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Old Jul 15, 2015, 07:04 PM
Doug Bartley is offline
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I am going to change my Tail heavy to tail low and agree with eye4wings. It flew already, very , slightly tail low, watching it yet again, it may have only been with the wind.
Since it already flew 98% good at 185mm, I'd still be tempted to go 165 instaed of 150.
fwiw Doug B
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Old Jul 15, 2015, 07:05 PM
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So let me throw my 2 cents (or would it be 2 pence?) into the fray... While watching the vid I did indeed see the tail slide a little in the bank. Given the way it flew and seemed to handle, I don't think CG or the incidences are to blame by any significant amount. I think the 'problem' (although I don't think it's really a 'problem') is that the vertical stabilizers are scale size and a bit on the small side (many times stabs get scaled up on models for this reason). There's just not a lot of surface area there to hold that long tail 'up' in the turns. I think the appropriate measure is to mix a bit more rudder in with the ailerons.
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Old Jul 15, 2015, 08:20 PM
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Oh no, not again!
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Okay, I'll leap into the fray. CoG is he Center of Mass, the balance point of the model. On the balance stand there is equal mass fore and aft. That's true wherever it is, so if you move the c/g forward it means there is more mass in the nose. Rule of thumb is that nose heavy planes fly poorly, tail heavy planes fly once.

In a WWII conventional aircraft, the wing provides all the lift, the H stab provides a leverage force to control the angle of attack. In normal flight, the stab should be providing some down force to keep the plane stable (hence stabilizer). When the angle of attack changes the Center of Lift moves, forward at higher alfa, rearward at lower. Since he c/g should be more-or-less constant in flight, moving the C/L aft puts it behind the c/g and the plane will tend to nose into a dive. When the C/l moves ahead of the c/g, the plane tends to pitch up, eventually stalling (or looping if you have enough power). The c/g is given as a range representing the limits of the power of the H stab to balance the C/L as the angle of attack varies . Exceed the c/g limits in either direction and the plane becomes uncontrollable. In the early stages of Lancaster development, the span of the tailplane was increased to 33 ft to provide better control.

So, Ken has already done the maiden. He knows the plane flies well at 185 mm. We don't now for sure what the fore and aft c/g limits are, just that 185 is within that range. If you move the c/g too far forward, you may find that there is insufficient elevator authority to overcome the rearward shift in C/L. This will occur most noticeably when the airflow over the tail slows down, i.e. when power is reduced. I'm guessing that he will most likely run out of up elevator in the flare, resulting in a hard landing. If it's too extreme, there won't be enough elevator to get off the ground, which is probably better.

My reason for expressing concern is just that. Keep the c/g close to where it works, and move it forward in increments as you explore the fight envelope.

My last note is empirical, no maths, just observation. Dragging the tail in a turn is an aerodynamic issue, not a function of c/g. I have an HK Mosquito that used to drag it's behind in turns and I was convinced it was tail heavy. After a lot of experimentation, taping fishing weights to the nose and tail, I found that it was in fact nose heavy, had to move the c/g more than 10 mm aft to make it fly and land better.

And my last little empiricism. I initially built my Sureflight Spitfire very nose heavy b/c I had read that Spits need a forward c/g. It flew nicely, but when I tried to loop it snap rolled like a bandit. I wound up moving the c/g a looong way back. Stanford Tuck wrote that the real Spit did the same thing, at the forward c/g limit it had a wicked snap roll.

Snap rolls and hard landings are not desired outcomes in the current circumstances.

And Ken, I may be totally off base, there may be no issues at all. I am just being cautious on behalf of your beautiful creation.

Regards,
Jeff
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Old Jul 15, 2015, 08:38 PM
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So, this looks like adverse yaw.. The plane is pointing outward, opposite to the direction of the turn. That also makes the tail look like it is flying low.

If the model flies well with respect to the CofG, experimenting by moving it back and forth a half inch at a time and test flying each time will give you a better sense with respect to how much up elevator is required for straight and level flight and in the flare on landing.

I'll include an article on adverse yaw as soon as I find it. Easy to fix by setting up the ailerons on separate channels as per the following...

P.S.

Taken from MAN

Aileron Differential: Why it’s so important and how to set it up
*************

Aileron Differential: Why it’s so important and how to set it up
Jun 02, 2015

For years, depending on the model setup, modelers often used offset servo output arms and bellcranks to achieve differential aileron movement. Today, however, using separate aileron servos and the aileron differential program menu in your computer radio has greatly simplified the task. But before we take a closer look, let’s first check out the mechanics of our model during a turn or a roll to understand why aileron differential is so important.

AERODYNAMICS
Typically, most models are set up with equal amounts of elevator (pitch up and down) and rudder (yaw left and right) control surface movements. But when it comes to ailerons, equal amounts of up and down (roll left and right movement), can cause the model to yaw in the wrong direction. Here’s why: When the ailerons are at their neutral positions, the lift and drag produced by each wing panel is equal and the model tracks straight ahead. But when a model has ailerons that move in equal amounts both up and down, the amount of drag (and lift) created by the wing panel with the down aileron becomes greater than the one with the up aileron. The panel with the aileron pointing downward moves up because it creates more lift. The opposite panel goes down (less lift) and causes the model to back toward the up aileron. But here’s the rub! Because of the increased drag caused by the upward motion, that down aileron wing panel also slows down; this causes the model’s nose to yaw in the opposite direction of the roll. The model yaws nose right in a left-hand bank/turn. This condition is known as adverse yaw. Without aileron differential, most airplanes require a certain amount of coordinated rudder to prevent, or at least minimize, adverse yaw while the model is banking through a turn. For sport and scale planes, this can be done manually or with a program mix-however, it won’t work in all types of flight conditions.

HIGH-PERFORMANCE PLANES
This adverse yaw thing is also an important consideration while flying aerobatic planes. Aerobatic pilots need to set up their models to react in pure yaw, roll and pitch motions. During a roll (whether it’s executed on a horizontal or vertical line), the model must roll axially without its nose yawing or wandering off the straight line of flight. Aileron differential helps keep the model’s tracking straight.

YOUR MODEL IS EXPERIENCING ADVERSE YAW IF:
The model skids through turns.
The tail drops during a turn.
The nose swings out of the turn.
It’s very difficult to roll your model in a straight line.
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Old Jul 16, 2015, 01:27 AM
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Thanks dangaras I do have some differential dialled in but can increase it a little

Thanks Jeff, I think what I will do is use the smaller main RX batteries I used on the maiden and re-measure the CG as the plane stands now.

In total there are 4 batteries driving systems on the plane, 2 for main RX, 1 for secondary RX and 1 for the sound system. I upped the size of the 3 RX batteries assuming I would need more weight in the nose post painting and fitting the rear turret but I guess with all the cockpit and front turret additions this has offset the rearward weight.

My gut still tells me the plane needed to be more nose heavy but perhaps 35mm is too far a shunt I think I will aim for the 165mm Doug suggests

Thanks everyone for the help
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Old Jul 17, 2015, 07:56 AM
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A truly outstanding build Ken.
I am building a T.Nijhuis Lancaster of a similar size at the moment. I have just measured the incidence of the tailplane and it seems to be about 2.5 deg+. I don't think this will alter the tail dragging in the turns, from what I have read that is how they fly so co-ordinated turns with rudder & aileron is the norm. Whether it will fly with a higher tail remains to be seen.
I had a similar tail down attitude on an SE5a which I tried to sort by altering the CorG, but it just flew worse. In the end I altered the tailplane incidence with the original CofG and it improved it drastically. Please don't cut that beautiful masterpiece apart based on what I say, I just thought it may give you something else to think about!!
Regards Ian
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Old Jul 17, 2015, 10:03 AM
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I would like to add. For all those that want to mix the rudder to the ailerons.

Dont

Learn to use the left stick for what it is for moving the rudder. When a full size airplane makes a turn The ailerons bank the airplane. The rudder makes the turn. A little up elevator is used to keep the nose from dropping A little bit of power might be added as well depending on the aircraft speed. The wing has a shortened effective span during a turn. The loss of lift must be compensated for with a little nit of power,/ a raising of the nose. and rudder is needed to keep the turn coordinated (tail in proper position)

When learning to fly full size. It applies to all phases of flight but it needs to be mastered to be able to land well.
The elevator controls airspeed.
The throttle controls the glide angle.



When the tail is low in a turn with our models it's because the turn is not coordinated. The airplane is slipping or skidding in the turn. Since so many of us tend to fly yak and bank. Proper use of the rudder is never truly mastered.

I do agree that the C/G was not the issue for the tail position during turns. But rather not enough rudder input. OH and the amount of rudder required will vary with airspeed.

Rick
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Old Jul 17, 2015, 11:31 AM
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Thanks Ian, I think I read somewhere about the Nijhuis tailplane having positive incidence along with a couple of other models its why I added some in, will you be doing a thread on your build would love to see it

Thanks Rick, I know you are right, in any case too much rudder mix just causes the model to yaw when you don't want it to, I have flown helicopters in the past so I have used the rudder in coordinated turns but not much recently

Family commitments this weekend so no re-maiden and looks like I will be working next weekend so its at least two weeks off
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Old Jul 17, 2015, 01:04 PM
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Hi Ken. I may do a post build log on here if it is a success. We will see.
Here is a link to it on another forum, but I am sorry, it is not in the same league as your awe inspiring creation. Can you tell me where I can buy some of your skill and patience, because I could do with a load of it!
http://www.modelflying.co.uk/forums/...82&p=6#PostTop
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Old Jul 25, 2015, 10:13 AM
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Hi Ken What going on here??? Nothing posted for days!! Are you alright? Family??
Doug B
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Old Jul 26, 2015, 01:56 AM
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Cheers Doug don't worry, busy with work but also doing some concept work on a 3.6m DH106 Comet build which I may be doing over in the EDF forum - http://www.rcgroups.com/forums/showthread.php?t=2463445, Ken
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Old Jul 26, 2015, 01:57 AM
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It has been typical UK weather here for weeks Doug... Workshop weather!
We had a day's solid rain on Friday and it's been worse further north from forecasts and so on.

The wind seems to have dropped through yesterday and into today though so don't give up hope!

Robin
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Old Jul 26, 2015, 02:47 AM
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Actually not that bad a day here today Robin, good enough for a maiden but I didn't get home from work till 11:00pm last night worked a 16 hour day flat out so I am bushed so not in the right frame of mind, might take my Black Widow to the field though
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Old Jul 26, 2015, 07:48 AM
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Good to hear its only weather and work holding you back. We hope to go flying here today, but first, I will help my bud install some rear brake rotors/pads on his Dodge Caravan.
I haven't flown now for approx. 10 days either,20-30klm winds, and 90F days with humidity in the 80% ranges make it hard to fly.
Take your time, be sure the plane is ready, and the PILOT. Doug B
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