Experimental EDF Jets and Other Ideas

[quorneng]

An interesting sequence of designs that almost exactly matches what Hawker followed going from the Sea Hawk to the Hunter.
Being a bit of a scale nerd I built them all one after the other and all to the same scale!

I only have one EDF with a true 45 degree sweep leading edge wing, apart from deltas, but it has lower trailing edge sweep than yours so the root chord is more generous. It has very mild handling characteristics but I suspect its light weight and low wing loading tends to mask any tricks it may have.
Unlike your design It has an all moving tail plane set low & flush with the fuselage underside. Tricky to do as it gets in the way of the tail pipe but its location does mean it is clear of the wing downwash when at high angles of attack so it remains effective.

 

[bisco]

nice fleet!

 

[telnar1236]

An interesting sequence of designs that almost exactly matches what Hawker followed going from the Sea Hawk to the Hunter.
Being a bit of a scale nerd I built them all one after the other and all to the same scale!
View attachment 253854
I only have one EDF with a true 45 degree sweep leading edge wing, apart from deltas, but it has lower trailing edge sweep than yours so the root chord is more generous. It has very mild handling characteristics but I suspect its light weight and low wing loading tends to mask any tricks it may have.
Unlike your design It has an all moving tail plane set low & flush with the fuselage underside. Tricky to do as it gets in the way of the tail pipe but its location does mean it is clear of the wing downwash when at high angles of attack so it remains effective.

Those are some good-looking planes. I like the very large trailing edge fillets on the middle one. It's funny how closely my design progression ended up reflecting that except for my very final one, although I think Hawker did consider a nose inlet design on paper but never built it.

I looked up a few videos of rc F-100s online, just to try and get a sense of what to expect, both of the discontinued Flex Innovations 90mm and the 3D printed 110mm version from mb-innov8tive and somewhat surprisingly, both seem to have very gently flying characteristics. I think my plane with its higher wing loading and shorter wing may get a bit more sporty in the stall, but I was expecting all of them to be a bit of a handful.
I will have fully flying horizontal stabilizers on this one too, but like you noted, they're in line with the wing. This is intentional since it puts them in the wake from the wing at low angles of attack which reduces drag at high speed, but as you noted, it also puts them in the downwash from the wing at higher angles of attack which could make things interesting

 

[telnar1236]

Got the 50mm version of the EDF mostly put together. I wasn't really able to work on it Thursday or Friday, but it's so simple it came together fairly quickly today.

I ended up using a 4s fan in it. It's enough bigger than my pylon racer that I think I can handle it if it ends up excessively fast and I was worried about it having enough thrust for a hand launch or a dolly launch (a dolly launch needs enough thrust to take off fairly quickly since you have no steering).
I also was sort of curious if blown wings might be a good option for this plane (since I haven't already set enough of a challenge for myself, I was curious about adding in STOL performance too). The short version is they would be super effective at reducing stall speed, but also super effective at reducing thrust and adding drag. I looked at adding them into the c1 version of the jet since it was probably the floatiest design of the lot to start with and changed the airfoil to a Boeing 103 airfoil to give it even more lift.

Power on stall speed would have dropped from 30 mph to 17 mph before even looking add adding slats but drag would be doubled vs the c3 and thrust reduced by a third.

So, while a cool idea, I don't think it is good for this project and I'll be stuck with using a 300 ft long paved runway still, which honestly, I'm fine with. Maybe once I'm done with this, I'll try to build a bush jet

 

[telnar1236]

As Quorneng suggested and as I was a bit worried about, CFD does predict a pretty nasty pitch up tendency, but at least it should be possible to mostly get around that by shifting the CG forwards. The blue dot is where I calculated the CG based on the wing AC at 0 AOA and the red dot is about where the CG needs to be to avoid pitch up according to CFD (200mm initially vs 140mm from CFD behind the wing LE at the root).

On the other hand, the good news is that with the forward CG, this design should be very much capable of flying around at high angles of attack using the all-flying stabilators to control pitch and roll. Flow separates from the wing tips which will probably make the ailerons ineffective at only about 12 degrees AOA, but the wing keeps producing more lift and doesn't fully stall up to around 25-30 degrees AOA which is better than I was hoping for.
You can see the mechanism behind pitch up in these plots - the first shows the wing root where the flow becomes turbulent and starts to detach from the wing, but the wing remains mostly unstalled.

Where at the wingtip, the flow is fully detached and the wingtip is completely stalled.

This results in the wing root producing more lift than the wingtip which you can see in how the wing root has a much lower pressure. Because the wing root is ahead of the wingtip, center of pressure moves forwards relative to the CG and the plane wants to pitch up.

This results in a slightly weird behavior where the plane is stable by fairly different amounts at different AOAs. A negative slope means stability, but the moments are the Mm/2 values from CFD so only the slope is really worth looking at. In zone A from 0 to 2 degrees AOA, the plane is behaving pretty normally. Then in zone B from 2-15 degrees AOA, the downwash from the wing results in a positive pitching moment on the undeflected horizontal stabilizer - this would have resulted in the plane being statically unstable at all AOAs with my starting CG so I'm glad I ran this analysis. Next, in zone C from 15-17 degrees AOA the horizontal stabilizer starts to escape the downwash and stability increases again. But then in zone D from 17 to 20 degrees AOA, the wing's pitch up tendency becomes apparent and decreases stability again, although the plane remains statically stable. And finally in Zone E, from 20-25 degrees AOA, the wing starts to stall more severely while the horizontal stabilizer remains fully unstalled due to its lower aspect ratio which means the horizontal stabilizer contributes a greater proportion of the lift when compared to lower angles of attack, and the stability increases accordingly.

@Piotrsko and @Zoom Master this is one of the few situations where having the stabilizer stall first is possible and would be extremely bad. In that case, the wing's pitch up tendency would be greatly amplified and the plane would become uncontrollable. In this case, I used an airfoil that had a slightly lower stall angle of attack on the horizontal stabilizer than on the wing, but more than compensated for that with the lower aspect ratio.

 

[telnar1236]

It's getting cool enough to start carrying my planes in my car again, so I stuck the 50mm prototype in my trunk and flew after I got out from work. It was mostly good. The good is that it was pretty fast, flew well at higher speeds, and didn't have any weird pitch up tendencies. It also hand launched really easily despite my worries, and I think I would have been ok on 3s after all. I didn't start the gps recording, so I don't know how fast it was, but it felt faster than my 50mm jet trainer which hits 75 mph but it didn't feel like it was hitting the CFD predicted 115 mph. Based on the elevator trim being 5 degrees up and the CFD trim and lift predictions (which are much more reliable than drag typically) it was about 100 but that could be off by a ways in either direction. I didn't really expect it to hit 115 and built a ton of room into the top speed projections for the 80mm version to hit 130 mph still, but it is a bit disappointing. If I can get all the kinks worked out, I may release the 50mm version as a resource since it's pretty nice to fly. I'll try and get video and an accurate gps reading soon.

Now, the bad stuff. Stalls get very exciting very quickly. It does not pitch up thankfully or even really drop a wing too badly, but it does lose directional stability. I'm not sure if I want to design this out or keep it in so I can do some crazy aerobatics - you're flying along slowly pulling back on the stick to slow down and then all of a sudden the plane is pointing 45 degrees off its angle of travel and losing speed very rapidly, then falling. It only takes about 40 feet to recover, but it was certainly surprising the first time it happened. I think for the next flights of the prototype I may try gluing on a ventral fin on to the bottom to see if that improves matters.

I also made a mistake when choosing the wing's incidence angle and didn't account for the trim forces from the elevator when I selected the slightly negative incidence. This makes it impossible for the plane to fly at its lowest drag condition and requires additional up trim beyond what I originally designed for, so I may reprint it with that fixed.

 

[telnar1236]

Also, has anyone tried making their own Li-Ion packs from cells they bought? I saw some 21700 cells that should be able to get 9000 mAh for the weight of a heavy 5000 mAh pack and about the same price while also being able to handle the current draw of an 80mm EDF but I don't know if it's worth it trying to mess with something like that.

 

[telnar1236]

The new ventral fin design. I added two smaller fins instead of one big one since one big fin would have likely hit the ground when the plane rotates for takeoff.

So far I've only modeled them on the 50mm version, since if they don't work there's no reason to add them into the 80mm version.

 

[L Edge]

When @telnar1236 showed his interesting work on BWB designs and his jet with the EDF in the rear, it caught my attention. Now his approach is based on flow studies, I use the white paper approach, evaluation of forces and moments and just build and test. For instance, how far back does he places the inlet EDF and what effects does the ram drag due to the thrust. One study is trying vortex generators to improve the system. Also in changing AOA and sharp turns is causing problems. So now EDF inlets really enter into the picture and boundary layers..

Next is the BWB dual/triple EDF is very interesting. They have a number of airfoils they are playing with my question is, how high above the wing and where do you place to reduce the ram drag? I see a few scale models are either in wind testing or flying, but not to much info is released.

So I went to my boneyard and dug up 2 models I want to play with. These are my funky flyers, and start to explore flying them in sorta BWB. First has linear bottom(orange) and eyeball airfoil and second has eyeball design of symmetrical wing. Prop will come off and 64mmEDF will be installed (on triangle) plus my "UDDER"bluefoam rudder since DT is not available. Going to see where it is center placed and what height above? to see if it flys?. Going to be ugly looking for all I am doing is chop and fill.
Second one will introduce 2 EDF's(now has DT) and see what happens. Hey, in a short while, I hope a sorta poor man's version of a BWB is working or out with the trash.

Will present results as to what happens.

 

[telnar1236]

When @telnar1236 showed his interesting work on BWB designs and his jet with the EDF in the rear, it caught my attention. Now his approach is based on flow studies, I use the white paper approach, evaluation of forces and moments and just build and test. For instance, how far back does he places the inlet EDF and what effects does the ram drag due to the thrust. One study is trying vortex generators to improve the system. Also in changing AOA and sharp turns is causing problems. So now EDF inlets really enter into the picture and boundary layers..

Next is the BWB dual/triple EDF is very interesting. They have a number of airfoils they are playing with my question is, how high above the wing and where do you place to reduce the ram drag? I see a few scale models are either in wind testing or flying, but not to much info is released.

So I went to my boneyard and dug up 2 models I want to play with. These are my funky flyers, and start to explore flying them in sorta BWB. First has linear bottom(orange) and eyeball airfoil and second has eyeball design of symmetrical wing. Prop will come off and 64mmEDF will be installed (on triangle) plus my "UDDER"bluefoam rudder since DT is not available. Going to see where it is center placed and what height above? to see if it flys?. Going to be ugly looking for all I am doing is chop and fill.
Second one will introduce 2 EDF's(now has DT) and see what happens. Hey, in a short while, I hope a sorta poor man's version of a BWB is working or out with the trash.

View attachment 253931

Will present results as to what happens.

Should be interesting to see

 

[L Edge]

Here is a video I explored with prop. Throttle up the plane gets up on level and stays on plane and doing a loop ends up wacky shape. This flight I explored gusty winds, slow flight ,stall, high alpha, and feeling out when a stall is approaching. I have decided to fill in prop area and play some up/down above frame with single EDF first. Glad we are dealing with low speeds, lots of major problems for speeds into the 500 to 600 mph.

 

[telnar1236]

Here is a video I explored with prop. Throttle up the plane gets up on level and stays on plane and doing a loop ends up wacky shape. This flight I explored gusty winds, slow flight ,stall, high alpha, and feeling out when a stall is approaching. I have decided to fill in prop area and play some up/down above frame with single EDF first. Glad we are dealing with low speeds, lots of major problems for speeds into the 500 to 600 mph.

Center of pressure tends to change dramatically on those cranked arrow designs at higher angles of attack. I think the prop helps keep it manageable by blowing air over the wing, so it will be interesting to see how using the EDF changes things. I've successfully flown a couple designs like that, but I've never really gotten them to a point where I like how they fly.

 

[telnar1236]

Winds were up around 20-30 mph this weekend, so still no flight on the v2 50mm prototype. I was a bit bored and I was given a box of old brushed EDFs a couple weeks ago to mess around with so I designed another plane around one of them using the laminar flow airfoil.

These are 5-bladed 2s EDFs (actually originally intended to run on Nicad packs, but thankfully a 2s pack is about the right voltage) and they only draw 10A and produce 80g of thrust, so the goal with this design was to keep it as light as possible while also reducing drag so that even with the very low thrust it can keep itself airborne. It will weigh in at a bit over 200g, so the thrust to weight ratio will be less than 1/2 which makes low drag absolutely critical (3d printing is heavy and even using LW-PLA, it's not a great approach to use for this kind of EDF, but I was curious if I could do it).

It's a bit of an awkward stubby looking plane, but that was necessary to keep the size and weight down. I wouldn't be shocked if this doesn't work at all with how low the thrust really is

 

[L Edge]

Center of pressure tends to change dramatically on those cranked arrow designs at higher angles of attack. I think the prop helps keep it manageable by blowing air over the wing, so it will be interesting to see how using the EDF changes things. I've successfully flown a couple designs like that, but I've never really gotten them to a point where I like how they fly.

I am going to try some new things I have learned during developing the A-10, Sr-71 and Dark Star. So I am hoping the exploring with the EDF will produce stability and see if the EDF looses thrust in high AOA or turns and causes stalls.
Loops at present required down elevator to get it closer to a circle, not "E" shape. So all you need to do how to correct for it.
More excited about using dual edf's to see what happens giving you yaw.

 

[Piotrsko]

Better stall characteristic/ slow speed handling going away from the 00xx series towards some of the 64 series. Believe the leading edge had an effect which according to my books got fixed by slightly cambering down the front which they say is the 64 series. Beyond my pay grade, but also stay away from retreating high points towards midpoint

 

[telnar1236]

Better stall characteristic/ slow speed handling going away from the 00xx series towards some of the 64 series. Believe the leading edge had an effect which according to my books got fixed by slightly cambering down the front which they say is the 64 series. Beyond my pay grade, but also stay away from retreating high points towards midpoint

Yeah, especially the thinner NACA airfoils don't stall too nicely. This is the airfoil I'm using for this design (my own custom airfoil) - it's pretty well optimized for low drag at rc plane scales and speeds, but it pays for that with a pretty abrupt stall and a lower max lift coefficient. Overall, it's not too bad to fly though, even at low speeds/higher angles of attack and it gives you a nice step up in performance.

 

[telnar1236]

Flew the v2 Super Duper Sabre 3 times after work today and the ventral fins are a huge improvement. It still isn't the most stable thing at higher angles of attack, but it now just wobbles but still flies instead of the dramatic loss of stability of the v1.

Top speed was 91 mph in this set of flights, which isn't bad at all for a 50mm EDF, but I suspect it has more in it

 

[L Edge]

Flew the v2 Super Duper Sabre 3 times after work today and the ventral fins are a huge improvement. It still isn't the most stable thing at higher angles of attack, but it now just wobbles but still flies instead of the dramatic loss of stability of the v1.

Top speed was 91 mph in this set of flights, which isn't bad at all for a 50mm EDF, but I suspect it has more in it

You should call it Super Saver Sabre. You are definitely a jet jockey.

For all future jet jockey's(haven't flown any yet) I invite you to run this video at .25 speed. Rerun from about 2 to 6 seconds, would you have the control necessary to get upright and adjust again and then go speeding off. Then jump to landing (2:40's) where you are coming in over the mound(left of the tree/bush?) and dives down and levels off real fast and then complete a safe landing.
Now reset to normal and notice his skills including far away, flying with clouds in background, flying smoothly across field, from takeoff to landing. nice turns with little drop and large turns.

He ain't no student pilot. Now for those that like speed. Top speed for this video was 91mph. Hey crank up speed to1.75 times normal. Play and fly the video will give you 160mph. That's what skills you guys would need to do this. It's tough when you have clouds behind you. So clean your computer screen and rate your skill. Yes, flying jets is more than learning that you need to account for spooling up.

 

[Mr Man]

I’ve flown 160 before 😁

 

[telnar1236]

You should call it Super Saver Sabre. You are definitely a jet jockey.

For all future jet jockey's(haven't flown any yet) I invite you to run this video at .25 speed. Rerun from about 2 to 6 seconds, would you have the control necessary to get upright and adjust again and then go speeding off. Then jump to landing (2:40's) where you are coming in over the mound(left of the tree/bush?) and dives down and levels off real fast and then complete a safe landing.
Now reset to normal and notice his skills including far away, flying with clouds in background, flying smoothly across field, from takeoff to landing. nice turns with little drop and large turns.

He ain't no student pilot. Now for those that like speed. Top speed for this video was 91mph. Hey crank up speed to1.75 times normal. Play and fly the video will give you 160mph. That's what skills you guys would need to do this. It's tough when you have clouds behind you. So clean your computer screen and rate your skill. Yes, flying jets is more than learning that you need to account for spooling up.

I appreciate the compliments but I'm really a pretty average jet pilot. They're certainly not beginner planes and I've been flying for a while, but they're also not too bad to fly. The takeoff looks extremely sketchy and looked the same every flight, but I barely touched the sticks - it rocks back and forth and looks awful, but then it recovers itself and flies off normally.

Orientation is every bit as much of a problem as you're saying though - it's pretty hard to see against the clouds. Incidentally this is why I had such poor luck with the pylon racer design - it's somewhat smaller and even on 4s about 20% faster and I couldn't keep track of it - as a plane the pylon racer was actually nicer to fly than the jet.