|Mar 12, 2013, 11:18 PM|
Read over this part again:
"The more forward you put the CG, the more you need up elevator to counter the diving tendency.
At slow speed, you need a lot of it, but the faster you go the less you need."
In other words, if you need more down trim at higher throttle settings, then that means the elevator is positioned too far up for that airspeed.
Think of it this way, if you're really slow, you might be using anywhere from 50-75% up on the elevator to maintain straight and level flight.
If you're at terminal velocity in a dive, using 50-75% up elevator is probably going to rip the wings off.
At that speed, you're probably using 5% or less up elevator to pull out of a dive, and when you level out you won't even need that much to maintain level flight.
The slower you go, the more that elevator has to deflect upward in order to hold that nose up, because the elevator loses effectiveness the slower you go.
Think of it this way, if you put 5lbs of lead in the nose, then you're probably going to need full up elevator just to maintain level flight (if that's even possible).
The faster you go, the easier it will be for the elevator to hold the nose up.
Your CG will be WAY forward compared to just having a battery up there.
Try it the other way, drop that 5lbs of lead in the rear cockpit.
The CG will be so far aft that your elevator will need to deflect downward to keep the nose down.
Position the lead weight so your CG is right on top of the center of lift, suddenly the elevator doesn't have to deflect at all, because it's balanced at the center of lift...the nose isn't heavy, the tail isn't heavy, everything is neutral.
At slow speeds, the elevator doesn't have to work harder to keep the nose up (or down) and at high speed it doesn't change attitude because the elevator is still in a neutral position.
When you change elevator position, you're effectively changing the horizontal stabilizer's angle of incidence (AOI) in relation to the wing.
More AOI means that the aircraft is very sensitive to speed changes, an example of this would be a 2CH model that uses thrust to change pitch attitude.
Less AOI (or zero relative to wing) means that airspeed does little to change pitch attitude, and in some cases it doesn't change it at all, an example of this would be a hotliner or racer.
So, if your CG is nose heavy, you have to compensate by using additional elevator trim, which increases the horizontal's AOI, and in turn makes it more sensitive to changes in airspeed.
If your CG is neutral, then the elevator is neutral, and your horizontal's AOI is closer to neutral as well...meaning that your model will be less sensitive to changes in airspeed.
I know it's a lot of info to digest, but you don't have to fully comprehend it...simply give it a try.
Move the battery back a little bit and see if it helps.
Remember that it's going to want to pitch up with the same trim settings, so be prepared to trim that out.
Keep moving it back until the pitching at speed issue is resolved or it gets harder to control and go back a step.
Moving the battery to a more aft location is far easier than changing the thrust angle of the motor, and based on my troubleshooting experience with aircraft, start with the easiest thing possible and work your way to the harder stuff if that doesn't work!
|Mar 13, 2013, 02:39 PM|
|Mar 13, 2013, 05:12 PM|
United States, TX, Johnson City
Joined Mar 2005
Thanks for the time and trouble there WTK and I understand what you are saying but yesterday I was flying at neutral trim for the most part, so I may having been flying with more throttle then needed. It is when I apply the throttle is what I am talking about at 1/2 throttle or below then I am a neutral in the trims. I did notice that when the throttle was past 1/2 the plane would climb like a homesick angle but then I have found that to be true with all biplanes must be the short nose.
I use Tower Hobbies plane balancer and in the past it has always read nose lite when I had used before so I may be a little nose heavy, so I will do a little experimenting very easy to do in the Waco
I will say that I like a plane to be a little nose heavy, helps it point into the wind, no
wandering when landing and makes the wing work. Plus when doing low passes something I really enjoying doing with the Waco it gives me much great control to have to hold a little up elevator on the sticks.
I will move the battery back and see how it flies I have added nose weight but then I have removed the wheel pants and the long nose prop nut.
|Mar 13, 2013, 06:59 PM|
Some guys like the way a nose heavy bird flies, nothing wrong with that!
Why fly your own model the way someone else wants you to fly?
I prefer mine to be neutral or just a tad tail heavy unless it's already a handful, then I give it some extra weight up front.
When I set up a model nose heavy, I make sure that the elevator has enough authority to lift the nose up and hold it in a stall.
No telling how many guys I've seen or read about that crashed their models on the maiden because they made it WAY too nose heavy to be on the safe side.
Airspeed drops and the elevator can't hold the nose up anymore, even though the wings are still making lift, it just pancakes into the ground.
Then when the plane is rebuilt, they land it at a higher airspeed rather than take some of the nose weight out!
So now they've caused several problems trying to play it safe:
-Elevator is less effective at slow speeds
-Elevator is sluggish at cruise speeds (loops are big, turns like a train, feels like you're on low rates all the time)
-Difficult or impossible to maintain a knife edge
-Incapable of entering an unaccellerated stall or spin
-Higher landing speed
-Due to a high landing speed, they can't flare properly and landings become a daring maneuver
-Nose-overs with tailwheel aircraft are hard to avoid
-Harriers and hovers...not a chance
No wonder why some models get a bad rep or why so many guys getting into the hobby or flying a new plane have such bad luck!
So long as you can hold the nose up in a stall, you've got all the elevator you need....if you can't, might want to back off the nose weight or add more elevator travel.
A curious thing about some biplanes is that they fly with the elevator deflected downward at cruise speeds, even if it's nose heavy.
In level flight, my Agcat flies with the elevator pointed down all the time, it's about the same deflection that would sustain inverted flight in a monoplane.
15-20mph before stall is the only time that it's in a neutral position during level flight.
Nobody knows for sure what causes it (that I've talked with anyway), just that it's a peculiarity of some biplanes.
Changing the incidence of the horizontal to allow the elevator to stay neutral makes it fly like a gnat with brick wings.
I'm thinking it has to do with the way the tail is positioned in flight.
The biplanes that have this trait all seem to fly with a very tail high attitude, and the belly from the wings back acts like the opposite of a wing, creating an aerodynamic force that pulls the tail down (like the underside of F1 race car bodies).
To make matters worse, the Agcat has a flat belly that allows even more...let's call it "negative lift"...to occur.
In order to combat this force, you need downward deflection from the elevator to balance it out.
Some guys think it might be from the drag caused by the upper wing effectively pitching the aircraft upward, which does make sense, but most biplanes have their tail surfaces higher than a monoplane too...and it doesn't explain why a high wing monoplane doesn't exhibit those traits.
Maybe it's a combination of the two, who knows?
I haven't noticed the effect with the Waco, but I have seen what might be the RC equivalent of it in the Durafly SE5a.
Further evidence for the flat belly/rounded belly theory!
|Mar 14, 2013, 08:36 PM|
United States, TX, Johnson City
Joined Mar 2005
I rechecked the balance of the plane working at 80mm from the leading edge.
I just weighted the items I removed from the plane the wheel pants weight 2.5 ounces removed the long prop nut didn't like it and replaced it with a 14/-28 dome nut (not acorn nut those have a pointed top). I had added 2.5 ounces now it is 5 inches forward but the long prop nut seems to really add nose weight.
I removed 1/2 ounces from the nose see photo, balanced at 80mm (book calls for 70-75mm) from leading edge,the tail was hanging down noticeably, put the 1/2 ounce back, balanced perfect, the plane flys fantastic now I am not going to mess it up.
What I did do, is what I had asked about before, that was putting two #8 washers behind to two top screws of the motor, I added them and that all but solved the climbing under throttle. Of course that solved the need of down trim and that solved the diving when I backed off on the throttle
Now after all of these years of flying I knew this and why didn't I address right away, brain dead.
Now I need to do the other Waco, I am wondering if the wheel pants cause a down force
|Mar 14, 2013, 11:15 PM|
I haven't assembled mine yet and now I think I may fly it stock to see any comparison. I too want to take off the wheel pants.
|Mar 14, 2013, 11:49 PM|
|Mar 15, 2013, 06:27 AM|
United States, TX, Johnson City
Joined Mar 2005
Let them sway in the breeze man.
No really if you take off the wheel pants be ready to add nose weight. The funny thing each Waco took different amounts, green & yellow took 2.5 the other took 2.00 I am guessing because I re-painted the planes that is the difference in the amount of paint applied to the tail feathers, on the G&Y I put it on pretty heavy to cover the mold makers. I had sanding the heck out of them, used primer and with the yellow paint they stilled showed up. On the Nazi Trainer the blue gray covered them very well.
|Mar 15, 2013, 10:42 PM|
Looking good WT! I was reading an article in the latest issue of Model Airplane News on a fellow that had just completed a scratch built electric Ag Cat. Soon as I saw it I thought of you! Don't know if you can open this or not but here is the link to the on-line magazine.
|Mar 15, 2013, 11:10 PM|
Couldn't view it
Not too many options for agcats in the RC world.
Pat Tritle has a short kit for an agcat:
It's beyond my skill/patience to build a model from plans, so I'm still stuck without an agcat...
|Mar 15, 2013, 11:47 PM|
That's too bad you could not open it, I have a subscription to the magazine so that is likely why you can't open it, sorry.
I just posted a video link on the E-Flite Beaver thread, I shot it with an on board Go Pro. If your bored you can see what "Canadian bush flying in winter" is like. If the Waco could take the camera, I would like to do the same thing with it.
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