Flying Wing Set-Up - Wash-Out and Reflex.

This article is about some of the finer details needed for setting up a flying wing for stable flight.

WASH-OUT:

I have read from several sources that wash-out is almost universally required for stability in flying wings.  I always used it on my conventional planes when I was doing nitro.  The one exception is still hanging on my garage wall in pieces, the victim or tip stall on landing.

Wash-Out is a twist built into the wing so that the wing tip is at a lower angle of attack than the wing root.  The main reason for using wash-out is to avoid wingtip stalls which usually happen on landings with disasterous results.

There are a couple of factors at play.  

First, airflow is not unifrom over the full length of the wing.  Tip vortex has an effect on the air pattern near the wing tip.  The vortex disturbance has the effect of the air near the tip striking at a slight upward angle as compared to at the wing root resulting in an effective higher angle of attack if the  wing is made flat..  

Second:  It is desirable to have an actual lower effetive angle of attack at the tip so that the wing root stalls first.  If the tip stalls first it usually happens on one wing first.  Immediately that wing loses lift and a sharp roll can occur.  There is not time to recover if this happens on a landing.

So there are two components contributing to the requirement for wash-out and they are addative.  If you need two degrees to compensate for tip vortex and two degrees for tip stall avoidance asside from the vortex effect then you need 4 degrees total.  

The amount required can be calculated precisely if you have all of the design data, an aeronautical engineering degree, a math degree, a wind tunnel and several weeks of spare time.  If we were putting Mom and the kids in these things that would all be justified.  In the real world of modeling science where only our egos and a small investment of time and coin are at risk rule-of -thumb engineering and trial and error will do.  The rule-of thumb value I have heard most often is 3 or 4 degrees. 

My flying wings were all built holding the flat wing bottom against the building board, so no wash-out.  I am going back and cutting the glue joints at tha backs of the flat bottom panel and then twisting as I reglue them.  I am able to get 2 to 3 degrees.  I think that will be enough in combimation with the undercambered tips that are common to the Flite Test designs.

Here I am using a long thin knife to cut the original glue joint:

The blade is extended to about 2" so it can "feel" the gap between the foam boards as I go. (This is a $.50 break away knifi from the special bin a the local Chevy dealer.  I resharpen it frequently with a dry wet stone. It has built about 16 planes to date and no end in sight.) 

Here I am holding pressure with my thumb forward and fingers back to hold the twist as the glue dries:

Both hands are required for this but someone had to take the picture.

On future builds I will get the twist established while gluing the spar, probably using a long tapered wood block along the trailing edge set to the right amount of twist.  That will be easier than reworking.

Here are the results on the first two wings: 

You can see the bottom of the wing at the root but disappearing at the tip.  These have between 2 and 3 degrees of wash-out.

Here is the one shown above being cut:

This is showing 3/4" positive at the wing root with zero at the tip.  The wing root is 13 3/4 ".  Drawing that and measuring with a protractor shows about 3 1/2 degrees.  In combination with the FT under cambered tips I think that will work. 

Without getting too complicated I think 3 degrees of wash-out is a good rule of thumb.

REFLEX: 

A flying wing is basically unstable compared to a conventional airplane because the major stabilizing component, the extended tail, is missing.  Conventoinal aircraft use the tail to push down to keep the wing at a small angle of attack to produce lift in balanced flight.  Whether the airfoil is flat bottom which itself produces a nose down pitching moment or symetrical which is neutral if aligned with the airstream, the horizontal stabilizer, elevator or both are used to push the tail down creating a positive angle of attack in the wing.  In the flying wing this function is asigned to the elevons.  They are usually set at a small positive angle (3 to 5 degrees) to create "reflex" in the wing.  The result in a flat bottom or under-cambered wing is that the front half of the wing has a nose-down pitching moment that must be balanced by the pitch-up moment of the elevons.  In addition the reflex has to create enough positive angle of attack to create adequate lift to accomplish take-off and maintain stable flight.. The amount required varies depending on the amount of pitch-down moment produced by the wing.  It will be small for symetrical wings, larger for flat bottom ones and larger still for undercambered ones.

Here are a couple of pictures showing one of my "One Sheet No Waste" designs with the elevons with 3 to 5 degrees positive:

This wing is flat bottomed with the FT undercambered tips on the outer 1/3 of the wing span.  I will be putting in about 3 degrees of wash-out.  I will start with about 4 degrees of reflex and do some glide testing.  If all looks well I will launch and fine tune with the trim tabs and then adjust as needed. 

UPDATE (3-18-14):

Below is a sketch showing the concepts of wash-out and reflex graphicly per a commenter's request.

UPDATE 3-29-15:

Note: I am attaching a copy of another article written later as an update to this atricle so you have the information without having to reference the second article.

HERE IS THE ENTIRE ARTICLE THAT WAS PUBLISHED ON 3-28-15:

Hello everyone:

This article is an update of my earlier article "Four Sheet No Waste 80" flying wing" that was published last summer.

Here are a couple of pictures of the model as it appeared in the original article:

The earlier article was a build article that I published as I was developing this design and building the first three examples but before extended test flying.  Last summer was my reintroduction to RC after a 3 or 4 year period of flying and then a 17 year gap.  I was pretty much starting over on the flying part.  Through the summer I flew a few times at a couple of local school football fields and stayed with some of my smaller designs.  I finally broke down and rejoined one of the local AMA club fields in September.  Before the weather turned I got about 25 flying sessions in and flew the "80" at least once in most of them.  I now have at least 25 flights on the prototype.

I built 3 of these, all the same, before doing any flying.  The first I built in a flurry of activity without photos as I was working out the design.  It was one of those "Honey, why did you stay up till 3:30 in the morning" episodes. The second I built exactly like the first but took time to take photos for the build article.  As I wrote the article I found that I needed a few photos that I had missed so I built a third to get those.  I figured I would need at least two and possibly all three due to accidents before I got up to speed with the flying but that went much better than I feared.  I am still flying the prototype with just a couple of minor wing tip bumps.

The wing flies quite well.  It is very steady in the air.  The elevons are responsive but not snappy.  It is too big and light for that.  It does fairly small loops and gentle to moderate rolls nicely.  It stalls out of large loops or extended vertical. Stalls are very gentle and recovery is very quick. I once got it into trouble in a cross wind flying too close to the flight line and had to roll it almost to 180 degrees to pull away.  It did the maneuver starting at about 20' from the ground and recovered after the adjustment at about 15' in the middle of the runway all in between 50' and 75' of distance.

The current motor is rated at 342 Watts.  I have a 500 Watt that I intend to try this year.  I think that will push the vertical performance. 

In the process of test flying I discovered a couple of items that needed modified to improve the design:

Modification #1:

Problem: Wobbly landing gear.

The first modification became obvious in the first two flights. The landing gear was OK for taking off but was not rigid enough for landing.  I had originally designed it with pods attached to the wing with skewers and landing gear attached to those with rubber bands.  The combination proved to be too flexible and the main gear collapsed on anything but a perfect landing.  The first modification was to make the landing gear and supporting pod permanently attached to the wing.  

Attaching the pod was simple.  I just removed the skewers, applied glue to all of the mating surfaces and reinstalled the pods in the original positions minus the securing skewers.  Spacers were already in place to facilitate this.  I then ran a small bead of hot melt along all of the joints to clean up the appearance and for extra strength.

The landing gear attachment was a little more involved.  The original design used a triangular bent wire arrangement where the rubber bands secured it to the pods.  The wire was 1/8".  I took the gear off and traced the  inside of the triangle onto 1/8" plywood.  I traced a second piece of 1/8" plywood to the outside of the triangle.  I then repositioned the gear and glued the smaller piece inside the wire triangle to the original plywood base.  I ran a bead of glue between the wire and the plywood insert.  I then glued the larger triangle over the first sandwiching the wire gear in place.  Finally, I ran a bead of glue around the outside of the wire attaching it to the original plywood base.

Here is the original installation:

Here is the modification:

This is much more secure amd does not collapse on landings.

Modification #2:

Problem: Failing landing gear attachments.

This arrangement held up well for a few flights but the paper then started to separate from the foam under the plywood base on one side.  To fix this I carefully cut the paper lose on three sides, leaving the fourth side as a hinge.  I then spread a generous amount of glue on the back side of the paper and swung it back into place. I proactively did this repair on all three landing gear positions.  I have not had any subsequent landing gear issues.

Here is a shot of the gear after cutting the paper and ready for glue:

The "after" picture is the same as the second picture above.  It looks the same before and after the repair.  

If I were doing this during the original building process I would cut out a rectangle of paper 1/8" smaller than the landing gear base so that it was glued directly to the foam instead of just to the paper.    

Modification #3:

Problem: All white was just not gettin' it!

My original article was published with an all white model.  I had published the article shortly after developing the design and had built three but had not painted any of them.  Most readers saw the article before I added the painted shots.  I later updated the article with a couple of color potos but in case you read it early and did not see the painted version here it is again.

The model was painted with oil based aerosol paint.  The top was done with bright yellow trimmed with fluorescent orange.  The bottom was done with the same orange trimmed with black.  Striping was added to the top using black duct tape. The color scheme works well for orientation since the bottom is much darker and the stripes are distinctive.  

Here are a couple if pictures of the paint job:

Modification #4:

EDF Conversion Preview:

Problem: It looks like a jet but it has a propeller.

One of my readers (Yogenh) asked if I had ever thought of using EDFs on it.  Up till that point I had not.  I had never donre anything with EDFs so the thought just never occured to me.  I looked into it a bit and decided that it might not be bad idea.  

I found a set of 64 MM EDFs on Amazon that claimed 750 g of thrust each and ordered a pair.  After getting them I could not get more than about 550 grams each so I am looking for repalcements.  There WILL be two EDFs on 80" wing #2 before the beginning of the flying season! (Better hurry up Dan!)

Here are a couple of pictures showing the plannned installation:

You can also see the modified landing gear supports on #2 before painting.

These are  just sitting in place till I verify whether I will use them or posssibly go to the 70 mm units for better thrust.    

The mounts are a simple 3" long "D" tubes made from Dollar Tree foam board.    The paper is removed from the inside of the curved part.  They are covered on the outside surface with 3 overlapping pieces of packaging tape. They are sized to allow the EDF and exhaust tube to fit snugly.  The tubes will be glued in place.  The EDFs will be a snug friction fit and will have a built-in stop to prevent them from sliding back too  far.

Here is a picture showing one EDF unit and one "D" tude disassembled:

This is the  same EDF unit assembly that was used in the "One Sheet No Waste Simple Fold Delta - EDF Version" published about a week ago.  It is made by hot gluing an exhaust tube made from an 11 oz powdered creamer bottle directly to the EDF housing with the wires exiting a small hole on the side (see that article referenced below for details). I will just use two of them for this. I mentioned in the earlier article that this unit could be easily moved fro one model to another.  This could literally be done in 5 or 10 minutes in the field. I will take two of the smaller planes ready to fly and move the motors to this before the end of the flying session.  

The motor wires currently reach just byond the rear corners of the "D" tubes.  I will make a "Y" extension cord to connect each side to the ESC which is inside the power pod at the center line of the aircraft.

The battery, ESC, servos and radio insallation are the same as for the original prop version eccept that the ESC must be 60A (or possibly larger with the 70 mm units).  The motor and prop were simply removed and the EDF units added.   The EDFs were positioned with the fronts of the "D" tubes matching the front of the two main landing gear plywood supports.  This position allows the CG to be maintained in the original position.  

Weight with the two EDF units will be about 1600 g which his about 125 g heavier than the prop version with the same batteries.

Of course the EDF housings and the landing gear modifications will be painted before flying (unless I get impatient and the weather is good).

When I finalize the installation and have a working model I will do a dedicated article which will show all of the details.  I just wanted to give you an idea where I was headed with it for now.

Please look at the original article for construction details if you are interested and missed it last year.  It is a very detailed step by step building guide. 

Well, that is it for now.  #1 Had all of the updates last September and has 20+ flights after modifications.  I am getting #2 ready as the EDF version for when the weather changes and we can get onto the field.  I hope that is soon (snow on the ground in western PA this morning).

Let me know if you build this or any of my other designs with comments here, or...  Send pictures and comments to my e-mail: dhark69@gmail.com 

Thanks for reading this and good luck with your spring flying.

Note: Please take the time to rate and comment on this and all articles you read.  Reader feedback is the most valuable tool that the writers have to improve their product.  The writers and readers will benefit from the improvements that result.  

Dan 

(END OF UPDATE)