Efflux Mk. II: Circulation Control and Drooperon Testbed

[Pieliker96]

This project started from a video I came across while researching boundary layer control - Once I saw how drastic the changes in the flow field could be from some surface blowing I knew I had to give it a shot.

I implemented circulation control through trailing edge blowing. A diverter door channels air from behind the EDF up into the wing. From there it distributes along the wingspan and exits through a slot, turned down by 45 degrees, at the trailing edge. This should in theory increase circulation over the entire section when active, and is also positioned to blow on the upper surface of the flaperons when they are deployed, F-104-style. EDFs are generally not good at dealing with flow restriction - though as long as you keep the outlet ducting at a reasonable %FSA it seems to work out. A test wing with an 85% FSA slot worked well, but the final version stalls the fan and has reduced flow (though still considerable) due to the ESC and wiring blocking some of the internal cross-section and constricting flow.



I'll also be testing 'drooperons' or leading edge flaps due to their use on one of my previous aircraft, the Cormorant II.

This will be the second installment of the Efflux Series: the first was an in-flight thrust measurement rig for a variable outlet area nozzle. The Efflux Mk. II uses the same avionics (graciously donated by @CampRobber btw) and powertrain as the Mk. I. The flight controller enables measurement of accelerations, airspeed, vehicle orientation and body rates, as well as providing control to many more servos (and with many more mixes) than a standard tx/rx combo.

Here's a walk-around of the craft, including a demonstration of the trailing-edge blowing system:

Goals (Drooperons)
-Demonstrate an increase in maximum lift coefficient with the flaps extended by drooping the leading edge
-Demonstrate an increase in maximum roll rate at high airspeeds by differentially deploying the drooperons along with the ailerons

Goals (Trailing Edge Blowing)
-Demonstrate an increase in maximum lift coefficient both with the flaps up and down with the blowing system active

 

[Flying Monkey fab]

I'm seriously impressed.

 

[L Edge]

This is what I did trying to simulate the Boeing YC-14. Still need to fine tune flaps position to get even lift.

Did another project called thrusters when I diverted flow out of single EDF by tube to the end of wing to prevent rotation while trying to hover. Also automated leading edge slots, so my suggestion to you is explore the slot deflection to prevent rolls. Then do your thing.

Looks like a fun project.

 

[L Edge]

2 questions:

1) What size EDF?
2) How about battery?

 

[Pieliker96]

2 questions:

1) What size EDF?
2) How about battery?

A 70mm EDF running off a 4s 3000. All-up weight is 1.45kg and area is 300 sq. in which puts cube loading at at 17(!)
The inlet and weird outlet ducting geometry also restricts the amount of power I can get on the ground - Only ~850W as compared to a free-air 1000W. Thrust-to-weight will definitely be below 1 and it'll take a good time to get up to speed - though that top speed should be rather high given the thin airfoil, minimized frontal area and just-over 100%FSA inlets. The Efflux Mk. I managed 70MPH all-out, I'd like to best that if possible.

 

[Scotto]

That is awesome! I love it. Bravo! The old video was really cool too.

 

[L Edge]

It will be very interesting in what the P static and P total will be at different locations. I even reduced the inlet blade gap (used 2 layers of scotch tape) and housing of EDF to reduce front end leakage (use a slanted water u-tube) to measure differences. Hope you have success.

You might need a 6s setup.

 

[Pieliker96]

It will be very interesting in what the P static and P total will be at different locations. I even reduced the inlet blade gap (used 2 layers of scotch tape) and housing of EDF to reduce front end leakage (use a slanted water u-tube) to measure differences. Hope you have success.

You might need a 6s setup.

We'll see how it performs on 4s - I'm expecting it to be more than adequate for getting around the pattern but not enough for high-energy aerobatics.
Reducing blade gap is a great tip, I'll give it a shot. I don't have any tubing on-hand so I'll just trust that it works
I also ended up adding some 40g-ish of tail weight to get the CG proper in the form of 6 quarters.

 

[L Edge]

We'll see how it performs on 4s - I'm expecting it to be more than adequate for getting around the pattern but not enough for high-energy aerobatics.
Reducing blade gap is a great tip, I'll give it a shot. I don't have any tubing on-hand so I'll just trust that it works
I also ended up adding some 40g-ish of tail weight to get the CG proper in the form of 6 quarters.

My static test was a full battery with a cheap 5 bladed fan with a huge gap. It hovered at 53% throttle and recharged battery and it hovered at 47% throttle after 2 layers of Scotch tape were applied to the housing.
Have you ever played with strakes to accomplish and improve high alpha?

 

[Pieliker96]

My static test was a full battery with a cheap 5 bladed fan with a huge gap. It hovered at 53% throttle and recharged battery and it hovered at 47% throttle after 2 layers of Scotch tape were applied to the housing.
Have you ever played with strakes to accomplish and improve high alpha?

I've never really tested them, though I have done dogtooth leading edges once. I'm not so concerned about high alpha here as I am high lift

 

[Pieliker96]

It didn't have the power to take off grass so I took it to pavement. It didn't have enough power to maintain any sort of semblance of climb rate, and (combined with a pilot-induced oscillation due to high rates and low static margin) quickly went in and out of stall, culminating in a violent snap roll (640deg/sec roll, 330deg/sec yaw) that sent it inverted into the tall grass. Damage was minimal and repairs were easy. The inlets were enlarged and allowed to pull from the airspace under the canopy, which netted a decent increase in thrust and acceleration in ground tests (0.4g -> 0.57g).



That brings us to the first flights, which went well. It is underpowered, but not fatally so. With a long enough takeoff roll, it can get off grass.

It achieved a top speed of 34m/s (76mph) in testing, which exceeds the Efflux Mk. I's previous record. Stall speed seems to be around 12m/s (27mph) based on preliminary flight envelope estimates.



I'd like more thrust, so I'm looking at further modifying the inlets or getting the high-speed variant of the fan disk. Once the performance is where it should be I can start collecting data with respect to the objectives.

 

[Pieliker96]

Well here's the inlet redesign. I added some eyeballed NACA ducts and sunk them into the fuselage to increase the inlet area to around 200%. This also removed the "cheater" hole, though I don't really need it any more.



With a ring of tape to reduce blade tip clearance and a new fan disk with a higher blade angle and thicker blade chord, I'm now up to 0.76g (including rolling resistance) of static acceleration. The previous redesign was 0.55g, and the original was 0.41g - And all this increase with just an ~8% increase in power consumption, now edging 1kW. Hopefully it'll have shorter takeoff rolls and a higher top speed!

 

[L Edge]

Well here's the inlet redesign. I added some eyeballed NACA ducts and sunk them into the fuselage to increase the inlet area to around 200%. This also removed the "cheater" hole, though I don't really need it any more.

View attachment 231118 View attachment 231119 View attachment 231120

With a ring of tape to reduce blade tip clearance and a new fan disk with a higher blade angle and thicker blade chord, I'm now up to 0.76g (including rolling resistance) of static acceleration. The previous redesign was 0.55g, and the original was 0.41g - And all this increase with just an ~8% increase in power consumption, now edging 1kW. Hopefully it'll have shorter takeoff rolls and a higher top speed!

View attachment 231121 View attachment 231122 View attachment 231123

Using Scotch tape, it is a way to improve thrust. Even found that with the tape, even ii the blades brush the housing, it destroys the Scotch tape rather than the blades.

Another tip I will give you is to shorten takeoffs, increase the nose wheel length to have a couple of degrees positive so airflow gets under the wing. Way I found that out was having a F-22 with variable LE flaps, it would bounce down the grass runway, catch the air under and when V1 was reached, it would jump nose high and almost stall out. Even when I automated the LE flaps,( found out it also reduces turn radius), I had the same problem.

 

[telnar1236]

This looks really impressive. I'll definitely be following your progress.

I tried something similar a few years ago. My primary goal was to use boundary layer suction to try and reduce drag, but I also messed with a blown wing at the time using air, both diverted from the EDF similar to what you are doing and using the separate pump I used for the suction system, to see if I could get improved lift. While I did get a better lift coefficient for any angle of attack with the blown wing, I actually found that I could fly slower without it since the lost thrust from the EDF (much more than yours) meant that I had to fly at a lower angle of attack any time except for landing, and, even on landing, it forced a noticeably steeper approach if I wanted to stay slow. Using a separate pump that could give higher pressure air got slightly better results since the pressure loss was so high in my system that the EDF didn't really achieve a large enough flow rate over the wing and I still got full power out of the EDF, but the added weight partially negated the benefits. And the boundary layer suction system ran into issues with weight and added power consumption meaning I didn't gain anything there either. I ended up giving up on both and just using slotted flaps to improve lift which worked pretty well and were a lot lighter and simpler. I think the biggest issue I ran into was the lack of a "free" source of high-pressure air like you get with bleed air from a jet engine.

But just because I did not figure it out, doesn't mean that you won't. Your system looks promising and I would love to see it work.

 

[Pieliker96]

Some footage from today's flights, with data visualization. Not much extra speed on the top end (up to 35m/s), but a good bit more power at cruise - I'm capable of maintaining altitude at half throttle now. Getting to know the aerodynamic limits and stall characteristics as well.

Updated cumulative flight envelope for the standard camber setting, now loaded to +4.5/-2.5g:


Preliminary envelope data for minimum camber setting (drooperons and flaps reflexed, will be expanded in later flights):

Flaps were doing some really weird pitch stuff, so I elected to postpone testing. At high speeds they give a big pitch-down moment, and a pitch-up moment at low speeds. There's also some power to pitch coupling due to the low pressure exhaust jet below the empennage. I'll work on sorting these out before going for the high-risk stuff.

I'm planning on getting maximum roll rate vs. airspeed data on the next flights, with the standard / neutral camber setting.

 

[JetCrafts]

wow that looks great

 

[JetCrafts]

try boundry layer suction or celera 500l delayed increase in width of fuselage and aerofoil to help with seperation

 

[L Edge]

Some footage from today's flights, with data visualization. Not much extra speed on the top end (up to 35m/s), but a good bit more power at cruise - I'm capable of maintaining altitude at half throttle now. Getting to know the aerodynamic limits and stall characteristics as well.

Updated cumulative flight envelope for the standard camber setting, now loaded to +4.5/-2.5g:
View attachment 231148

Preliminary envelope data for minimum camber setting (drooperons and flaps reflexed, will be expanded in later flights):
View attachment 231150
Flaps were doing some really weird pitch stuff, so I elected to postpone testing. At high speeds they give a big pitch-down moment, and a pitch-up moment at low speeds. There's also some power to pitch coupling due to the low pressure exhaust jet below the empennage. I'll work on sorting these out before going for the high-risk stuff.

I'm planning on getting maximum roll rate vs. airspeed data on the next flights, with the standard / neutral camber setting.

What's the cost of equipment?

 

[Pieliker96]

What's the cost of equipment?

Around $130, though what I'm doing with it currently could be done with less:
-Matek F765 Wing (now discontinued due to chip shortage, $70)
-Matek ASPD-4525 Digital Pitot Tube ($40)
-RDQ BN-880 GPS/Compass ($20)
If you're looking for a data collection system for your projects I'd be willing to donate a flight controller and any sensors you may need. I think the planes you've built are interesting in the concepts they explore and I'd like to see more of that into the future. @L Edge DM me if you're interested.

 

[telnar1236]

Some footage from today's flights, with data visualization. Not much extra speed on the top end (up to 35m/s), but a good bit more power at cruise - I'm capable of maintaining altitude at half throttle now. Getting to know the aerodynamic limits and stall characteristics as well.

Updated cumulative flight envelope for the standard camber setting, now loaded to +4.5/-2.5g:


Preliminary envelope data for minimum camber setting (drooperons and flaps reflexed, will be expanded in later flights):

Flaps were doing some really weird pitch stuff, so I elected to postpone testing. At high speeds they give a big pitch-down moment, and a pitch-up moment at low speeds. There's also some power to pitch coupling due to the low pressure exhaust jet below the empennage. I'll work on sorting these out before going for the high-risk stuff.

I'm planning on getting maximum roll rate vs. airspeed data on the next flights, with the standard / neutral camber setting.

The pitching moments you describe sound about right. The pitch down is because of a larger airfoil generated pitching moment similar to what all flaps cause at small deflections, while the pitch up is because of a combination of flow separation and an interaction between the wing and tail and suggests that you're still getting flow separation on the flaps at those deflections and angles of attack. A higher aspect ratio wing with a shorter chord should avoid such large high-speed pitch coupling, while a T-tail could mostly avoid the low-speed pitch coupling but would result in a design that was vulnerable to pitch up and it might not totally eliminate it.

I was also thinking about what you were saying about increasing roll rate using the drooperons, and I think you might see something similar. Flaps (and downwards deflecting ailerons) both increase camber and effective angle of attack (by shifting the chord line) but drooperons decrease effective angle of attack while increasing camber. Therefore, you might see a small decrease in roll rate at low angles of attack by deflecting the drooperons and ailerons together. However, they should definitely help at high angles of attack where they should enhance aileron effectiveness and reduce adverse yaw.