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Old 05-31-2013, 03:24 PM   #1
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Default Wing incidence and the taildragger takeoff blues

OK, I have a '70s clipped wing cub aerobatic trainer, the symmetric wing gave it a faster takeoff speed and landing speed than I really liked, so I made an adapter so I can run a flat-bottom non-clipped wing with some dihedral, and it worked pretty good today.

The symmetric wing was set of 0 incidence relative to the Hstab, my setup sets the flat bottom of the new wing at 0 incidence relative to the Hstab.

Q: Should I be running a little +or - incidence? manual/plans has no info.
It's pretty sensitive in pitch.

I'm guessing a little on CG, it might be a little forwards, when I trim for level flight at a given power setting then dive with that power setting, the nose comes up a bit when I release the controls.

Now the takeoff part- I have to hold the tailwheel on the ground for a bit until it gets enough speed that the (I think undersized) rudder will start to do its job.
I get up some speed, then transition to full throttle, then let the tail come up, then let it build speed on the ground or let it come a little off the ground.

When I pull up a bit to gain some altitude, it rolls left or right, had some close calls there. I think this may just be that I'm still going slow enough that the Vstab just isn't really doing much, and maybe it is sensitive enough in pitch that I give it a tiny bit of up elevator and it just pitches up too much.

It's also one of those things where I got some really good takeoff rolls in, just held the tail down, kept it nice and straight, got it flying just off the ground, then was able to take it up nice and straight. Then later, it started veering, so I'm not sure if it's me, or if maybe a wheel got tweaked a little from straight.

One mistake I made was holding the tail on the ground too long, got going too fast and she lifted, pitched right up, and I just kept it from stalling with a boatload of down elevator.


Q: if I just want better stability from the Vstab is it better to add some area in the front, or better to add some area on the rear of the rudder portion?


This has the type of tail-feathers where elevator and rudder are longer closer to the fuse, and are shorter the further from the fuse.


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Old 05-31-2013, 05:32 PM   #2
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From that 'wings wobbling on takeoff' description, I would suspect it's tail heavy. But I'm no expert and these things are tough to call without actually seeing it. By changing the wing airfoil, it probably affected the wing's center of lift compared to the original wing, so the desired CG would probably change, too.
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Old 05-31-2013, 05:44 PM   #3
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I flew this plane with the other wing in a for-certain nose-heavy config, and it then also acted as a tail-heavy plane will. I don't know what makes this one so elevator-twitchy.

The CG is decently far forwards on the wing, at about 1/3 the way back from the front.

What makes it harder to tell where 'correct' ever was, the instructions for the plane say to set the CG with the wing off.

I should have mentioned the plane will go left or right, so it isn't quite the usual plane-goes-left. My Fokker D-VII taught me fairly well how to keep a taildragger (well, tailwheel) straight on takeoff.

What I really didn't like with the other wing was that I had to get it going really fast to get it off the ground, the rwy is a bit bumpy and non-level (in all directions) so fast with the wheels on the ground is iffy.

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Old 05-31-2013, 05:48 PM   #4
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Is the veering just on the ground? Maybe some toe-in of the gear could help that.
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Old 05-31-2013, 05:55 PM   #5
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I think it is not veering until the weight is off the gear, and I think not until the nose comes up a bit- I can do fast taxis and 'aborted takeoffs' getting the plane just up and it goes straight pretty well.

I got used to tweaking the gear on my Fokker, that one is very sensitive as the gear has no give, where this one has a bit. That Fokker likes toe-out, I tried in/neutral/out on it. The gear on this plane I left how the previous owner had it, neutral.

It's possible I gave the gear or a wheel a small tweak that won't be apparent until I get it on the bench.

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Old 05-31-2013, 06:04 PM   #6
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Well, I'm out of ideas. Hopefully someone more knowledgable will see this and help. I'm curious as to the fix.
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Old 05-31-2013, 06:06 PM   #7
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25% to 30% MAC is typical... 33% is getting a little too far back for some aircraft.

The wing wagging on takeoff may be an indicator that BOTH ailerons are deflected down slightly. Try 1 to 2 turns on the clevices pushing them up.
It could also be "dutch roll" which is sometimes cured with increased rudder/fin area.
It could indicate sloppy linkages in the rudder also.

You can add area to either the rudder or the fin with about equal effect for combating the tail wagging. Its often easier to do one than the other, so take your pick.

If that plane has hinges that make removing the rudder easy I probably would pull the rudder and add the area to the leading edge of the rudder. Shaving the bevel off the rudder then gluing on 1/2 to 1 inch more balsa stock the thickness of the rudder should be fairly easy to hide. I'd make that choice because its easier than dealing with changing the curves. Just one suggestion of how to add area.

You shouldn't need 100% throttle to take off. Your first post's description of the takeoff sequence sounds to me like an under-powered model that has trouble achieving adequate speed.
What is the plane's weight and what is the power?

Flat bottom wings will tend to pitch up as the aircraft gains speed on a 45 deg dive test. The wing's airfoil gains lift with airspeed and that looks like a flat plate tail.
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Old 05-31-2013, 06:17 PM   #8
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I haven't measured the CG, it's somewhere fwd of 1/3, I think. I'll measure with some precision over the weekend.

The ailerons are dead flat with the underside, but if deflected they do drop too much- I need to use an angled control arm so they drop much less than they do (relative to raising).

I'll try adding some area to the rudder an easy way as a test and see what effect it has.

The linkages are all tight.

The plane has plenty of power, under 5 lbs with a 700KV motor on 4S with a 12x6. I have tried (with the other wing) a 12x8 and a 13x8, I'm running the 12x6 to get it handling well to help lessen prop-wash effects. Stock on this plane when it was gas was a 10x6.

All my planes I've been a WFO-takeoff pilot, it may be this is the first plane that gives me grief with that habit. I get 'em rolling on part throttle then when the fins start to bite I give it full throttle.

I think it's not tail-heavy as it needs a good bit of trim up to fly level at moderate speeds, but that's not definitive. It had this same pitch sensitivity when it was very nose-heavy.

As far as I can tell other flat-bottom wing planes with a plate Hstab have the wing bottom parallel to the Hstab.

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Old 05-31-2013, 06:32 PM   #9
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Originally Posted by CNY_Dave View Post
The plane has plenty of power, under 5 lbs with a 700KV motor on 4S with a 12x6.
Sounds pretty marginal to me, I'd have the 13x8 on there right away.

Sounds a bit like you are pulling the plane off the ground with inadequate airspeed and/or climbing too steep and losing airspeed.

hen I pull up a bit to gain some altitude, it rolls left or right, had some close calls there. I think this may just be that I'm still going slow enough
This seems a text book case of flying right on stall speed and 'tip'stall' causing toll one way then the other.

As noted earlier, 1/3 chord on the CG is probably could easily be too far back.
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Old 05-31-2013, 06:56 PM   #10
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It will climb quite steeply on the 12x6 (and accelerate pretty quickly), the 13x8 was just about enough to hover, but tended to trim the grass a bit. I need to get some big Dave Brown Light Flight wheels on there, like on my bipe. 12x8 is a decent medium for the most part, the prop doesn't seem to stall on a static test so it has a bit more static thrust.

Without someone watching it's hard to tell the sequence, what I'm not doing is running out of runway, so it is quite possible I'm pulling up a bit early.

It's even possible it veers a bit and then I have to pull up early to clear the tall grass on the edges.

I do know I'm finding it difficult to maintain a prolonged high speed on takeoff with the main wheels on or just above the runway.

I think a little (+) incidence on the wing compared to the Hstab might be better, thoughts?

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Old 05-31-2013, 06:59 PM   #11
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Relative incidence between wing and stab (decalage) varies quite a bit with the way the plane is intended to be flown.

Flat bottom airfoil is more typically flown as a trainer and with some + decalage, sometimes more than 5 deg. This makes the plane pitch up significantly with increased airspeed so when a beginner gets it into a dive it pulls out itself. CG would also be expected to be further forward than normal with this much, forcing the nose to fall as soon as the plane starts to stall.

You can fly just fine using a flat bottom airfoil with 0 decalage.

**********

Prop size doesn't mean much. But 700kv on 4s means you should be spinning the prop above 10,000 rpm (if the motor isn't overloaded) and that should be plenty of power with the 12X6. That's approaching the power of a .61 glow which would be plenty for a 5 lb plane.

Sensitive in pitch indicates either the CG too far back or too much elevator authority. Try dialing in some expo since you indicate it had the same issue with a more forward CG. It doesn't look like it has oversize elevator.

It may be just flying like a Cub...
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Old 05-31-2013, 07:08 PM   #12
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DX5e, I have the tiny bit of secret-setting expo turned on, but it is very minimal.

The stall behavior I have checked a bit, and it is pretty mellow, it was with the symmetric wing as well.

I have a lot of experience with 4S on a 770KV motor, the 700 is quite similar, it has a very nice 'crackle' sound during a static test even with the 13x8.

I guess I'm having trouble correlating the relatively large up-elevator trim required for level flight with the CG too far back.

It takes a lot of weight to change the CG just a little, I hope it doesn't need nose weight. I have some freedom on the wing placement, I'd rather try moving the wing aft a little and make some new adapters to keep the declage neutral.

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Old 06-01-2013, 01:48 AM   #13
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Originally Posted by CNY_Dave View Post
I have a lot of experience with 4S on a 770KV motor, the 700 is quite similar,
Well there's the thing. Power is proportional to the cube of RPM, so all other other things being equal a 700kv motor will produce 3/4 of the power of a 770kv motor... So not really so 'similar'?

Having said that it should still be adequate power to fly a cub, but it sure sounds like inadequate airspeed.
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Old 06-01-2013, 11:14 AM   #14
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Normally a flat bottom airfoil is given a little more incidence than a symmetrical countered with downthrust on the motor. The incidence gives stability and the downthrust counteracts ballooning under power. Then you shouldn't need up elevator trim as you are having to do.
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Old 06-01-2013, 11:46 AM   #15
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Originally Posted by Panther View Post
Normally a flat bottom airfoil is given a little more incidence than a symmetrical countered with downthrust on the motor. The incidence gives stability and the downthrust counteracts ballooning under power. Then you shouldn't need up elevator trim as you are having to do.
'flat bottom' wings automatically have positive incidence (relative to the zero lift incidence) if you set them up with the bottom surface 'flat', typically something in the 3 to 5 degree range.

See attached.


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Old 06-01-2013, 01:22 PM   #16
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3/4 the max theoretical power, if the KV ratings are accurate, and if a whole host of other ifs. I'm way too lazy to compare resistance specs and measure actual KV.
By 'similar' I meant what I was actually getting as far as static thrust and general performance.

Would the difference be less if either setup could operate the prop near it's stall speed?


I measured the incidence more accurately, the wing profile yields +2.1 degrees above what the flat bottom is set at, maybe I'll set the bottom a degree up and see what I get.

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Old 06-01-2013, 04:56 PM   #17
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Dave,

I still think you are 'barking up the wrong tree' worrying about incidence angles etc. The behaviour you mention (dropping wings to left and right) sounds 100% like the plane is flying stalled, which has nothing directly to do with incidence angles or the vertical stabiliser and everything to do with flying speed. The fact that you say this behaviour only happens when you give up elevator basically confirms that the plane is stalling.

It just sound to me like you don't have enough flying speed.
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Old 06-01-2013, 05:59 PM   #18
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Downthrust isn't really added due to wing incidence. Its added due to the motor being mounted below center of drag, tending to force the nose up if you don't have the motor angled down.
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Old 06-01-2013, 06:26 PM   #19
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Originally Posted by JetPlaneFlyer View Post
'flat bottom' wings automatically have positive incidence (relative to the zero lift incidence) if you set them up with the bottom surface 'flat', typically something in the 3 to 5 degree range.

See attached.
At last !! Tks JPF ... you beat me to it ...

Absolutely ... a flat bottomed wing especially Clark Y actually has +ve incidence by virtue of it's chord line when set bottom at zero.
Where people have quoted +ve set-up - this is for trainers .... intermediates with such wings usually have wing set more zero to HS..... to allow you to fly a little more spirited.
It is not normal to set a flat bottom wing past it's zero bottom set-up ... this can introduce some interesting characteristics !!

To the people that said symet wing is set at zero - that would create a model that flies seriously nose up ... only a speciality model designed for aerobatics may have approaching zero. a symet wing by virtue of it's being symet must have +ve angle to create an offset to enable lift.

...... To OP ... you have to also consider that the lift coefficient of a flat bottom wing is far higher than a symet ... so you have to play with model trims etc. to get it to fly right with each - they will have completely DIFFERENT trims etc.

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Old 06-01-2013, 06:31 PM   #20
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Originally Posted by fhhuber View Post
Downthrust isn't really added due to wing incidence. Its added due to the motor being mounted below center of drag, tending to force the nose up if you don't have the motor angled down.
Oh Dear .... so you discount any other factor out of the hundreds that would incur a thrust line ?

Downthrust is added to combat many different factors - one of them being wings tendency to pitch up and balloon, stabs at back are primary to stop this, but often need help from the prop department ... when a wing generates lift - it also wants to pitch up etc.

If you don't believe me ... check out models with prop shafts exiting ABOVE fuselage C/L and drag....


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Old 06-01-2013, 08:02 PM   #21
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The incidence question was co-incident in time, I didn't suspect it as causing my takeoff issues, I just wanted to make sure I was guessing right on how to set it.

Other than a slightly cocked tail-wheel I think it's probably just me.

I think I'm having trouble transitioning from holding the tail down with up elevator to plant the wheel to releasing up elevator and not having it go airborne.

I'm going to practice getting up to takeoff speed and keeping it down on the ground a bit, then aborting. I think flying off small fields made me a bit gunshy about staying on the ground too long, and I have to train that out of myself.

Full-throttle is more my habit than necessity, although my draggy WWI bipe does much prefer it, that really needs some oomph to gain speed, and the rudder is a bit worse than this plane at low speeds.

I'm going to leave the wing bottom flat (parallel with the Hstab). The stock wing is set at 0 incidence with the Hstab (center of leading edge to center of the strip aileron at neutral position), and with the stock wing it did need a fair bit of up elevator trim.

This thing was sold as an aerobatic trainer.

As to thrust centerline, on my trainer (which was a bit overpowered) I did mount the prop centerline as high as possible, used a mid-plate between 2 sets of standoffs to move it up. It was above the fuse centerline, and with a bit of down-thrust it did seem to not balloon too badly. With the 770kv motor on 4S and a 13x8 or 13x10, it went fast enough to fear for the wing (well, I was worried ).

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Old 06-02-2013, 01:27 AM   #22
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As flown with the flat-bottom wing, CG is 29.7% back from the leading edge.

With a sizeable lead weight at the firewall I can get it to 25%.

I looked at the EL trim and I actually ended up with it at neutral elevator on the last flight, I might try taping on the weight and doing some tests.

The CG is about 3 inches back from the wheel axle/runway contact point with the plane level.

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Old 06-03-2013, 05:42 PM   #23
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Today I put in a good bit of practice before actually taking off, it's still a bear to keep straight because the tailwheel has very little traction.

Best technique ended up being to accel slowly with up elevator to keep the tail wheel planted so I could keep it straight, get up to a fairly constant speed and let the tail come up when the rudder has a shot at actually doing something, then adding a bit more juice and just letting it come up on its own.

This ended up with about a 50% success rate due to not keeping it straight and aborting, but aborting while still on the ground.

The tricky part is that with the tail down the liftoff speed is only just over the speed where the rudder starts to work, I had a couple unexpected liftoffs where a small gust of wind lifted it right up, I was able to get the nose down and feed it power and just take off.


After flying a bunch of t/os and landings I did more practice, getting a feel for liftoff speed with the wheel down.

A pretty good gust lifted it up while doing this, and unfortunately pointed the 5lbs plane with the whirling scimitar of death right at me, so I had to just kill power and let it drop, I got the nose down but it was rolled pretty good, and it did the wingtip-hstab-wingtip-nose cartwheel which popped off the vstab and broke loose the hstab.

Not a difficult fix, but bummer. Better than stitches, though!

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Old 06-05-2013, 11:23 AM   #24
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There is another point .... taildraggers are often set-up for ' clean ' surface take-offs ... meaning they are often a little too far back.

My Skymaster bipe is prime example where I changed the main UC to plate alloy .. the wheels are well ahead of bottom wing and about similar to old CoG point. But what a dog to T/O from grass ... slightest bump and over the nose goes.
So I turned the gear 180.the legs have a slight curve form to them - which I fitted curving back .... now they curve fwd and now the wheels are slightly ahead of old CoG position ... cured.

BUT moving TD mains too far forward can affect ground handling by increasing the tail moment and then you get swerving around all over place !! ... try any short nose WW1 job !!

Normally Rudder will take effect BEFORE average TD lifts ... it's only on small rudder or high lift wing models that you may find lift-off before rudder has full authority .... NOT a good situation ... you MUST keep that bird ON the ground !!

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Old 06-05-2013, 03:51 PM   #25
CNY_Dave
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This plane shows no tendency to tip forwards, not at all. The gear is I think in the location on the plans, and the gear is all the way forwards, the mount is right up against where the fuse bottom tilts up to the cowl, and the mounts are slightly swept forwards.

The axles are right at or just ahead of the wing leading edge, so it sounds like someone has already done what you did (in the plans or no). If I reversed them they'd be back by 1 to 1.5 inches (and still 1.5 to 2 in front of the CG), I'd take a little tendency to tip over in favor of better handling on the takeoff roll.

I put a tailwheel on my WWI Fokker D-VII as I was at the time flying off pavement, and that taught me, lets say, quite a crapload about keeping a tailwheel plane straight. The fokker has a slight tendency to nose over but it rolls pretty straight.

The key for me was reading that the tailwheel to a degree doesn't really set heading, it sets the heading change rate, and neutral steering really just gets you a steady heading change rate. If you get the rear headed back towards straight you can't just release the rudder/wheel when the plane is straight, you have to give at a little stab in the opposite direction to arrest the heading change.

Ask me why your DX5e is doomed... and how to fix it.
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