Merlin Man

Hi,

I am completing a Seagull 120 sized PC9 at about 11 lbs. The wing is slightly tappered and unfortunately has no washout.. It has a relatively high wing loading of approx. 35 oz./sq.ft.

Reports from other modellers are that this model can have a nasty habit of stall and wing drop when the enginedead sticks.


Can I improve thissituation by pitting in a small amount of"up" aileron on each wing? ( Or , should it be"down" aileron ?)

I know that doing this is only a" bandaid" solution but I am trying to avoid rebuilding the outer wing panels to include some washout.

(Most of my efforts are centered on preventing a deadstick situation but, as we know,that is not always 100% possible. )

Any advice would be appreciated.
Cheers,
Merlin Man.

rhall999

Oh dear, where to start.

Well, I will do my best. First off, the problem is not the deadstick itself. The problem is that people allow the speed to drop too much causing a stall. Any aircraft should be able to somewhat succesfully be landed deadstick, BUT, you need to get the nose down and keep the speed up, otherwise you will get the stall, and in the case of the airplane you have, one wing drops on you. In all the years I have been flying I have seen way too many people have an engine fail, and they don't get the nose down to maintain speed. The other thing to watch out for is pulling the nose up to try and "stretch" the glide to reach the runway. DONOTDOTHIS. 99% of the time you will be better off to land a ways away in tall grass or bushes that to try and stretch the glide and end up stalling and spinning in. With the high wngloading, you absolutely can not try and glide it like, say, a J-3 Cub. You must maintain the speed to avoid stalling. Yes, the rate of descent will scare you a bit , but resist the temptaion to pull up. Avoid steep turns as well, that will cause the whole situation to worsen drastically.

Anyways, you are wondering about washout. Moving the trailing edges of the ailerons up a wee bit might help, but there are other, (maybe better) ways of doing it. (By the way, if you set the ailerons so the trailing edges are down a bit, it will make the tip stall problem worse). Is the wing built up and covered in monokote or some such thing?? If it is, you can hold a bit of twist into it and shrink the covering with a heat gun. This will cause the covering to hold a bit of washout into the wing. Another thing that may help is a small turbulator strip along the leading edge on the outboard half of the wing. You would have to do a bit of research to get the location and size of the turbulator right. Of course, the best way is for it to be built in from the start, but I can understand you not wanting to rebuild the wings if they are already done!!

I would try doing the covering trick if you can, at least thats where I would start. Anyone else have an Idea for Merlin Man??

Lnewqban

If the ailerons are full semi-span, you cannot add wash out with them.

If your model have operational flaps, they will be the most effective device against stall-one-wing-drop.

Rodney

Yes, if your ailerons are on the outer span (not strip ailerons) you can get some washout effect with setting both ailerons slightly up when the control stick is neutralized.

Lnewqban

Informative reading here:

http://adamone.rchomepage.com/index6.htm

TimBle

A small amount of Down Aileron i.e. the aileron must be slightly above the surface of the main wing will add some washout. I have this on my FW190 and it has certainly improved the glide performance in a deastick situation. it also has othe benefits like reduced drag since the aileron does not deflect downward as far as it would upward. its also a form of aileron differential.

Merlin Man

Hi All,

Thanks for all your replies. I will try the aileron "tweak ".

I think Rhall999,youmay,need to lighten up a tad. Incidentally, I have been flyingR/Cfor 30 years, have an Instructor rating and know about keeping nose down and speed up. I respectfully suggest you shoud not assume everyone you talk to is a rank beginner.

Anyway, again thanks for you imput.

Cheers,
Merlin Man.

rhall999

My apologies, I did not intend to come across that way. However, I will say thatwith the comment that "other modellers" have said it has a problem when the engine quits, and then asking about aerodynamic fixes, led me to believe that you were fairly new to the hobby. Plus, the reason I made such a long speech about the importance of keeping the speed up is because that is the root cause of the tip stall to begin with, and by going directly to trying to fix the wing drop instead of just preventing the stall also led me to believe that you perhaps were new, and didn't realize what was really happening. Again, sorry to have offended you.

Besides, maybe some of the gazillion people who post videos on youtube of their "radio glitches" and "wind gusts" may read this and learn a bit stalls and spins. As many times as I havewatched guys crash (not just deadsticks either)because they have stalled and spun in and then blamed it on radio glitches tells me also that a few more r/c pilots need a bit more instruction in basic aerodynamics. A quick perusal through youtube r/c crash videos will demonstrate what I am saying

My background just for fun:
15+ years of R/C flying (been around it for 30 years with my dad though)
20+ years of control line
Free flight...heck, I don't remember when I scratch built by first stick and tissue rubber power model.
I also hold a private pilots license, as well as an aircraft maintenanceengineer (canadian equivilant to A&P + IA)license

Trust me, when I say that I have seen a lot of "thud...f**ks" that were blamed on radios, I mean it. In your 30 years I'm sure you have seen a lot too. The most common, and you will recognize this senario, is turning from base leg to final, and the airplane suddenly drops one wing, and goes nose down into the ground. I don't know how many times I have seen this, and then listened to the "old heads and experts" at the field discuss that spot where there is always interference, and how to avoid flying through there because there must be some kind of "strange transmission" passing throught "that zone right off the end of the runway".

HighPlains

My Feedback: (1)

Maybe not.

Merlin Man

Hi Rhall999,.

No problems, perhaps thiswasan oversensitive reaction on my behalf !

Yes, I know what you are saying Rhall999, especially regarding the "radio blackspots" that cause constant problems at our field. !!............
We have an area that some of our members call "The Bermuda Triangle" right in the middle of our most common downwind leg !! You would have problems counting the number of"radio glitches"
that occur here, strangly mostly on the landing phase of the flights !!

( Actually,this is the real motivation for confirming this "washout cheat ". I just don't want to be another victim of the Bermuda ...LOL.)

Happy Flying,
Cheers,
M.M.

BMatthews

MM, the aileron tweak is better than nothing but it's not a proper substitute for proper washout.  Is there no way you can heat and twist the wing to induce some washout?  Have you tried this yet? 

When I googled for "seagull pc9" I got back pictures and reviews of an ARF that uses strip ailerons.  Unless you modify the model to use wider outboard only ailerons and inboard flaps then reflexing the stock strip ailerons won't do anything at all but increase the stall speed and make it even more likely that you'll get a tip stall.  This would occur with reflex because by reflexing the entire wing panel by angling the ailerons up you're removing camber from the airfoil.  Airfoils with less camber can't reach as high a lift coefficient before stalling.

So unless you can warp the whole wing to gain some washout I'd suggest your only two options are to

A)- misguided by your first post wording or not, follow rhall's advice and keep the speed up until that final flare just before touchdown.  You can aid this a lot by using the sailplane flyer's trick of using only small amounts of aileron at lower speeds and just be patient with the slower roll response.  At low speeds the down traveling aileron increases that wing's effective angle of attack and that is often why folks can suffer from tip stalls in the landing approach if they are flying slow and toss in lots of control deflection.  It may partially explain your "Bermuda Triangle" effect you see at your field if there's no big metal fences or metal roofs in the area.

B)- Modify the wings to move the aileron hinge line more forward by trimming off about 3/4 to 1 inch of the wing's trailing edge, re-cap it with 1/4 or 3/8 wide balsa as a hinge base and build new ailerons and flaps that are separate and about 2 to 2.5 times as wide as they are now.   The outboard 50% will then be conventional "barn door" ailerons and the inner 50% would be plain flaps.  In use you can deploy the flaps down about 5 to 7 degrees for takeoff and the same or up to 90 degrees for landing.  By angling the flaps down you gain some useable increase in lift from the inner portion of the wing and at the same time induce a useful degree of washout twist.  This occurs because the flaps increase the angle of incidence of the inner portions while the outer portions are at their original angles of incidence.

While it would likely mean stripping the covering and redoing it in order to do these mods and install the controls I suggest that the fun to be had by adding flaps to this model would justify the work involved.  Your choice though.

Merlin Man

Hi,

Thanks for your reply BMatthews,
My PC9 has the i/3 wing ailerons so there is a chance for the Tweak to help.

I agree that this is not a proper solution to washout. No, I have not tried the wing panel warping at this stage as I have seen this done before with only temporary results. The wing eventually "Remembers' where it was and returnS ( if only partly) to its shape.
However, I have not dismissed this idea entirely.

I have been giving more thought to the wing restructure as you have suggested.but all this goes against my orinal purpose of a quick assembly, and fly it approach I was planning for this model. LOL.

I guess I need to be disciplined enough to make myapproachs andlandingswith authority.

I am still thinking about all of this ??I appreciate your inputs.
Cheers,
M.M.

rmh

I don't believe in washout
Except for very long (high aspect ratio wings)
Why?
Once the airfoil is compromised by twist- the TOTAL lift at any given AOA is lessened. Also once the wing loading reaches a critical point - No amount of twisting aileron repositioning etc., has any real value . Speed will help however.
Simply put - you can NOT improve lift - just redistribute the effective lift at any given speed, by twisting /altering camber..
So part of the wing is working -and part is NOT working as hard - once washout is introduced
I know the theorists won't agree with this .
Sorry about that but 35 oz ft -on 11 lb model -is over the top

BMatthews

Dick, I do agree with your take on washout but there's no doubt that in some cases it can make a model that is nearly uncontrollable at slower speeds into a model that can at least be flown safely and enjoyed. 

I believe that you're so focused on how washout would affect an aerobatic model that you're missing the goal of using washout on a model where it'll be right side up for a vast majority of the time when flying.  For example scale models of such radically tapered wings such as the DeHavilland Comet racing plane pretty much needs a lot of washout along with an airfoil fudge to be safely flown at lower speeds near the stall.  And a lot of the between wars airliners such as the DC3 are not much different. 

But yes, it's a crutch and yes it reduces the overall lift.  But if it can make the model fly more safely for the landings and initial takeoff then it's a reasonable tradeoff.


In this case though I agree that it's a two edged "cure".  By inducing some washout through aileron reflexing we're reducing the camber in the airfoil over the tip area of the wing.  In fact if it's a symetrical airfoil we are actually making it into an INVERTED cambered airfoil.  This means that the outboard portions won't be able to lift as much and the stall speed of the model will actually rise.  And with such a high wing loading already we're just making a bad situation worse.  So even with some aileron reflex added in MM is still going to have to deal with a faster stall speed by adding his reflex.  This is why I'm more in favour of adding flaps to the model.

MM, if it uses proper width conventional ailerons then half the work is already done.  You'd only need to modify the center section to add flaps.  And the good news is that you can elect to do this later if you find that it's the pig at landings that Dick and I suspect it'll be.

rmh

I will go along with that -with one caveat
Wing loading
Thereare some loading limits that simply can't be exceeded
On smaller models these limits DECREASE at a exponential rate (for practical purposes
In this case an 11 lb model with 35 oz ft loading
a nice workable number would be 25 or under
a friendly number 15 and under.
Will it fly at 35?
sure but it can be a handful as speed bleeds off
A model which has 35 oz ft which flies easily will typically have 1200 or more sq inches and a flying weight in high teens or low mid 20s (lbs)
I have built scale stuff from frankly awful kits which weightwise were dreadful
The head shaker is that using decent building techniques -most of these will be 25% lighter.
The result will beasy to fly and still be scale as examples:
A SCALE P51 can be built which flies like a pattern- plane ditto for a Spitfire
There are even tiny foam models of these which fly decently -not great because they are actually at the high end of wingloading
I do build scale stuff - got 2nd at the Nats once in scale ('84)Dalotel scale model- 11 lbs 935 squares

My 1990 sTOC Buckers were SCALE (clipped 9%) 1800 squares and 16.5 lbs rtf . really hate to see builders try to fix oveweight models with "magic airfoils " This approach has a low incidence of success

Lnewqban

Merlin Man,

Do you know how and when this happens, as you have been told?

Right after the engine stops or during flare at landing?

Merlin Man

Hi All,
I agree with the points made by all above and realise that the basic problem is the wing loading. being an ARF, I can't really do much about this

Lnewqban, as for the exact details of these crash incidents ,I'm affraid I am at a loss. except to say that they appear to stall immediately after deadstick, and not necessarily on landing..
Of course , I have no real idea of the experience or otherwise of the pilots reporting these events.either.

I am going to purchase a new incidence meter. ( mine 's been damaged ) Close eyeball examination of the wing incidences last night seems to suggest to me that there could be even wash in !! Certainly an unattractive view from the wingtip !!

So now I am thinking of wing surgery . What do you think about this ?........ removing a narrow strip of covering from along the top and bottom of the main spars and cutting through the ribs right behinh the spars. This for the last 6 or so bays involving the aileron region.
Then I could jig the trailing edge to say 2* (?)wash out, trim and reglue ribs. Are there better ( easier) ways that you know of ??

I know that flaps would be great too, but at this stage , I don't really want to commit to this step. The model is building into a very presentable plane otherwise, I don't think I would bother with thinking about changes at all.

Thanks,
Cheers,
M.M.

LargeScale88

I agree with everything stated above. When you move to bigger aircraft (20+pounds, 80+ Wingspan), you are gonna want to have washout in the wing itself. It is very critical in the bigger planes because of the higher wing loading.

Jason

Lnewqban

For that wingloading, your plane will stop flying below around 30 mph and max AOA (assuming CL=1 for that condition).

Keeping the airspeed above that value during dead stick situations and landings is critical.

The attachment shows that the cubic loading is not terrible, but within the Sport range.

An intermediate best solution for any tip stalling plane as the one described here are split flaps, because they can be added to the bottom of the wing without much surgery and change in appearance.

Why doing it?

Read "5.5 The Effect of Flaps" at this website:

http://www.av8n.com/how/htm/vdamp.html

How to do it:

http://www.warbirdsresourcegroup.org...0convert2.html

rmh

Well -flaps CAN NOT add lift- Just configure for best lift at a given speed -also the drag increases-
However the fuselage will be at a lower angle when it touches down

Lnewqban

Washout cannot add lift, it only passes more load to the root, which must increase AOA to compensate, stalling ahead of the outer section of the wing (the very tip section can never develop enough pressure differential to stall).
Of course, the situation reverses during inverted flight.

Now, flaps add lift by increasing the coefficient of lift, since the airfoil becomes more cambered (with all the associated pros and cons).

Copied from http://www.av8n.com/how/htm/vdamp.html

"So, the question is, what do flaps do? Well, that question has no less than six different good answers:

Extending the flaps lowers the stalling speed (the bottom of the white arc).
Extending the flaps increases the wing’s angle of incidence.
Extending the flaps effectively increases the washout, since on most planes the inboard sections have flaps while the outboard sections do not.
Extending the flaps increases drag. This is helpful during landing, but unhelpful during climb and cruise.
Extending the flaps perturbs the trim speed. This is an undesirable side effect.
Extending the flaps lowers the allowable top speed (the top of the white arc). This is another undesirable side effect."

ron ward

merlin man... just wondering and pardon the hijack fella's, one question and it's back to flaps....
are you called "merlinman" for the aircraft engine, the sailboat or the archery equipment?

Rick.

I'll chip-in if i may. Just had a look at the model specs - its a portly 71" low winger with a design AUW that can reach 12lb jeez!

Well, heres my advice, maybe worth a try for less effort than a wing re-build!

Add leading edge SLATS to the outer 1/3 of each wing. simple to make, just curved pylons 1/2" deep supporting a curved sheet of 1/16 x 1 1/2 balsa. (umm, not to sound condescending, but do ensure you remove thin strips of film where the pylons glue onto - they won't stay stuck to profilm too well Lol).
The actual profile/intake/exhaust areas arn't too critical to get the slat effect. 1/2" front opening closing to 3/8" rear opening should speed up the flow and help it negotiate the camber better. an if you don't like them its easy to cut the pylons off and put a strip of covering film over the 'scars'n returning the wing to its previous state. True leading edge SLOTS would look even neater, since they are built INTO the wing, but requires surgery to the leading edge whereas slats are an easy 'add-on'. I've added them to several models over the years and found they work well and whilst there's undoubtably an ideal slat shape/width/position, they are surprisingly effective even as just a quick n'dirty effort.

Many full size aircraft use them of course (eg: Fieseler storch in WWII and modern airliners). Most full size have retractable slats, but thats not necessary or desirable (difficult/heavy) on our models.

Try the slats first, if successful in stall tests, you can keep them, or cut them off and go for the tidier slotted version - if they don't help (which is unlikely) you can cut them off anyways and no harm done!

Rick

brjensen71

My Feedback: (1)

Use differential aileron throws, less down throw and more up throw, if you have a computer radio and a servo on each aileron you can make it selectable.

For landing use 100% aileron differential only (you will want to experiment with this, you need a lot of up throw), it lifts the aileron, no down aileron. This lowers the lift and drag on the high wing, and does not attempt to create more lift and drag on the low wing. Hence no increase in AOA across the aileron section of the low wing and no advancing or creating a stall on the low wing.

While the use of the rudder is always good to tame tip stalls, sometimes it is just not enough.

I have used this on a couple planes with nasty tip stall characteristics, and it really works. I could stall the inner part of the wing, and still maintain a wings level attitude.

If your not careful, you will still slam the airplane into the ground, but your wings will be level when you hit.

On high wing loaded airplanes, landings often come close to stall speed and AOA, this will help give extra ability to control roll without inducing a tip stall.

rmh

It is apparant by now that the BASIC problem on this model,is wing loading.
Having done a number of "reviews" on models from this manufacturer, It seems surprising that they continued with some very highly loaded designs
The scale Tucano is a nice design as a model, and built correctly will fly fine -in scale porportions
My fix would be strip the model bare and cut out every gram not needed.
Ain't going to happen tho
So keep the landing and take off angles as low as possible (meaning get speed up before take off and FLY it onto the field.)