Brokenprop

My Feedback: (6)

Maiden flight on my H9 Corsair wasn't what I expected today. I let the tail come up and allowed the plane lift off the blacktop by itself. That part was beautiful. Then it started to climb. I fed in a little down. No response. Repeat, repeat, repeat. About 100 feet up it took some down, a bunch. I fed in some up. No response. Repeat, repeat, repeat. Suddenly gobs of up. After about four of these elevator cycles it wasn't 150 feet down field.

The bird was not going to find neutral. The ailerons responded slowly just like the simulator version. Chopping the throttle lost the little control that existed.

The only reason there is a plane left is some "restored Prarie". The vegetation prevented the inverted diving plane from reaching the ground. I broke a $26 18 12 prop on the RCV 120 and cracked the 'scale' antenna mast.

Everything is set up to the book. I was on high rates. H9 thought the elev. servo might be bad, their only suggestion.

I'm not sure what to try next. Doing the same thing again will surely yield the same result. Should the box have been labeled Free Flight?

Thoughts?

alan0899

G'day Mate,
Did you do a full preflight, & range check, before the flight. If not, why not?
Was the elevator working on the ground, & was it set at neutral & with the recommended throw.
Was the ailerons working properly, & in the right direction, at the recomended throw.

GotSky

Sounds like a dead on board battery to me. Slow servos and can't find center. What battery are you using and what are you using to charge it. Have you tried to charge the pack since you brought it home.

Brokenprop

My Feedback: (6)

Battery is fairly new NImh and was charged for 14 hours on a 150 mah wall wart the night before. The surfaces are extremely responsive with Hitec digital servos. The elevator will lift the plane off the stand and the mechanical retracts will break your finger.

With the antenna collapsed, range on the ground exceeded 100 feet. It took 6 ounces of lead on the tail to balance the RCV 120 in the nose. The plane is balanced fore and aft as well as right to left. Controls are centered, free moving and set at the recommended throws. All controls were responding correctly before and after the fllight. During the flight, both ailerons and throttle were responding, I didn't try the rudder in the death dive. The push rods on this ARF are only 4" long with the servo at the tail and are very stiff 4-40 hardware with ball joints, no slop.

I was wondering if the prop wash from that big prop could be affecting the elevator response. It doesn't seem to bother little rubber models to have a prop almost as large as the wing. The plane tracked great on takeoff and didn't seem to want to roll or yaw during the flight, just toggle from up to down.

I test flew an Ultimate for a friend last week that had a bit of a tendency to do the same thing, but we discovered he had the wing incidences wrong and there was enough control to at least get it back down in one piece so we could correct it.

With only one wing and a fat leading edge, I don't know what could cause the toggling.

You are correct that it behaves like a low battery or slow servo on the elevator.

Campgems

I have a Rupert's Dad, a scratch build from RC Model World magazeing. It's a great flying plane, on day I had a battery issue and ended up dropping the plane in pond. THe only damage was the lower LE sheeting which had to be replaced. I finally got it repaired a couple weeks back and the maiden was totaly white knuckle. One aileron servo's arm slipped a tooth and the plane wanted to cork screw. I got that taken care of and every thing checked out and the next flight was nearly as bad as the first. The roll was gone, but I just couldn't get it to fly level. A touch of elevator resulted in extreme response and as it is a plane that you have to fly every minute, it was all over the sky.

The problem was that I had bumped the switch for high rates and I had full travel at that setting with little expo. Once the addrenlin (sp ?) kicked in, there was no such thing as a smooth stick move.

Set up you low rate on the elevator with maybe 20% expo and set the throws per the insturction for low rates. It sounds like you may have had to much expo and to much throw and mix in a little panic and you are all over the place. I've been there too many times myself.

All of this assumes a good, slop free mechanical linkage setup and that the controls are all mechanical trimed with no trim on the TX.

Brokenprop

My Feedback: (6)

Your suggestions are really appreciated, thank you. No expo is programmed and the linkage is really solid. Control throw is by the book, 7/8".

Something that dosen't seem right is the balance point. The wing cord is over 13" at the root. The CG is supposed to be 5" back from the Leading edge. Have you ever had a wing balanced that far back? That's about 40%.

Seems tail heavy to me. The TE sweeps forward and the LE is straight.

Campgems

You balance in relation to the mean cord, so on a tapered wing, it will be somewhat different than a rectragle shaped wing. Take a look at this site and put your numbers in and see what it shows you. http://www.nasascale.org/howtos/mac-calculator.htm and

http://www.nasascale.org/howtos/cg-calculator.htm

Don

rainedave

My Feedback: (1)

How do you have your pushrods set up? They should be attached to the innermost holes on the servo arms and the outermost holes on the control horns. It sounds to me like it's a bit tail heavy and your pushrods are not set up for good resolution. That alone will make any plane twitchy and difficult to control. I would balance it between 25-30% using the calculator Don linked to, and make sure your pushrods are set up as I described. Using the outermost holes on the servos and innermost holes on the control horns defeats the purpose of using a proportional radio.

David

Craig-RCU

I originally balanced my H9 Corsair at 4.75" and felt that this balance point was close to if not at the neutral stability balance point because no trim changes for hands off straight and level flight were needed from low through high speed flight. I recently experimented with moving the C.G. back to 5" and found that my Corsair was wanting to pitch nose up at slow speed and dive at high speed. These pitching tendencies were mild and easily controllable so, I think that a 5" C.G. is only a tiny bit tail heavy. If your balancing technique was inaccurate in any way, you may have inadvertently set your C.G aft of 5". Because the Corsair's gear retract aft-ward, you have to balance with the gear retracted too, if you weren't already aware of that. I liked 4.75" better than 5", personally. Elevator control is going to be sensitive at these C.G.s so, I recommend lots of exponential.

I once experienced some weird servo behavior caused by a partially unplugged servo extension. The servo sort of behaved "lazily." The servo would swing past center after quickly centering the stick on my transmitter by letting the stick-springs on my transmitter self-center from full stick deflection.

I had some problems with NiMh batteries holding charges. I found that NiCd batteries hold charges better. That was a few years ago and I haven't used NiMh since so, I don't know if NiMh battery technology has changed since then.

A friend had trouble trimming out his ailerons in flight. Afterwords we found out that the stepping for the aileron trim had been set to some huge increment that made fine trimming impossible.

Nathan King

Don't you mean white thumb?

Campgems

Didn't stop at the thumb. Went all the way to the elbows. Dry cotton mouth and any other description. I've got dents in the TX case to prove it.

Don

Jezmo

In my experience, prop wash tends to help elevator response. An example is when hovering if you blip the throttle up while giving a control input the result can sometimes be more than expected. 3Ders will know what I mean. Moving the CG farther aft makes the plane more responsive as well so balance shouldn't be the issue. It almost sounds like some sort of radio problem to me, whether it be glitching or something going bad. If it's balanced that far back and the servo has enough torque to move the control surface while under flight load the plane should respond accordingly. I have had planes that were balanced way aft and they porpoised badly but the movement was fast not slow like you described. This is going to be very interesting when you figure it out. Keep us posted.

P.S. As Campgems said use the software located at the link or use the one at http://www.geistware.com/rcmodeling/calculators.htm . Look on the left side of the page and he has a number of very good calculators.

mjfrederick

This isn't true, it makes the plane unstable, which makes it seem responsive.

Craig-RCU

I think that Jezmo meant his comment to be taken in the context of the range of C.G.s that are not unstable. Aft of the neutral stability point is where your comment is true.

da Rock

What affect the CG has isn't that simple.

Moving it when it's ahead of the neutral point does quite differently than moving it when it's behind the NP. And wings have aerodynamic centers which are not where the NPs are, and moving the CG relative to those is another issue.

Jezmo

Thank you Craig, that is what I was driving at in the post. Yes Rock it is complicated but I meant it in more simple terms. When a manufacturer of a sport type plane gives you a CG range and you start out at the forward CG the plane is more sluggish and as you move it back near the aft CG called out in the plans it gets significantly more responsive without becoming unstable. Yes I am aware of what happens when you move it beyond the NP. My CG Sukhoi has been there and it was a handful although it was easier to hover.

da Rock

Actually, it's not that simple.

Moving the CG aft changes the stability. And in most cases with our models, actually does make the elevator more effective. However, the change in stability is very seldom into anywhere approaching instability. And the change in elevator effectiveness is very easily dealt with by modelers with some experience and understanding.

With quite a lot of our models, moving the CG aft makes the model appreciably more efficient, and changes the stability only slightly. But the predictability of the result is not a simple thing at all.

Lnewqban

Brokenprop:

Just one stupid question:
Did you check the range of the radio with the engine on, at different rpm, and with both antennas oriented in the same line?

In those conditions, and having another person manipulating the radio, you could see on the ground any slow and non-centering servo's response.
There is no reason for the model to behave differently in the air.
I would check vibration effect, electrical interference on the receiver antenna due to wires too close to it, and also metal parts touching each other.

Regards!!

mjfrederick

This statement is only true of models flying with 0 degrees incidence on the wing which require them to fly in a relatively tail-heavy condition in order to maintain flight and not have a large amount of up elevator trim. A properly-trimmed aircraft will end up with a CG more forward than others. Also, once an aircraft is properly trimmed the reaction to changes in CG and incidence become very predictable.

Wilki01

I noticed that you are charging with a wall wart charger. Is that the only way you have of charging your batteries, do you have a good way to charge them with, such as a Triton or whatever known brand name charge from your LHS that shows what it put into the batteries and has a way of loading them to show condition. You should have someway of testing them at the field as well. I would definately look at those batteries before anything else, It is most important that you absolutely know the condition of your batteries before even thinking of flying, then consider other things. You can change anything you want on the plane, but ultimately if you didn't prepare the batteriess, well the ground comes up quick.. You can charge all month long with a wall charger and still have a bad battery pack. I just grounded my Hangar 9 1/4 Scale Cub gasser because my flight pack developed a bad cell, now it's just a waiting game till the new one comes. I can fly it and take a chance or just replace charge, test and if ok fly. Instead of charge and crash. Take my adivice at face value, not worth much, but the whole idea is safe fun flying.

Wilki

da Rock

And that's why "it's not that simple" was said.

Changes are quite predictable. But they're not predicted accurately with sound bytes.

For example, when the CG is quite forward of the neutral point ("relatively nose heavy"), but behind the aerodynamic center of the wing and you move it aft, the predicted behavior is quite a different magnitude than when the CG was also ahead of the aerodynamic center of the wing and is moved aft (also a "relatively nose heavy" condition).

Quite a lot of our sound bytes are not universally true.
To say that moving a CG aft makes an airplane unstable, is an example of a sound byte that is too simple, and more often incorrect. Also, moving the CG sufficiently to make an airplane unstable would make the airplane uncontrolable, but would do it in degrees. As stability is measured, so is controlability, and so is responsiveness.

da Rock

The incidence of a wing is it's angle to the fuselage. An incidence of 0 degrees means nothing to pitch stability. And having 0 incidence certainly doesn't require a model be balanced tail heavy. Nor do properly trimmed models end up with more forward CGs. And the trim of an aircraft has no bearing on the predictability of changes in CG and incidence.

Campgems

Da Rock you are correct. All of the angles are in reference to an imaginary line down the plane from the center of the prop. It really doesnt matter where that line is drawn, or at what angel, it is only the relation of the cord of the wing, the cord of the horziontal stablizer and the engine thrust line that matter. The fuselage it's self has little to do other than to hold things together.

A 0 degree wing incidence could be the same as a 20 degree depending on the reference line. That isn't imortant, but the relation to the engine thrust line and to the horziontal stab is. And, if it's a Biplane, to the other wing.

Don

da Rock

And what importance would you think it would have to pitch stability?

The formula for pitch stability has 4 major players in it: 1. The size of the horizontal tail. 2. The leverage that tail has, or it's distance from the wing. 3. The wing's area. 4. And the wing's chord. Nowhere in that formula is incidence. Incidence really doesn't affect pitch stability.

Campgems

I was just pointing out that it is the relationship to each other rather than to an abratray reference line is what's important. Pitch stability wasn't part of my converstion. I'll leave that discussion to those with a better knowledge of the subject.

Don