markhamregular

The PT 40 trainer that I built (first plane) had a tendency to go nose up. A friend at the field added some weight on the nose and it flew better.
However, it was still going nose up andanother guy considered as the best at that field just took a quick look and told me that I had a positive incidence
which cause the air to flow over the stab and cause the nose-up tendency.
He recommended I put a little foam on the back of the saddle so the wing would be inclined more forward.

Is it possible to note a positive incidence by just looking?

Should I follow this guy's advise?

Thanks,

Harry

Rodney

Yes, it is possible but not with any great accuracy. It is not clear what you mean by "it goes nose up". If you mean that at high throttle it noses up but when at a lower throttle setting or glide it acts normal, then it could be that you need to adjust the thrust lineneeding more downthrust. If you can trim it out at cruising speed (about half throttle) then cut the throttle. If the nose drops a bit and it enters a nice glide, your incidence is fine. If it drops the nose and then pitches up as it gains speed in the dive, you may have to much incidence. The best way to tell is to have one of the more experienced flyers at your field trim it out for you. He should be able to give you a clue as to whether to check CG, incidence, thrust line or???. If you have ailerons on it, one trick that often helps is to have both ailerons slightly up when the ailerons are centered. I've seen this trick often make a trainer much more pleasant to fly.

Lnewqban

I agree with Rodney.

More than one thing may be wrong, and you need to learn each.

I see no reason for that person, who is "considered as the best at that field", not to take more time than "a quick look" with your trainer and teach you how to trim it properly.

It is much easier to trim a plane "hands-on" than from a distant forum like this.

Just ask for help at your club, somebody will help you properly for sure.

cfircav8r

My Feedback: (1)

A trainer has positve incidence built in, usually about 5 degs. This causes the plane to go from, lets say trimmed out level at half throttle, to a climb when power is increased. The reason for this is to aid in self recovery, a trait that is desirable in a trainer. All aircraft are trimmed for a specific airspeed, and when the throttle is changed the A/C will nose up or down to maintain that airspeed unless trim is changed. Changing the incidence will help lessen the nose up tendency as will down thrust. This will have the added effect of reducing the self recovery trait. If you are no longer using this as a trainer then both will help, but not eliminate, the nose up tendancy. Just make one change at a time, that way if it has a bad result you will know what caused it, and make small changes until you are happy with the results. I would not go past about 1deg of positive incidence and 5degs down thrust. Any more than that and you may have some very bad, ie. unrecoverable, tendencies. As far as Rodney's technique of determining the need for down thrust, if you chop the throttle it will nose down excessively and start to porpoise until it stablizes, reguardless of down thrust. If you smoothly reduce power it should just go into a nice descent with no elevator changes needed. Just wanted to add that to avoid possible confusion. Rodney's points are excellent as well as Lnewqban's. The trick of raising the ailerons slightly, reduces the angle of incidence as well as changes the curviture of the wing, and is an easy fix.

pimmnz

Actually, the models response is a desirable trait. Learning to fly, and most scale types, full size as well, do this. It means that, to climb you simply add power, and to descend, throttle back. No elevator or trim changes need to be made, and come landing, when you need throttle to control the rate of descent and elevator to control the airspeed, then it all works properly. If, as a student this is learnt early then all landings/circuits with any model type become much easier. Leave well enough alone, and let the 'experts' modify their own models.
Evan, WB #12.

markhamregular

Thank you all. I notice that expert advice can vary. I did notice that a lot of experts in the field tend to advice on modifications or try to do maneuvers that trainer planes were not designed to do. As soon as that expert started to fly my PT40, he tried to make a roll but that plane is not good at rolling for it tends to get back to its original position.

I will put back my foam on the wing sadle and adjust the throws to the level suggested by the manufacturer for my first solo landing this weekend. And maybe after, as you guys suggest, I may make some modifications one at a time.

You guys have been great. I appreciate it.

One last question: is there a book considered the bible of R/C modeling? In my other hobby boating, we have one that has been around for decades and cover in detail from A to Z. I assumed RC may have one too.

Thanks,

DagTheElder

Hi,

Check out these two by searching RCU:

1. Andy Lennon
2 .Basid Aeronautics for Modellers 2nd Ed by Alasdair Sutherland BSc-
Published by Traplett Publications, UK. Rare in North America but should be readiliy available in the UK.

Good luck

Regards

markhamregular

Thanks for the tip...

Also I forgot to mention... to the credit of that expert who told me about the wing incidence, he was the only one to find out that the right wing was higher than the left (this aeronautically challenged writer is not very handy and his first kit is certainly not perfect). The expert told me that’s the reason the plane is not easy to turn left, so he advised I use more rudder on my turns, especially left.

Dsegal

Go to http://www.flying-models.com/ Select Higley Books and order "Getting Airborne. Vols. 1 and 2.

Lnewqban

I don't know of one.
There are many that are just very good, but mostly outdated regarding electronics and electrical power.

I liked this book, which I found in my public library, very much:
"Take off" by Alex Weiss

With the Internet today, there is more information available about RC modeling and flying than what anyone has time to read:

http://homepages.ihug.co.nz/~atong/

You will find a wealth of good information here in RCUniverse.
Research the Beginners, Aerodynamics, Tips and Techniques and Questions forums.

Best luck with your first solo landing!

cfircav8r

My Feedback: (1)

The only problem with experts is many are very good at flying and set up, but can be lacking in experience or ability to teach new pilots. It is easy to forget the challenges new pilots face and the new skills that are just being learned. For an expert, trainers can be frustrating to fly due to the A/C constantly trying to re-establish straight and level flight. This trait is what makes them ideal for learning, but can hinder advancement when the student is ready to move on to more advanced flying. When you find you are trying to fly the A/C beyond its capabilities, and feel the need to modify it, you should probably move on to an advanced trainer or an easy sport plane.

rmh

some new flyers new a quick visual on CG and incidence angle of attack and what they mean in actual practice
The new micro flyers such as the one ounce VAPOR can be used to easily demonstrate this
I learned it with paper gliders when I was a kid
the basics can't be skipped over if you really want to understand what is happening.
a text book full of how to's is fine - for some
but the new models DEMONSTRATE this and that is a learning approach which is much easier for some. The airfoil stuf is all well and fine but misses the basics.

BMatthews

Moreso than the wing on it's own it is the angle that you have between the wing and the stabilizer that produces the nose up reaction to speed. But linked to that is the location of the balance point. Or Center of Gravity (CG) a many of us also call the balance point.

The trim angle needed between the wing and stabilizer is linked to the balance point location. If the BP is well forward you need a greater angle. Further back and you need less angle. Too far back and the model won't want to recover from a dive on it's own.

The trim is somewhat variable ONLY because we typically use elevator trim to slightly alter the trim angle. Note that with a model such as your trainer you can only adjust the elevator trim for level flight at one power setting. Or more accurately you're setting the level flight SPEED. The speed coming from the power setting. Lower the power and the model will go into a glide at nearly the same trim speed. Raise the power above the level flight pointand the model will react by climbing at an angle that maintains the trimmed airspeed.

By moving the BP forward like your one expert did he moved the trim speed to a slightly higher setting. But your other expert that suggested foam under the trailing edge did not know that you had already put in the nose weight. The forward BP from the added noseweight and the lesser wing incidence angle from the foam will be working at crossed purposes. To finish the foam under the wing mod the proper second step is to remove the added nose weight. If you don't then all that will occur is that you'll add enough elevator trim to cancel out the reduced wing incidence angle.

If you're at the point where you are flying well enough that you are finding that the model's desire to "balloon", or lift the nose with added airspeed, is getting in your way then the proper modification is to trim theBPBACK about 3/8 to 1/2 inch. Then work on shimming the wing until you don't have a lot of down trim in the elevator. It is the BP that drives the need for a matching wing to taildifference angle that affects the flying. The wing to tail angle needed is purely a product of the BP locationand it is the BP location that sets how strongly the nose up pitch in response to power or diving speed will be. Thewing to tail trim anglethat actually does thepitching actionis slaved to theBP location by your desire for the model to fly level at some cruise airspeed.

Does that sort of help to tie things together for you? I've attached a simple all sheet balsa glider plan. If you build one you can play with a lot of the features I've discussed. You can also alter the BPback and forth and then play with the elevator trim to see how as you move the BPback at some point the model will no longer recover from a stall or a dive. Then as you move the BPforward in 1/8 inch increments and re-trim the elevator for a slow and stableglide how it becomes marginally stable with the first move, and more stable with each additional move. At the same time try a HARDstraight forward throw with each setting. Note now at first it does a gentle rise of the nose and a long upward curved path before falling off in a stall. But as the BPgets two or three more steps ahead and the elevator is re-trimmed to compensate how a hard launch produces a tight loop that tries to hit you in the back of the head. This is the result of the link between the BPand the wing tostablilizer trim angle forstable level flying speed that I've been talking about. If you understand how this all works then YOUwill be better than most of the experts at your field.

If you joined the wing center joint together with some angle difference there really isn't much you can do to compensate for that which will work over the whole speed range. You can try to warp the wings so that they compensate but in the end the best option is to cut the center joint apart and re-do it with the two sides dead even. Depending on your construction abilities at this point you may find it easier to just use the plans from the kit and build a new wing. If you go this route consider it as a first step into scratch building. If you want to try it then you'll get no lack of help from here for advice on how to do each step.

Lone Star Charles

markhamregular

Charles,

Tiy said "When flying, I have trimmed the airplane to level flight at half throttle."
How do you do that? Do you have to use the trim everytime when you fly?

Thanks,

Lone Star Charles

HighPlains

My Feedback: (1)

A lot of experts don't know what they don't know. But that seldom stops them.

The real experts know what they don't know. There is hope for them.

markhamregular

Thanks BMathews for all your explanation. Thank you all others for your kind advice.
I have to rely more on what the plan says than what experts on the field are saying.
They may be right, but as Mathews noted, they might not know what I have done before.

I put my foam back.

For instance, last night I measured the throw on the elevator and it was 1/2" while the plan says it should be 1/4" for a single rate radio. So I moved the clevis to a different hole. No wonder by just touching the elevator I almost stalled the plane on 2 occasions on take off.

The plane is now ready for my first solo landing and I am doing it this weekend.


 

Lnewqban

I believe that this reading will help you prepare for that first solo landing:

http://masportaviator.com/2004/01/17...ng-techniques/

http://masportaviator.com/2004/03/25...ding-approach/

Rick.

BMathews - i like your little chuck glider 'aerodynamics lab' - i might make one for my boy (if i can resist putting micro radio on it Lol).

harryangus - just my 2cents (or 2 pence worth) - most trainer types are trimmed with a deliberate excess of +ve incidence and a way too far forward balance point, for stability (as has been mentioned) and have that annoying tendency to rear-up under power, yet dive maybe on the glide. As BMathews said, the proper cure is a slightly more rearward balance point accompanied by less incidence - but try a little at  a time because the plane will become less stable in pitch. A little down thrust is no bad thing, but excessive down thrust will make the plane briefly want to dive when the throttle is opened a bit quick - cos the down thrust effect is instant, but the balancing aerodynamic forces only come in as the airspeed increases. Another fudge around 'cure' is if you have a programmable radio and can programme mix in a little down elevator as the throttle opens and a liitle up elevator as the throttle closes - i've used this trick on a slightly too 'bouncy' vintage cabin type plane.

good luck with the solo flight!

BMatthews

Rick, if you'll be using it for serious flying and not just test gliding to find out how the trim works I'd suggest some 1/8 sheet for the wings so it'll withstand throwing it hard and the inevitable "tough love landings"   You could even install a peg about half way between the nose and leading edge and extend the fuselage stick about an inch behind the stab and glue a wrap of medium sandpaper on it.  Then two or three #64 bands looped together to form a long bungee would send it aloft with fine style.

Harryangus, the elevator trim in the case of sailplanes is our throttle.  As such it is a primary flight control.  On the old movable style levers I used to extend the elevator trim button by drilling and threading in a 1 inch long 2-56 screw so I could find it easily.   An airline pilot with previous jet fighter experience that used to be in our club used his elevator trim frequently to trim for different power levels and for landing approches during flights with all his power models.  In full size airplanes the elevator trim is also a very frequently used control.  Almost to the point of being as important as the joystick or wheel.  A lot of us modellers see it as a one shot correction for new models and use all manner of tricks to try to avoid using it.  But it isn't any sort of admission of inadequecy to use it a number of times in a flight.  To me it speaks of a pilot that knows more than many that he's willing to set the trim for such things as landing and takeoff and again for the "sporty" part of a flight.

Foxman

[size=3]When you put some packing under the leading or trailing edge what you are actually doing is changing the angle of the fuselage and stab.
At a given speed and weight the wing will fly at the same angle whatever you do with the packing on the body.
What you have changed is the angle that the body flies at.
Adding weight to the nose to stop the nose up tendencies is probably the wrong thing to do.
The elevator has to have more "up' dialled in which tends to make it climb even more as the speed or power is increased. Fly it inverted and see how much down you have to push in. Should only be a little bit. Get help with that.
Getting the cg in the correct place is the only place it should be.
Your other problems are due to it being designed as a trainer and should not be regarded as problems if you are a beginner.
Find a good teacher, not a show off who wants to demonstrate "his" flying abilities with your model.
Just because you fly good doesn't mean you teach good.
Jim Fox

rhall999

Incidence vs. CofG is one of those neat sort of give and take relationships. They work together hand in hand to give you the flying characteristics you want. BMatthews has explained it pretty well, but he hasonly lightly touchedonone other very inportant factor that plays into the whole thing as well. Airspeed. He sort of alludes to it by mentioning to throw the glider hard, and when he talks about the trim speed. I am going to try and explain in practical terms how airspeed creates a neat balancing game between CG and angle on incidence.

Here is what happens, as we know, if you move the CG back, you have to decrease the incidence (or trim by adding down elevator trim). Now, when the aircraft is flying, the aft CG will try to "push the tail down", but the incidence (or down elevator trim) will hold it up, and away you go. But, here is where the airspeed comes in to play, you will have it trimmed to a certain speed as has already been explained, and you decide you want to slow down. The incidence or trim (aerodynamic forces) will be less effective due to less speed, but gravity of course staysthe same,so the aircraft will become unstable, wanting to nose up and stall. (less incidence or more down would help here) Then you get scared, push the nose down to pick up speed, and the aerodynamic forces become stronger again holding the nose down, no you find yourself in a dive gaining speed and you are thinking your elevator servo is jammed because it will not pull out. (Now your extra down trim or negative incidence is a major problem!!)

Lets look at a nose heavy airplane. You have it all trimmed out for a certain speed, and you want to slow down. Throttle comes back, airplane slows down so aerodynamics become less effective, and the nose starts to drop. A you slow down more you find yourself running out of up elevator to keep the nose up. (More incidence would help here) So, you giv'er full throttle, speed up so the aerodynamic forces can overcome gravity, but since you are in a dive you gain speed quickly, and next thing you know, the airplane is going straight up!! (Your extra up trim or positive incidence may even cause it to loop right over on its back). Of course it will only go straight up for so long before it stalls, and goes straight down, speeds up and starts over again setting up a wonderfull porpoise until....kerboom.

As you can see, it is all a big game of balancing. To keep basic trainers stable and easy to fly, designers typically go with the second scenario to provide a more stable aircraft. Once you are more proficient you will start to see these things all happen on your own. Once you have found a happy balance between incidence and CG that works at both ends of the speed envelope,then it all becomes a game of trimming for different speeds, depending on how you want to fly that day. Yes, elevator trim is constantly being adjusted.

With the neat little "aerodynamics lab" that BMatthews gave us, you can demonstrate all of these by setting up the gider to glide nicely at a certain speed with a certain CG, and the throwing it harder or softer. You will find that it will do what I mentioned above everytime, for the reasons that BMatthews has explained. Great fun. Another fun experiment is to build some good ole' rubber powered free flight planes and get good at trimming them. This is how I started out and I firmly believe that I am a better pilot because I learned all about trimming, CG, and lots of other things from them.

Hope this helps a bit to visualize a real life example of what is going on.

PS: I used bold type so that you can easily see where the aerodynamics change due to the airspeed.

markhamregular

The question is: why would a manufacturer goes to great length to create something like this just to give a plane an ability to turn back to its original position? If after all, all other planes will require you to touch the aileron the opposite way to bring the plane back to its position, why don't they design trainers with just a high wing with a slight higher angle on top than the bottom.
Why go so far as doing a plane with a flat bottom wing, some exagerated incidence and a dyhedral that makes the stupid wing look like a V.
End result is that they create a plane that flies totally different than any other plane the newbie will use in the future.
And they call it a trainer? They take a good thing and make it so exagerated that it becomes a drag.

But then again, I am just newbie who has not solo yet. 

rhall999

Ahh, Great questions!! It is actually quite simple. When everyone is just learning, they WILL screw up a time or 3, and things will start to happen fairly quickly, and the "student pilot" will gauranteed get disoriented and not know what to do. So, the airplanes are designed in this manner so that all you have to do is let go of everything, throttle back, and the plane will right itself, and then you can get your breath, and carry on flying. As a beginner I can, with 100% certainty, assure you that when s**t starts getting out of hand, you first instinctive reflex actions will bethe wrong ones. So, the plane is designed to get you out of troube on its own. Otherwise you will most likely end up walking up and down the runway with a bag collecting bits. The other bonus, is that airplanes designed like this are slower and easier to land for a beginner.

Not quite sure what you mean by this, but I assume you mean a high wing witha bit lessdihedral than a low wing.

Ahh, not so. Airplanes still fly like airplanes, some are just slower and easier to handle.. Its just that sport planes, we'll use a Goldberg Tiger 2 (or even an Extra 300) as a common example, are too fast (so everything happens quicker) and will not recover by themselves. If you get into an "unusual attitude" (to use a full-scale term) a sport plane such as one of the ones mentioned, will stay there. They will not recover themselves. Once you have mastered the basics of flying with your trainer, you will also have developed the correct instincts and reflexes to get yourselfout of trouble in this sort of situation.