Slotted flaps

[Henni]

Hi everyone,

Does anyone perhaps have a link available of any more detail on how to make these:

http://www.love4aviation.com/DYNAERO/Te ... +2003.html

I can come up with a rough idea, but I'm sure there is a better way. How is the forward airfoil of the flap made & how is it attached to the rear section? I guess it would be the same as the leading edge slats on some microlights. If someone can share how those are made and attached to the rest of the wing, it would also help.

Yes, I'm aware of the serious trim changes caused by these, however I'm still interested in giving it some more thought. The Black Shape aircraft uses the same, and so do many other designs like i.e the Pilatus PC6.

Cheers all,

Henni

[Renegade2]

Hi Henni
I don’t know your background, so please forgive me if I am teaching you to suck eggs. The biggest problem with a mod like this is the engineering challenge because this is not a minor modification and has serious aerodynamic and structural implications that need to be resolved.

You need to consider the following as a minimum:
1)    The loads caused by the flap hinge moment.
2)    Flap limiting speed
3)    Tail blanking
4)    The elevator’s ability to compensate for the increased pitching moment
5)    FLUTTER.
If you have already done or are prepared to do the engineering, I have some ideas on how to construct them. I have often thought that this is the best way to get extremely effective flaps without adding the complexity of flap tracks, but the plane should really be designed around them from the start.

[Henni]

Hi my friend,

What I have is this, this, this, and this. They are for fowler flaps. I intend using exactly the same area only for slotted flaps, so the elevator slipstream should be okay. I need to make my rudder higher for my larger canopy, hopefully it will put it back in the moving air. My inboard wings has small split flaps built into them. If needs be I'll leave that part as is for the sake of rudder authority. I know it's not everything you've required, but it is a good and educated starting point at least.

As for flutter, see how they balanced theirs - I intend to do exactly the same.

Elevator authority: I think my KR2 has about the longest tail of any built to date. The initial pics in my project page clearly shows this. I am willing to increase the span should it be required. The Blackshape light aircraft has rougly the same tail length, uses the same double slotted flaps right from the start of the inboard wing.

Hinges should be strong enough and they attach to a boxed wooden rear spar. Off course this needs to be verified with proper calculations. So my flaps will be exactly the same as those above, but they will only be of the double slotted type. Stefan offered a while back that he has proper engineers willing to analyse any design at a cost of course. When I know exactly how to make these, I'll create the drawings & approach him for the stress & other design verifications.

I would be thankful for any help/advice you would be willing to give. After I know how to make, I'll have proper drawings prepared and then approach someone like Stefan or similar to help verify and also with the fine tuning. If it can at all be made to work on my project, I'll be elated.

Kind regards,

Henni

[Renegade2]

Hi Henni

There is some excellent information on flaps here: http://naca.central.cranfield.ac.uk/rep ... rt-938.pdf

Before you do anything further on these flaps, please discuss it with Stefan's engineer. He could save you a lot of heartache and if you are lucky he will not charge you for his advice. It is really important to do a preliminary investigation before you waste time and money on something and then find out the hard way that it doesn't work.

My background is mechanical engineering not aeronautical but this is my take on what you want to do - first lets look at the potential benefits: (These are just rough estimates nothing has been calculated but it should give you an idea)

Lets say you increase the effectiveness of the flaps by 30% - you may be lucky and do a bit better than this, but without access to a wind tunnel or C.F.D. it's not an unreasonable estimate. The flaps cover 40% of the wing span. The improvement in the maximum lift coefficient for the entire wing is 0.3 x 0.4 = 12% - say a 3 to 4mph reduction in stall speed. This is quite a significant change, but it doesn't put you in the S.T.O.L. category.

Now lets look at the potential downside
1) lets say your engineering analysis shows that due to the increased pitching moment and rearwards movement of the wing centre of mass, you need to beef up some wing ribs, the wing skin, wing attachment fittings and the flap linkage. This results in a 25kg increase in the structural mass, and as the maximum all up weight is unchanged, it means that your useful load has been reduced by 25kg or 35l of fuel which means you won't make Margate in one go if you have a passenger

2) You will have spent over a hundred hours on engineering

3) You will spend over a hundred extra hours on construction + the additional cost of materials - ignoring the possibility that you will mess something up and have to do it again and buy more materials

4) You may increase the drag to such an extent that you may not have sufficient engine power to do a go around with full flaps which means when you deploy full flaps you are 100% committed to the landing - maybe not, but it is one of the things that needs to be evaluated

5) For the first flight and every flight thereafter until you have fully explored the flight envelope, you or your test pilot have an increased risk of crashing - fact

6) You will waste weeks or months of your time getting the CAA to approve the mod

Please understand that I am not trying to be negative. As long as you understand that this a HUGE change. From an engineering perspective it's equivalent to changing from a piston engine to a turbine or increasing the maximum all up weight by 30%. When you modify a plane you effectively become the designer with all the responsibilities that entails. If you have your heart set on these flaps, you either need to do the engineering yourself or find someone to do it for you. If you approach the universities, you may be able to find someone who is willing to take it on as a project for no or very low cost. The engineering has to be done before you modify anything because it may be completely unworkable. The CAA will want to see it anyway before they grant you an authority to test.

The original designer very likely considered and rejected complex flaps and hundreds of other things during the design process. The designer always tries to design the best and safest plane he possibly can within the constraints he works within. The assumption the designer makes is that you will build the plane according to the plan. If you make structural or aerodynamic changes, you may as well be building your own design from scratch.

I think if you really concentrate on keeping the (non structural) weight down and build neatly and accurately according to plan you will achieve the same performance gains and you will have a plane that is safer and has a higher resale value.
There are literally hundreds of safe mods that you can do to the plane that will make it better and won't endanger your life - think of things like seats - everything except the mounting and seat frame can be changed to make them lighter, more comfortable and better looking than standard - the choice of instruments and the way they are mounted. You have options with the selection of wheels brakes and tyres, engine, propeller and accessories as long as you follow good aircraft practice.
Sanding the interior and giving it a nice coat of paint can look really good and is a lot lighter than vinyl, leather or carpet. Composite aircraft can gain over 50kg in filler and paint so there are lots of potential savings here too.

[Henni]

Hi my friend,

Thank you so much for all your advice. I doubt if any aircraft have ever been designed that received more modifications than the KR2. There are a great many flying worldwide, and just about everyone is different. They are flying with Rotax two strokes, four strokes, Jabirus, VWs, Corvairs, Continentals, Franklins, Subarus and maby others. Engine weights range from 90 to 260lbs and output from 60 to 180hp. Some have no flaps, others split flaps and even fowler flaps. Some have belly air brakes. Some have spoilers and no ailerons. Some have fuel in nose, some under cowling, some under seats, some behind seats, some in stub wings, some in main wings, some in both. Canopies range from square to round. Elevators differ on many, have been extended, rudders differ on many. All the newest builds use a completely different laminar airfoil, not at all approved by the designer. Some are built from wood, foam & fibreglass, some only of fibreglass, some only of wood and some of wood using only fibreglass skins. The tail has been lengthened, the firewall moved forward, the fuselage rounded. The under carriage range from anything you could imagine to anything you could imagine. Some, even here in South Africa, had water ballast in the tail that has to be drained when flying with a passenger. Some have had their main wings extended using foam covered with a little fibre glass only for main spars attached to the current boxed wooden spars. There have not as yet been a single fatality as a result. Some have their main spars built from wood caps with wood webs and wood ply skins, others from laminated foam & fibreglass sections. Some have been modded to have dihedral right from the fuselage, others only from the stub wings. I'll show you pics of some KR2s that you would no longer reckognize the original design in it at all. Many, many a good idea came as a direct result of all of the above.

There have only been two KR2 fatalities in South Africa. First one as a result of someone adding lead in the spinner to compensate for a rear cog that came off, took off the prop, moved the cog beyond the maximum rear position and the result speak for itself. The 2nd one, by all indications, was as a direct result of an instructor that tried to turn back to the runway under a engine out condition after take-off in direct conflict with his own teachings. Of the many flying over here, this says a lot for their safety record, mods & all.

You should really have a look at my project page. I have my share of ideas and am busy building all of those into my project. It will open up the doors for many future builders to do same if they should wish.

I'm just filling you in on these, not being hard headed at all. I think the KR2 must be a mechanical engineers worst nightmare seeing what has been achieved already by mere amateurs like myself. I can immediately think of extending the flap hinge brackets all the way from the forward spar, link to the rear spar and then to the flaps. It will act as wing fences (yes I know fences are normally on top of the wing) and will most definitely eliminate any spar twisting and/or other structural required mods. And all it will take is some light aircraft grade aluminium angled pieces & I have many of those. As for power with full flaps, the MCR4S has only 100 hp, and has to haul four big adults to safety under the same conditions. Mine has 97hp, weighs much lighter, has shorter double slotted fowler flaps for a lesser wing length as well as a lesser wing chord section and needs to haul only two to safety on nearly the same power.

So yes, I'll approach Stefan with some drawings. Thank you for the link and advice. I'll look into it. Can it be done on my project? Yes!!! It has been done before and it works. My two case studies are the MCR4S (yes, T-tail elevator) and the Blackshape of which the combination of everything is similar to mine. It has slotted flaps right from the fuselage and it does not block out either the rudder or elevator. Mine will have slotted flaps only from the end of the stub wings outwards, leaving the airflow around the main fuselage more or less unobstructed. The other concern of spar twisting can be sorted out with ease and without adding any real significant weight to the overall project. As for wing twisting, I really cannot comment and someone would have to verify for me. The KR2 does use two solid boxed wooden spars, that by itself helps a lot. The wings have been designed for plus and minus 7 g's at gross, I'm sure that must also count for its rigidness. So I'm sure I'm safe, but will have those verified.

Back to my main question: It is easy to make the main flaps. It is easy to make the secondary airfoil shape for the flaps (same as a KR2 wing, elevator, rudder or whatever). It is not easy attaching these two once they are made, neither is it so simple to make them if I need to attach the two spars for these two individual sections to each other before I make them. Hence my main question above. Those who know how to build a foam/fibreglass wooden spar section will exactly understand my specific problem.

To attach these two pieces at both ends is a piece of cake. To attach them at any other section is where I need some valid ideas.

All the best my kind friend,

Henni

[Henni]

Hi again,

Found two VERY interesting links:

http://www.docthrock.com/Wings.shtml

http://www.melmoth2.com/texts/Pictures.htm

All the best,

Henni

[Renegade2]

If I was going to build these flaps I would do it like this:

1) I would build the slats as part of the flaps – see the picture of the DC9 flaps below. The slats and flaps have a common hinge point, and there is no flap track.

Flap Hinge.gif (4.78 KiB) Viewed 3966 times

flaptypes.gif (22.25 KiB) Viewed 3966 times

2) In order to get the benefit of the extra slot the flap deflection must be high – they probably have got to go down 60 or more degrees. This means that the hinge point has to be far below the wing – you are going to have to make completely new flap hinges and a new flap linkage. The force on the flap linkage will be a lot higher so you might need a longer Johnson bar or go to electric operation.

As far as building them from fiberglass is concerned you have two options.
The engineer will be able to give you the layup schedule.

1) Use a hot wire cut foam core. It’s really easy to make a hot wire cutter – just google it. You can only cut cores accurately up to about a meter long so you have to glue the cores together into the length you require – this is the way most of Burt Rutan’s home builts are made. You will need ribs and spars between the core sections anyway so this is not a big issue. After you have the cores and ribs / spars all made up, you glass the outside and then sand / fill / repeat until you get a decent surface finish.

2) Find a furniture shop that has a CNC router. Get them to rout a plug out of MDF. I would do two plugs – one for the top half and one for the bottom half both having a flat base. You then sand and finish the plug until it has a mirror finish and then lay up fiberglass moulds. When you have the moulds, you lay up the top and bottom skins in the moulds, glue the spars and ribs into one half, laminate all the joints and then glue the top skin onto the bottom skin. After gluing you generally have to reinforce the outside of the joint with more glass cloth. Although eliminating the cores makes the flaps lighter, there is a lot more work involved. You probably need to use honeycomb or Rohacell cores in the skins to get the weight savings anyway.

3) I think it would be fairly easy to make a jig to rout out the plugs yourself. I am thinking of a rail that the router can slide up and down on along the Z-axis. You need a frame that sides along the rail and is adjustable in the x and y axis. You then set up the router, take a cut, re-adjust, take another cut etc until done. The router will leave some slight steps, which have to be sanded out by hand.

Composites are not hard – just time consuming and messy. There is a huge amount of sanding and mess involved though and be prepared to make some scrap during the learning curve. I would estimate that you spend 10 hours sanding for every hour you spend laminating. AMT composites in Kempton Park have an excellent 1 day course on composite lay ups and vacuum bagging. They also give free advice. Don’t even think about working with epoxy resins unless you have an accurate digital scale that can measure to 0.1 gram. This is really important. If one batch of resin does not cure properly you end up scrapping the whole part.

[Henni]

Hi my friend,

Tx for the info. I am familiar with foam/fibreglass components. This is not my 1st KR2. I understand exactly what you mean.

Keep well & tx once again,

Henni

[Henni]

Hi all,

Found this interesting write-up:

From: http://www.maddyhome.com/krsrch/thread?tt=f&on=1&hit=11

Subject: Flap Magic

Somebody opened the door by asking what the pros and cons of various flap
types are. So here is one viewpoint, hopefully not a blast of blustery
opinion.

A flap can be a multi-purpose thing. It can create some extra lift at low
deflections and extra drag at large deflections. In any case, adjusting the
angle of the trailing edge of the wing also directly changes the pitch
moment (nose down) created by the flapped part of the wing. This is a very
powerful influence, and should be carefully estimated for aircraft of the
configuration of the KRs (low wing, maybe short coupled, not very powerful
trim capability). Why? Because the extra drag below the center of mass also
creates a nose down pitch moment, and the semi-high horizontal tail reduces
favorable downwash coupling of the wing and tail.

In some extreme cases I have looked at, another complication enters the
picture. You would like to achieve higher lift flaps down than flaps up.
Ideally, then, you will get the maximum lift out of the flapped part just
before stall, while the rest of the wing is somewhat less close to achieving
it's maximum lift. This gives you the lowest stall speed. This is not
guarenteed.

Some flap/wing geometries can't get good lift out of the unflapped part of
the wing. If the flap is a powerful type (like a Fowler arrangement), the
incidence of the flapped wing can be approximated by the chord line from the
dropped trailing edge to the nose of the airfoil. So the flaps can
powerfully increase the effective incidence of the flapped part of the wing.
In a relative sense, the unflapped part is now operating at a much lower,
maybe negative incidence. In some cases this effect limits the chord of the
flap more powerfully than any other single factor. Make it too big
chordwise and your total maximum lift decreases!

Another thing. If you design the flap properly, now the tips of the wing
are loaded and may stall. Ailerons at the wing tips mean that leading edge
treatment may need to be examined to avoid nasty rolls at low altitude.

But a proper design will avoid all this, since the flap for a KR should be
there to do two things: 1. Increase drag to provide better landing control,
and 2. Lower the nose in landing to provide better visibility. If the plane
is built or loaded too heavy, the extra speed on takeoff and landing is the
smallest concern.

So my vote is with a good, light split flap arrangement. Good drag control,
incidence and pitch moment change is equivalent to half the flap deflection
of a regular flap (average of top and bottom surface at TE). Fowler is out
of the question. Plain flaps likewise not a good idea.

Walter Lounsbery
POB 54266
Hurst, TX 76054
(817) 285-8520
Walt@Lounsbery.com