I have a 1/4 scale extra 300. It is for the most part, 3d capable. I'm running a 1.8 c.i., 4 stroke glow engine, with an 18x6 prop.

I heard for good 3d flight, you need a thrust to weight ratio of somewhere between 2 or 3 to 1. I'm sitting somewhere around 1.5 to 1.

I'm running the biggest engine suggested by the manufacturer of the airplane. How do I go about reaching that higher ratio?

With a more powerful engine.

Always try getting the biggest and lightest engine possible

gee, thanks

It's the truth man.

You need more thrust if you want the ratio to go up to 2:1 or higher. Only way to get more thrust is with more cubic inches. I don't even know if it's possible to get 3:1 out of an airplane this size and still have a flyable airplane. You could put a 50cc gas engine on it and get 3:1--but it wouldn't fly.

You could try and lower the weight of the plane. I'm guessing you are flying an ARF. It's REALLY hard to get any weight out of an ARF.

You could build a kit and lighten it up. It's easy to drop as much as a pound on a 25% kit, but it costs $$ to do it. You have to replace the ABS parts with fiberglass. You have to purchase contest balsa and replace all your sheeting with the contest balsa. You can even replace the fuselage doublers with balsa instead of using the supplied lite ply. Buy carbon fiber gear and wing tubes.

Light airplanes will always fly and 3D better--but you better be really good at landing. Once you manage to pull a pound out of a plane this size--it's like landing an egg shell. [sm=confused_smile.gif]

Are you using a Saito 180? Thats a darn light engine for the power it puts out. I don't know how much more the Saito 2.20 weighs, but you might look at that option. Or, go for a 2-stroke like the Moki 2.10.

Take that motor and stick it on a Ultra Giles and you will have your power to weight.

With that motor, you want no more than 12 pounds. 10 is stupid power.

I don't think many 3d planes have 3:1 thrust to weight. 2:1 is lots and I find 1.5 is a little too little. 3:1 is a Hacker A20 on a 10oz foamie, a Saito 100 on a 4 lb Mojo. At some point it gets silly. What would you power a 30lb 40% plane with if you NEED 3:1?

1fasthitman,
Look around at just about every forum and look for posts that mention engine size. There are posts everywhere that mention using engines that're appreciably larger than the largest suggested one for the airplane.

Reading those threads should give you a good idea just how much oversize you can go.

Also, another thing that'd be valuable for you would be to research engine displacement to weights. OS is presently marketing a newly designed 1.20 2-cycle as being "the size of your average .90 2-stroke". It's only about 5 ounces heavier than their 91. Why would you think they're doing that? And look for matchups like the OS 61FX--OS 91FX. They're basically the same weight. Actually only .7oz difference. There is actually less value to making an ultra light 91 for use on 90 size airplanes than there is making a 91 that'd balance up front on a 60-size airplane.

These guys are all leadng you correct...

However, also important is wingloading... you can put a DA100 on your bird and have tremendous power-to-weight ratio... but it'll fly like a rock (if at all).

Truth is... in order to get the VERY best power to weight in an ARF... you need to buy an ARF known to be light and install the higher end powerplant.

A great case in point was the Extreme Flight Yak... 15.5 lbs with a DA50 was aaesome... Maybe around 2:1 or a little over. It's just not feasible to expect more.

In reality, 3D is more about a combination of features. Like in computer games when you develop a character... you get "100 points" to distribute among 4 or 5 attributes.

Well let's do that with an aircraft and set maybe 4 attributes:

Weight
Thrust
Wingarea
Stability


You have 100 points to distribute among these 4 things to get your plane...

Imagine setting them all to 25%... a nice even balance. No one attribute is more important than the other.

But let's say you want MORE power to weight... you might set:

Weight :30
Thrust :30
Wingloading :20
Stability :20

Since you only have 100%, you notice to get a better power to weight ratio, that you need to lower the weight (increase importance) and thrust... BUT wingloading and stability suffer. These are all inter-related... and increaseing/decreasin one attribute will effect the other to some degree.

For instance you want the weight to be lower... Well either it accepts a smaller engine that puts out less thrust, or the airframe is smaller and the wingloading suffers or it's too heavy and the plane want's to snap more.

Let's go for wingloading and stability:

Weight :20
Thrust :20
Wingloading :30
Stability :30

Here we want wingloading and stability higher priority. So we get a larger plane and smaller engine... but now we don't have 3D power.


But occasionally there are airframes that offer good weight and engine ability.

Like the new 74" Yak from Extreme Flight (or like the 68" and 87" were) or Quique 73" Yak. Other options might be the Giant U Can Do, Chip Hyde Tunnel Vision, Fliton Extra, Funtanas, etc.

Planes that are designed ultra light for their size... These planes get "bonus points" like weight and wingloading +10 so you effectively have 120 points total.

An engine can offer bonus points too... like the "who knows when it will ever get here" BME 55 that weghs a good bit less than other 50cc engines. Or the Saito 82 that weighs the same as the 72. Thrust +10


Then there are $$$ points... points you can BUY like lipos, CF gear & spinner, titanium pushrods. that add say +5 to +10 to weight.

I hope this analogy is clear...

Basically the BEST starting point is to select an airframe KNOWN to be lighter than the others. 1/4 scale is an odd size... a little heavy for glow (sometimes) and a little small for gas (sometimes) 50cc gas is awesome 3D in a 15-16 lb airplane in 1300-1400 sq in.

Then select a good balance powerplant of P:W.

Finally determine the $$$ items you can put on it to drop weight...

Do these 3 things and you'll have a more satisfying 3D aircraft.

Hope this helps...

J. David

Not all 18x6 props are equal.

Try an APC 18x6 wide.