the wing skin deflecting downward as we saw on the dark a one so because we have the top Wing skin fixed that defines somewhat of a fixed separation point for the flow it can easily follow this top Wing skin all the way to the trailing edge of the wing so that allows the split flap to produce a little bit higher maximum lift coefficient compared to the plain flap we want the flow to follow the air foil and be deflected down at the trailing Edge that doesn't exactly happen with the split flat but at least it's not going uh crazy and turbulent and getting chaotic right at the hinge line so that is what what allows the split flap to perform a little bit better than the plane flap same case is true it's simple and lightweight and what's interesting with the split flap is that it's well suited for composite construction we can easily build this uh basically a flat plate for the split flap with Composite Materials by making a sandwich type structure by sandwiching a foam core with carbon fiber on both sides and that makes a really stiff rigid structure that's both uh strong and stiff in torsion and bending which is what we need when we deflect this flap into the breeze it's a little bit harder to make a split flap out of riveted aluminum because you have to add in a bunch of ribs and stiffeners to stiffen up the flap and the wing skin above the split flap it's not impossible to build a split flap with riveted aluminum construction you'd see it maybe in some of the old warbirds like the Spitfire the P40 and the Hawker hurricane uh but that added complexity and added part count with riveted aluminum is probably why we don't see it more commonly but for the dark a 1 with carbon fiber composite construction split flap is great for manufacturing neither of these designs represents the Pinnacle of what we can achieve for increasing lift with the flap design for that we might graduate up to some of these more involved flap designs like the slotted flap or Fowler flap slotted flap is characterized by having an actual physical slot between the fixed portion of the wing and the flap and that slot allows high energy air to bleed from the underside of the wing over the Leading Edge of the flap and that helps the flow stay attached as the flow passes over the top Wing skin and the downward over the flap and that is what allows the slotted flap to achieve a little bit higher maximum lift coefficient compared to the plain flap or split flap something around maybe 2.6 we would see this is going to be a little bit more complexity both in the design and Manufacturing because we have to put some thought into the geometry of this slot and the geometry of the Leading Edge of the flap and also the hinge mechanism because we want this flap to uh deflect a little bit aft and open up the slot as we extend the flap down so that air can pass through but then we also want it to close up and close that slot we're in Cruise flight so we're not getting extra drag from the slot uh with just parasitic losses through the slot and cruise so between the geometry of the slot and the hinge mechanism we have to put in a little bit more effort with the slotted flap design uh because we could in theory design a slotted flap that works a little bit worse than a plain flap or split flap which uh right out the gate give us pretty decent lift increment the added lift that we get with the slotted flap gives it more appeal for takeoff and landing and that's why we see it pretty commonly in some of like the Cirrus or Cessna type aircraft if we really want to up our lift coefficient we might go to the Fowler flap which is sort of a mashup between some of these designs and the special thing about the Fowler flap is that the flap actually translates aft all the way to the trailing edge of the wing and then deflects down so when the flap translates aft that increases the wing area that alone would increase lift but then when we deflect the flap down as well that further increases lift and we might expect to see a maximum lift coefficient of something around 3.0 all these lift coefficient numbers are pretty rough they're ballpark numbers but I thought it'd be good to put numbers down rather than just making uh hand waving statements about the comparison of these lift coefficients the fer flap is going to have the highest complexity of these designs because we have to design a hinge mechanism that allows this translation to occur as well as angular deflection in the flap and then we also have to have an actuation mechanism that can accommodate that motion so that motion is normally accommodated with uh linkages or tracks and then the actuator is usually something like a ball screw all that Hardware adds weight it's still worth it because the amount of extra lift we get definitely offsets the weight of the extra Hardware but it's just more complicated to design this fall flap design is seen pretty commonly in some of the old bombers like the B-52 and b29 I've covered how these different air foils compare for Lift what about drag that's where things get interesting the split flap is going to have the least amount of drag for a flap retracted configuration because it has the cleanest external profile there's no seams or hinge lines on the top Wing skin to trip up flow the only sort of disturbance we have is on the lower wing skin where we have the hinge line here other than that it's a very clean configuration in Cruise so it's going to give us the lowest drag and that's one of the reasons we picked the split flap is because it feeds into our high-speed long range mission for the dark Arrow one all these other flap types are going to have either multiple seams or hinges hanging out in the breeze so the plane flap flap and follow a flap are all going to be a little bit more drag than the split flap and cruise what about for a deflected configuration when we drop the flaps what does the drag look like there so we saw there's somewhat of a hierarchy here for lift with the plane flap being at the bottom and the Fowler flap being at the top for lift that same hierarchy somewhat exists for drag but in the opposite direction with the plane flap and split flap presenting a little bit more drag than the slotted and Fowler flap if you look this up you'll see sometimes some conflicting information between how the split flap and plane flap compare for drag it's often stated that the split flap has more drag than the plane flap I think that intuitively makes sense it looks like this should have more drag because you have this separated trailing Edge almost sort of a a wedge profile that's separating flow and that seems like it should present more drag than the plane flap uh that's not exactly the case but this often gets misstated even in some of the fa pilot training material it says that the split flap has more drag than the plane flap but if we measure how much drag we get for a given increment of lift the split flap actually comes out on top of the plane flap now you don't have to take my word for it on this there's actually published publicly available wind tunnel test reports that have compared the split flap versus the plane flap on different air foils and shows that the split flap comes out on top for lift versus drag so I'll leave some links to those reports in the description of this video if you want to do some further reading and understand this a little bit deeper anyway enough of the rant on the drag I also want to show the mechanism or the actuator that we created to make the Swip flap deflect downward we're going to cover that in the cad world and I'll hand the discussion back off to Keegan for that to get a better idea of how the flat mechanism works I'm sitting over at the computer and I have the cad model pulled up it's a little tricky to see what's going on with everything installed in the aircraft but we can get a much better sense of all the components involved by isolating them in the cad world and ghosting out the things that we don't need to see so right now I have highlighted all of the main linkages and components that are involved in actuating the flaps as I mentioned this is a manually operated system that's controlled with a flap handle in between the pilot and co-pilot but how does that motion of the flap handle actually translate all the way to the flaps so I can actually move the flap handle here with my cursor back and forth you can maybe already tell what's going on here but the flap handle is connected to a series of torque tube and linkages through all all gang together and that motion translates through them all the way through the flaps so what enables that is this main torque tube that you see here that the flap handle is connected to and then the other components that are connected on the ends of it that help translate that motion all the way back to the flaps since we have a left and right flap this all has to be connected together and they have to move in sequence so as I move that flap handle the torque tube rotates moves Bell cranks and linkages and translates that all the way out to the flaps most of the components that you see here is a combination of machined aluminum or machined carbon and then some off-the-shelf carbon fiber tubing there were a couple parts that we had made uh through an additive manufacturing process we don't have an way to additively manufacture here in house so we teamed up with isometry to get these parts made out of house and they're actually the sponsor of this video so if you've watched our Channel you probably I already know a lot about xometry that they do manufacturing on demand where I can upload a cad file of a part and get an instant quote and an instant lead time for that part what you may not know is that zoetry recent released a update called teamspace within teamspace not only can I see the orders and quotes that I've made but also the ones that my team members have so I've added the team members from dark Arrow here and I can see their quotes and their parts that they've ordered so right here I have a quote that's been made by Riley he quoted it and he ordered it and there's actually these bearing blocks right here but in addition to just being able to view it I can also create a copy of that order and if need be without having to bug the rest of the team members I can submit another order so if you're part of a company that has multiple team members that are ordering parts through xometry and you want to consolidate that all to one area and streamline your workflow highly encourage you to check out teamspace and and if you want to learn more there's a link in the description overall we're pleased with the flap design and the flap actuation mechanism there are a few things that we want to tweak like the lockout positions on the flap handle those will likely change between now and flight testing wrapping up work on the flaps is exciting to us because it represents one of the last major systems to finish up before focusing additional effort on ground testing leading into flight testing so thank you for watching we'll catch you in the next one

Channel: DarkAero, Inc

Views: 144,549

Keywords: experimental aircraft, carbon fiber, experimental aircraft build, darkaero, darkaeroinc, carbon fiber plane, airplane build, experimental airplane, experimental airplane build, experimental airplane kits, homebuilt aircraft, homebuilt airplane, homebuilt aircraft construction, building an experimental aircraft, building an airplane

Id: 9MWlKlI00Gc

Length: 13min 35sec (815 seconds)

Published: Fri Dec 22 2023