As always empty:)

thailazer - now ask why.

Kurt

Honestly I would go with a 12 x 5. That might make you uncomfortable though so maybe an 11 x 6. You really don't need 7" of pitch on that plane though. It's too much. If you go with the 12" prop be sure you've got ground clearance. Again, if it were me, it would be worth it to go to larger wheels but I'd also make sure I was using very light wheels.

On the subject of CG and it's effects :


Most primary RC trainers are designed to self recover from pilot input, though today electronics can provide this function. This meant that when flying level, there is one throttle setting that will bring the aerodynamic forces into balance. It the model flies faster, then it climbs, and when going slower it loses altitude. Over a period of time the model when upset will eventually return to level flight, although it will still have both long and short periods of pitch oscillation. This is also similar to the force set-up of most general aviation aircraft.

Having the GC more forward amplifies this effect, making the model more sensitive to changes in speed. The most common method to compensate is to add down thrust, but not always the best method. As noted by several, decreasing the wing incidence can also reduce the ballooning effect. But moving the CG back and reducing the elevator travel is also effective when combined with other methods.

In this digital world, aerodynamics is one of the last true analog functions you may encounter.

Most 40 trainers do not require a 40 size engine. A 25LA is more than enough power to fly that PT-40 unless your flying from a cow pasture with foot high grass. You don't need unlimited vertical performance. The added bonus is a better cg. And yes rubber bands are your problem. If you keep the 46ax on your plane use 8-10 bands per side ,if you use the 25LA 6-8 per side

I think the last three posts make some good points. I would never say that the CG of a trainer should be at 25% of chord. There is no such thing as a universal CG point; it depends on the design.

As pointed out above, if the CG is too far forward then the throttle will have a bigger effect on pitch. You really don't want that for training, or anything else.

Also, the .46 AX is WAY more power than you need for training with that plane. So the throttle control has a big effect on power. Combine that with a forward CG and you have a recipe for big pitch changes every time you move the throttle. That will cause your plane to swoop up with full throttle. The guy who used a TT GP42 made a good choice. An OS .46 LA would be fine too. Probably the .25 suggestion would work too, but I'm not as sure about that.

Also, it is much better to use lower pitch for a trainer, and for many other planes. With less pitch you can use more diameter. I would try a 12x4 prop. It will slow down approaches and give better, easier control for landing.

Jim

Nose heavy does more than just create pitch sensitivity with speed. It also makes for a plane that has to land hot in order to keep from bouncing down the runway. It's the #1 setup problem I see with student pilots. Half the time, I can't even make a smooth landing with their planes after they've gotten on the internet and read the advice to go nose heavy for stability. A proper CG will result in a stall that mushes down slightly but still retains some elevator authority, which makes for nice slow and smooth landings.
I'll disagree with the recommendation to go with bigger props too. Trainers aren't slow flyers, and they aren't particularly draggy either. A 12x4 makes sense on a .40 size Cub, but it leaves very little room for speed on a trainer. On windier days, speed is your security. There are few things more frustrating to a beginner than fighting every little wind gust in a plane that won't go fast enough to handle it. The mind gets dialed in to the speed the plane goes, so there is no need to keep it low for students. An 11x5 is a good trainer prop on a ball bearing .46 engine, or a 10x5 or 10x6 on a true .40 or a bushing engine in that size.

Props are cheap and good to experiment with. I have consistently found that lower pitch props give me plenty of speed and excellent control on landing. To each his own, but to know what your own is, experiment.

Jim

Why is a beginner trying to learn when its that windy you need a rocketship to fight it. I disagree with your teaching method. Are you an instructor? When I teach the first thing I do is throttle to half and trim before I even give the control to the student.Guess what size pitch the Eflite trainer prop is? It's a 10-4 3 blade which is the same as a 11-4 2 blade for a 40-46. Maybe your brain can catch up to the speed but most can't. I never flew a nose heavy plane that was pitch sensetive only the opposite,loss of elevetor control at slow speed.

"Pitch sensitivity" is not the right term. Sensitivity, meaning a little movement of the elevator stick causes a big pitch change, is definitely caused by a rearward CG. However too much pitch change in reaction to THROTTLE is a result of a CG too far forward.

Also, I definitely notice that having the CG too far forward makes for hotter landings. There is no reason to move the CG farther forward than what the manufacturer of a trainer recommends. People keep reading "forward CG flies badly, rearward CG flies once" and some get so afraid of a rearward CG that they overcompensate and balance too far forward. The correct CG is better than either too forward or too rearward.

Interesting discussion. First off, while I think we nailed the reason for the ballooning, I think it is bad advice to recommend to a relative newcomer to start adjusting the rigging (incidence, thrust, decalage etc.) of a perfectly proven design. Especially without the ability to measure these things to know exactly what is going on. i.e. having an incidence meter.

I have found it best to have a lower pitch prop on a trainer as it gives more efficient thrust. I own this engine, and an 11-5 would be a great choice. There is no reason to have speed in your trainer. That creates a lot of issues. The controls will get more sensitive, you have to turn around faster, to keep from getting to far away, and overall, they will not have as much time needed as they would otherwise to make correct decisions. Also the lower pitch gives good breaking effect to slow down your landing approach.

While we are drifting somewhat off topic, let me take us further. I am an instructor in my club. One of the things I would like to emphasize to all of the instructors out there, is to emphise flying right hand AND left hand patterns and landing approaches. I cannot believe how many pilots in my club that cannot land to the right. This is a bad practice to maintain, and it takes practice to get over it. It is really not safe to not be able to fly both ways, and I feel you have not completed your training until you have the ability to land both ways.

As I said in my first post on this thread, I have never cared for the design of the PT-40, even though I knew the designer. It is too stable to be a good trainer. The best thing would be to take out the radio and fly it as a free flight. Eventually the wildly excessive dihedral was reduced, but with the decalage and balance when built strictly to plans makes it a poor trainer when compared to an Sig LT-40. I think that suggesting a few changes to improve the handling is not out of line and well within the abilities of anyone with a loose association of the English language.

Another thing I have noticed in many threads. There are distinctly different areas of the country, and each have different requirements. For instance, a guy that flies at sea level where there is little wind off of pavement does not need the same set-up as the modeler that operates a mile higher where the wind blows flying from grass. The mild breezes (10 mph) that drives people away from some fields would not even be mentioned at others. Having lived in many areas and belonged to at least 7 or 8 clubs over the years, plus doing contest flying at many dozens of fields in the US, you would not believe the differences in each group of modelers.

I think that most trainers have too much positive incidence for my tastes. A cambered airfoil such as those found on almost all trainers already will climb with power, without exacerbating this with more positive incidence. I personally like them set to zero relative to the stab.
I cannot remember if I have flown a PT 40, most likely I have, but I do not remember a problem. It would not surprise me if it did though. I think the worst one I have flown though is the Nexstar. They over thought that design, and stock they fly like cap.

That is a good point about altitude. It makes a huge difference. So while a .25 might be good at sea level, it most likely would not at mile high, such as where I fly. Not sure about altitude and wind, but we sure get our share of it. We have a lot of calm days though as well.

One design I always heard great things about were the GP trainer series. The 20, 40, and 60. This was a Joe Bridi design and had a fully symmetrical wing. In spite of that out of the ordinary design difference, they are known to be one of the best trainers ever designed. I always loved the Telemaster as well.