Info:

Hangar 9 J3-Cub 80"WS
OS61FX swinging 12x8MA prop
Plane weight fueled = 6.5lbs
CG slightly nose heavy at 3-1/4" per mfg. specs

I am thinking a prop change would help slow this bird down some and still give it a good pull in the air. I have an 11x10TFwooden and a 13x8MA on hand. Would either of these props be out of range for my engine and how would they affect it's performance? I also have access to an 11x7MA as well.

Probably a dumb question, but hey, that's why I am posting it in the Beginner forum.

I have the H9 and have had the same problem. Trying using the bigget prop with the lowest pitch in your engines range. Also check your idle and make sure your not balanced overly nose heavy

You may want to balance your plane. If it is nose heavy that will require higher landing speeds and cause your plane to stall at a slightly increased speed.

OS Recomends the break in prop to be 11x7. For flying sport aircraft you should use: 12x6, 12x7, 12x8, 13x6, or 13x7. Only if you have a Tuned Pipe use: 12x9, 12x10, 12x11.

That being said you may want to check you low end idle RPM. Does it pull your plane while its in the ground? Or can you idle in place? Also use a prop with alittle less bite such as a 12x6 or 12x7.

Moving the CG back will help a lot. It will allow you to pull the nose up at low throttle settings, which will slow you way down. With the CG forward, the elevator has to work too hard to keep the nose up and get the wing to the high AOA you need for slow flight.

Lower prop pitch is also a very good idea, some good suggestions above.

Try a 13-6 or a 14-6 Master airscrew, or better yet, an APC of the same size

APC even makes a 15 - 4 prop

Try a prop with a lower pitch. You said you are using a 12x8 now. Try a 12x6. If that is still to fast try a 12 x 5.

Recheck your balance. You do not want the plane nose heavy - it makes the landings faster.

It is not uncommon for the aircraft to have a problem slowing down to land and stick to the runway with the engine at idle when you have a light aircraft.

The thrust of the engine at idle (especially with some of the high power-weight ratio combinations we fly) can exceed the aircraft's minimum poer to maintain altitude. Add in ground effect and you can have aircraft flying in "the backside of the power curve" (where you point the nose down slightly and the plane accelerates and climbs with no throttle change) that just won't settle.

Lower prop pitch is often the only way to allow the aircraft to land while the engine is still running. With the lower pitch, at the same idle rpm, you don't create as much thrust. Increasing pitch and reducing diameter can sometimes help... but there are other tradeoffs when doing that, which may be a disadvantage, especially on a Cub or a design such as the Four Star which won't bennefit in (or from) top airspeed.

Being able to hold the nose higher due to a further aft CG can put you further into the backside of the power curve... and that does help sometimes. Being able to maintain pitch control at a lower speed (all kinds of explainations as to why... all add up to less downforce from the tailplanes needed with the CG change) and have the aircraft "wasting" more power due to higher AOA giving more induced drag. Caution here though... you can get in a place where the added power needed to prevent slowing to stall at that higher AOA makes the aircraft exhibit the above nose-down = accellerate+climb phenomenon more clearly...

********

A book I recently found (well... given to me by my mother out of some of my father's old books...) explains why this stuff happens pretty clearly. It doesn't address being unable to slow down specificly... because the book is intended for full scale, and at idle, they never have more power than needed to stay aloft.

"The Proficient Pilot"
AOPA 1980 (Author listed in a dfferent topic here... "Great Book!"... its in the car at the moment.)

I would try a 13-6 MAS. It will give you lots of pull if needed and act as a brake at idle speeds. I am pretty sure the 13-8 would overload your engine. My 2 cents. [8D]

Is this frame built for the 40 size engine size?

Yep. Horizon recommends a saito .54 or 40-46 2s if I recall the specks. Funny thing is with the battery and gear up as far as it would go it still need 2-3oz to balanace with a .OS 91 surpass

I have put a 13x6TFwooden on it to try and see how it reacts. Will have flight tests tomorrow afternoon, weather permitting (been raining here[:@]). I have 4-5 oz lead on the nose atm to balance at the recommended starting CG. I will remove a little lead and rebalance it a little further back. I know it's built for a smaller engine, but the 61 was all I had on hand, and it fits with very little mod.

P.S. Why are all of FHHuber's posts awaiting approval?[sm=confused.gif] In my short time as an RCU member I have received a great deal of welcomed assistance and advice from his posts. Clear the air!

Fly through a tree. Always works for me.

Andy



(Don't fly through a tree.)

Remove the "noseheavieness", and use a 13X5 prop.

Jetts

I would even go with a 14x5 on that set-up. That plane should cruise nicely with a bigger lower pitch prop. I flew one with a .70 4stroke, it landed at walking speed, and took off in about 2". Very light plane to fly, and doesn't take a whole lot of wind to make it move. Good luck.