mithrandir

My Feedback: (2)

Seems this is further complicated as the tailwheel of the P-51 is set further fwd making it rather short coupled as well!!

I have heard of some of the guys flying the narrow main's BF109 and Spitfires using a gyro on the tailwheel....

maybe not a bad solution?

AA5BY

On a recent build of a Spacewalker, I lost the steering arm springs and replaced them with some evidently too springy... ground handling went very south. Replaced them with much stiffer springs and handling was fine.

BMatthews

Zedhead, as you found from practical experience the best final location depends a lot on the reality of the field being used.

The info I gave up above worked for me on the sort of fields I've flown on. But each field is a little different.

The whole ground looping thing is also highly model design dependent. I've got old timer models where the wheels are way up front near the prop and they don't tend to ground loop either. But they also don't "enjoy" the sort of power to wingspan that some like to use on some models. They are also big and light so if they do get a little off there isn't a lot of mass for the rudder to deal with to get them back in line. I suspect all these things come into play.

rmh

That's giving up!
The relative wheel to rudder throw is typically the problem

AA5BY

The plane is finished and a flight report will be forthcoming after maiden opportunity though half of outing opportunities this month have suffered too much wind to fly.

The plane finished out fairly well, though heavier than hoped/wished... how often is that... eh. However, it was gratifying that a self designed scratch build balanced so well (at least to the theoretical balance point). After batteries were set forward, it needed two more ounces on the nose, which were acquired by trading the wood prop for a heavier composite.

Flight report soon.

AA5BY

The plane got its maiden and the original concern I raised might be valid.... will know more after a few more flights. To refresh, I was concerned that if the mains were too far forward, it might leave the tail difficult to come up and with a lot of wing area the plane might have a nature to get airborne as soon as the wing provided adequate lift rather than when the pilot decided to pitch the nose up with some elevator.

It did indeed go airborne without any elevator input though two clicks of down pitch were needed to trim the plane for level flight under power. More flights will tell what change that trim has on the take off. Another point that must be remembered... I fly off a rough landfill that was regraded last season and it is lumpy and quite often a plane will launch itself airborne after hitting a bump. So one takeoff can't be trusted to provide a solid character.

It also bounced once on the landing but I'm not sure the CG is dialed in yet or whether I had the right match of power until touch down.

Otherwise, the plane flew quite well. A full maiden report over in the Giant Scale forum.

http://www.rcuniverse.com/forum/m_11...m.htm#11069897

da Rock

How much tail weight really isn't the best question to ask when you have a ground handling problem. The answer is going to differ widely based on a number of details. I've got some WWI electrics that aren't as heavy as the battery that goes in a buddy's Pattern model. How far a tail wheel is aft alters how much force it'll put down. There are things that would have an effect and make the tail wheel weight approach really difficult to answer.

What works better is simply finding where your model balances and drawing a line from there to the main wheels' contact patch.

The drawing shows what works very well for conventional gear. It suggests that 15-20 degrees is the range, measured from where the CG gives good flying to where the tires touch the ground. The picture shows a model that has proven to porpoise on most landings. The model pictured has it's mains touching about 23 degrees from the CG. It's a good flying plane, and in fact was balanced 'safely nose heavy' from the mfg's suggested CG location.

The drawing shows what the aircraft industry has known forever.

HighPlains

My Feedback: (1)

Once again, hang the airplane upside down by the main wheels, and the angle that the airplane hangs will give you where the CG is relative to the wheel placement. Check empty and with a full tank.

AA5BY

I grasp your well made point about the problems in using the weight on the tail as a benchmark. After firming up CG, I'll check the angle from cg to mains and if out of range shim the struts aft.

AA5BY

Maiden video added

http://www.youtube.com/watch?v=QINu1...ature=youtu.be

rctech2k7

My Feedback: (1)

You have a nice build result, congrats on your maiden!

I finally got a chance to read this, plenty of very helpful and good information here. Just wanna comments on the earlier part of the thread. I agree that if the CG has moved backward, as the center of mass moved backward then the distance from CG to the gear will become longer and shorter to the tail arm, therefore you'll lose ground stability, both directional and horizontal. However there are other factors like on horizontal, the vertical and horizontal distance of CG to the gear and the amount of ground resistance. Also the the location of lift at the wings in the air and the load by the gear creating difference in moments with respect to CG.

Very true. Takeoff is one of the critical part in flying where load and controls are being trasfer to the wings. Therefore we wants aerodynamics to rule, takeover to above safe limit before getting into the air. Last time I test a model on takeoff by adding leading edge flaps on the wings thinking it might create some kind of nose down effect because the leading edge wing is pointing downward. Actually, the effect is true but the lift is purely aerodynamics.

AA5BY

The video shows the tail not coming up and the plane lifting off without elevator input. Two clicks of down trim were given for flight trim, which might have an effect on helping to bring the tail up... but I'm thinking the mains might have to be angled back some.

The landing is less clear about the cause of the porpoise. It could be simply a bounce from stall too high or it could be return to flight when the tail drives down when mains touch. Too hard for me to call.

AA5BY

Two more flights this afternoon with good results. The two clicks of down elevator trim had a positive effect and perhaps in conjunction with a little faster throttle run up has the tail coming up now with the plane skipping high spots on the runway until elevator is given for climb out. Both flights yielded a very nice takeoff requiring elevator to break ground effect.

And... I reduced the flap position to about 10-15 degrees and increased the elevator down mix and luckily hit the magic trim for no pitch change and used flaps for both landings. The glide slope was good and provided excellent soft and stuck landings both times. Thus... it appears that the gear position is satisfactory as is.

AA5BY

After a few flights, it was obvious that my original suspicion was right... that a generous wing area will in conjunction with a tail that is hesitant to come up will commence liftoff prematurely. It seems that the ability of the tail to come up before liftoff was very close because with a prop that yielded less torque but more speed, the tail would come up but with a prop yielding more torque and less speed, the plane was airborne before the tail would come up and I prefer the higher torque prop otherwise.

Though the plane does fly off the deck well and seemingly safely this way, it is disconcerting and might someday pose a problem if power should fade at the wrong moment. So, the strut has been shimmed aft and will be tested.

I was interested in seeing if the numbers were similar to those that DaRock posted... and the best I could tell by using a straight edge and angle gauge... the before angle was in the lower twenties and has been reduced to the upper teens. Exact numbers would have to come off the plan and the effort wasn't made.

As an aside, guys at the field suggested the airframe resembles a Zero and giving thought to the comments later,... it does turn quickly and climbs well on the wing. With a 35cc and 8' span at 16.5 lbs it is not overpowered but it does fly well and does a aerobatics well though it accomplishes them with slower speed lift rather than fast speed and power.

AA5BY

The mains were moved aft about 1.25" and the results were tested today with four flights and it did well. Ground handling was still good and the tail comes up fine now and will not take off until given elevator. There is no tendency to over on its nose but I'll reserve final adjustment after taking off in a little longer grass. I'll try to plot the current position of the wheels on the drawing and report back the angle when time permits.

da Rock

The tail wasn't lifting on the early takeoffs. Could have been the gear was so far forward and the ground stance so nose-up the wing was flying before the tail had enough lift to heft the CG up against gear that's too far forward. Hindsight, but that does match up with the results you got from moving the mains aft. The farther aft the CG is (or the farther forward the gear is ahead of the CG), the more effort the tail has to give to lift the CG or rotate on the takeoff roll. Gear too far forward and takeoff speed has to be up there before the tail has enough muscle to get the tail up. Wing is often flying way before that.

When gear is too far forward, landings often porpoise for the same reason. When the wheels touch, the planes momentum is heading down. The momentum acts through the CG. It's heading down, but the wheels stop the front from going down. The CG has nothing to stop it and keeps heading down. It being aft moving down, and the gear being forward and not moving, what does the plane do? Pitches up and heads skyward.

The fact you've got no tendency to nose over proves the gear is far enough forward. The fact it didn't porpoise with the new gear location suggests it's not too far forward. The fact the tail comes up easily now suggests the gear isn't too far forward and is far enough back.

Kewl job of designing.

Back when I was with Lockheed, I could go watch windtunnel tests. Talking to the engineers I got the following 'insider info':

They used thousands of hours of computer time running all the magic formulas.
They created models from that and used hundreds of hours of wind tunnel time finding out how the formulas did.
Then they took the prototype out and found out how it actually flew and fixed all the things that were wrong.

Excellent job on that plane.

cfircav8r

My Feedback: (1)

Sounds like a real winner. There are many production kits that have more issues than you seem to have had on the maiden flight of an untested design. I call that a "job well done."

AA5BY

Thanks for the kind remarks... the plane is mostly flying quite well. There are however two issues that have my attention. The first is probably quite simple and has to do with loop tracking, which is off a little right now. Needed is lateral balancing and double checking tracking of elevators.

The second is slightly more serious. During the four flights today in hardly any wind, the plane gave a shot across the bow of warning when it dropped a wing during the last landing of the day. My landing routine has been ten degrees of flaps with appropriate down elevator trim mix for no pitch change when applying flaps with low power. This seems a modest flap setting, but does shorten the landing by perhaps one fourth. They provide a very nice glide slope all the way to the deck and today after the wheel relocation, the plane sticks to the ground... all requiring very little if any flare.

On the last landing this evening, out of habit, I offered a little flare at about one foot from TD to see the plane balloon to three feet and drop a wing, scuffing it very slightly on the grass. Had the tips not been rounded... it might have caught. I was fortunate to avoid a serious mishap.

My first thoughts before sleeping on things is to reduce elevator travel and increase expo now set at 60%, move CG forward a tad and reflex the ailerons up some with flaps engaged. If that doesn't stop the nastiness and test show it to be flap related only... the flaps will be abandoned. The landing switch might be set for a few more rpms as well.

da Rock

hmmmmmmmm

My first inclination would be to take it up high and try to duplicate that stall. It only happened once and could have been a couple of things with that one approach. A slight gust, wind could have shifted, you got it slowed down more than the previous finals.....

One thing that sticks out is your 60% expo. That's a lot. Some systems get real snakey when the expo gets into the upper ranges. They will start acting like on-off switches. Or we start actually flying our slow flight with the stick about 1/3 from center because center doesn't do spit any more. I've seen TXs that didn't seem to move the surfaces at all around center when the expo was up there. The guys wound up flying the plane from 1/3 stick out. They'd adapted and didn't notice they never gave the stick just a touch when a touch was all that was needed because the surface never moved. You might get a buddy to watch your TX/sticks while you fly.

I found one TX that needed the EPA taken back to 100% to get the expo to work right. It was a witch to fly that sucker but the owner had gotten used to it... mostly. And he thought he'd just gotten old and lost his touch.

AA5BY

Yep, will surely do some testing up high to see whats up.

Interesting thoughts about too much expo... I generally run 40% to avoid doing dual rates but out of concern that the elevator on this plane might be sensitive because the stab really ought to be about four inches further aft, the anti was upped on the expo. btw, the stab is where it is because of hauling issues... would have had to leave the tail gate down.

Once one sleeps on things... and processes more fully what has taken place... perspective is often clearer. Something noted yesterday while doing consecutive rolls was that the feeding of elevator (both down and up) produced a slight tendency to over correct. In conjunction with adjusting the gear strut angle back, another prop was also used that didn't allow use of the two ounce spinner that had previously been used... though I thought the larger prop was likely nearly a wash in weight being two inches longer and having a thicker hub.... it might not have been. Combined, (just checked) the two changes have moved the CG 1/4" aft and likely increasing elevator authority.

At play also... the last flight of the day was a more extended flight leaving probably four ounces less fuel in the tank than previous flights.

The initial CG for maiden was set on the point prescribed by the aeronautical calculator and had seemed good and it now seems wise not to come aft of it any.

BFoote

I didn't see the real reason upthread for why many model RC planes with tail draggers have such poor handling on the ground. The reason is super wobbly rear tail landing gear. An angled tire turns. In fact you could steer your plane by angling the tire on the rear and not pivoting it if you wished just like how its done on a road grader. Most RC modelers a) take off from grass and therefore don't need much of a tail wheel and therefore are unfamiliar with them and b) simply throw a small wire down and throw a wheel on it and expect it to work.

Landing gear systems need rigidity. Unfortunately this requires weight. Something many modelers are loath to do. Put a rigid tail wheel design on the rear and you won't need two servos anymore. You might need dual rates set for ground verses flying of your rudder. Designing a rigid tracking suspension system is hard to do. It requires more than a simple thrown down wire.

da Rock

It's gotten to be a lot easier to add on a decent tailwheel. There are a number that provide decent strength and when designed like this one: http://www3.towerhobbies.com/cgi-bin...&I=LXBN38&P=ML

extend the leverage the wheel has for holding it's heading. They usually have a different response so you don't always need dual rates to deal with taxi response versus rudder air response. This is the light weight model of a group that Tower sells.

BFoote

Should have added rigidity in main landing gear is also an issue as well as for tricycle gear, not just tail draggers. Rather it is usually more noticeable on tail draggers as most throw a 1/16" wire down at some odd angle and expect it to work well. Have 2 axis the wheel can bend and turn throwing the ground handling all to whack. Add in surface roughness that on a full scale airplane wouldn't be anything to worry about and on an RC airplane throws the airplane all over the place as every exposed rock in the concrete waggles the wheels in regards to pivot angle and tow in.

da Rock

Another detail about tail wheels could be mentioned as long as the issue is their effect on takeoffs.

Taller tail wheels often help make ground handling less of a threat and more enjoyable. In the real world, I believe they jacked up the Corsair's rear for exactly that reason. They also help models react better when some of us firewall the takoff.

AA5BY

Didn't get to fly the Sport Thirty-Five this weekend but might tomorrow afternoon on the Fossil Fliers Tuesday afternoon session. Several changes (going against the rule of one at a time) were made.

1. Moved CG forward 1/4" from original point and 1/2" forward from last flight

2. Reduced elevator throw (each way) from 1.75" to 1.25" as well as reduced expo from 60% to 40%

3. Added 1/4" up trim to the ailerons in the second flap position (both flap positions have the same setting). So, flap1 is just 10deg flaps whereas flap2 adds the crow. This might require adjustment of the elevator mix in the flap2 position

4. Corrected non linearity in elevator servos... reduced the slave to 95% throw each way

5. Mixed a slight bit of throttle increase with flaps, which prevents pulling the power off too much as this engine will idle down to hardly any thrust