I'm using NACA inlets on the jet for cooling. (about 8 inches long, to equate to about 3 sq. in. of intake area) I'm using two. (one on each side) If you want to make your own, the info is at http://naca.larc.nasa.gov/reports/19...gi?thumbnail15

By the way, Merry Christmas, Ben. http://homepage.mac.com/mikejames/rc...7_06_hover.mov

Whow, nice hover! You know - it is a nice looking airplane too!

Rear engine stability is straight physics. If the plane is flying straight and level, then the free stream air is essentially hoizontal as it approaches the prop. If the aircraft pitch is positive (nose up), then the prop deflects the air slightly downward as it passes, so the equal and opposite reaction on the prop is upward.......or in the nose down direction. The greater the pitch, the greater the reaction for a given power setting. A negative aircraft pitch has a nose up reaction in the same fashion.........so overall you obtain a positive stability from rear props.

Front props, and jets with intakes forward of the CG have a negative effect on stability for the same reason.

This is the same stability factor that got Nothrop's flying wing (and gave Edwards AFB it's name.....after the pilot). They built the wing with pusher props and it flew fine. They then tried it with jet engines, and the loss of the pusher prop stability was enough to make it unstable. This was before the modern active stability systems we have

CJ

Thanks Paul for the photos. The scoops are smaller than I expected them to be.

The J21 plane had liquid coolers in its wing with air inlets in the leading edge of the wing. I could have a similar solution with tunnels for the air back to the engine compartment. But I do not really like the idea of having big holes in the leading edge as it would make the structure of the wing more complex. But it can be done. However I do not know how much it would change the characteristics of the wing. It will definitly change the airfoil.
Mike, the NACA link does not work for me. I would like to read it. Maybe it is only temporarily closed.

/PO

peso, about the only pusher with a liquid cooled motor I can recall is the Do-335, which had serious cooling problems with the rear motor.
It also had an ejection seat, which it needed when the rear motor caught on fire.
There's another condition endemic to the configuration.
The c.g. is WAY back unless you add a lot of nose weight.
My twin Kadet has something like 30 oz of lead in the nose due to the motor location and the tail booms.
I fly a lot of pusher camera planes, with tail booms.. these need a very extended nose to get a reasonable c.g.
This one has the motor basically over the wing, with just enough clearance for the prop, and a 3 pound camcorder in the front. It still needed nose weight.

You triggered my interest. Don Stackhouse at DJ Aerotech had an interesting summary with respect to a low inverted pass with a tractor prop setup with the airplane dragging the vertical fin on the runway.

"If the propeller disk is angled slightly upward as I just described, it will also be making some additional nose-up forces. A propeller in an angled flowfield makes sideways forces, just like a wing or tail surface. This is why forward mounted props (like most tractor propeller installations) are slightly de-stabilizing in pitch and yaw, while aft mounted props tend to help stability. You can see this on the old Northrop flying wing bombers. The original XB-35 had pusher props and no vertical fins. When they converted these to the YB-49 jet version, they had to add small vertical fins to make up for the loss of the yaw stability contribution of the props. "

"The downward moving blade in your slightly nose-up inverted pass situation sees a slightly higher airspeed and a slightly higher angle of attack than the upward moving blade, so the aerodynamic drag of the downward moving blade is slightly greater. This creates an upward force parallel to the plane of the prop disk, which tends to pull the nose up if the prop is ahead of the pitch axis (usually the C/G, or in this case the tip of the fin).This force is probably negligible in this situation (although every little bit helps!), but can sometimes be very significant. For example, on the V-22 Osprey tilt-rotor aircraft, this sideways force is supporting a large portion of the aircraft's weight while it is transitioning between helicopter and airplane mode. "

It isn't the result of the air being deflected downward as it passes. It is the result of the asymmetric loading on the propeller disk. The descending/lower blade in the pusher setup has a higher angle of attack than the upward/retreating blade which causes differential lift and drag effects and results in a force parallel to the prop disk. That transfers to the CG and results in the stability increment.

It brought back a memory of test flying a model built by a friend of mine. He had an interesting horizontal tail location and size and wanted to see if it worked. The CG was calculated by him. Right after liftoff I realized it was unstable in pitch so I pointed the nose up and wobbled to altitude to try to find some ideas before I ran out of altitude. Finally he and I decided to call it quits and try to get it on the ground. When I throttled back it suddenly stopped the pitch oscillations and although sensitive was controllable. It was the prop/power pitch stability increment - in this case it got worse with power in the tractor prop configuration.

Tall paul,
The J21 also had an ejection seat and a Daimler Benz 605 liquid engine. And it had cooling problems. 297 planes were built between 1945 and 1949. The engine was also built in Sweden. And many modifications were needed.
And you are right the C.G is a problem. Fortunately the wing is swept. Otherwise it would be even worse. I have calculated that 20 oz will be needed. That is 10% of total weight. Not good but acceptable I think.
The J21 is seldom seen as scale model. And that is not without reasons. And then I have not mentioned the landing gears. They are sooooooooooo long.
A feasibility study would say no. But it is a challenge. And after having built RC models for 30 years I am glad this kind of stuff exists.

Might it be a good candidate for a brushless electric motor? Put the batteries up front instead of lead. 3 blade prop on an outrunner type motor design. It would require cooling but maybe not as critical as a liquid power plant - put a small cooling fan on the same shaft as the prop. Jason Schulman (spelling?) certainly has pointed out the usefullness of the electric system (if you ignore the massive costs of batteries).

my two pattern twin boom setups wer swept wing--and I swagged the weight (actually got really close) such that the prop fell spot on the best cg of aprox 27% .
So all the mass was concentrated on the cg and there was NO moment arm on the prop -which I thot could minimize possible flywheel or precession or whatever from the prop.
The use of a good small brushless is a terrific idea for prototyping
for one - the power to weight is truly amazing and no vibration etc., it is truly one of the best experimenter's power plants.
You don't have to build the old conventional structures with a million cut out parts either
use cheap CF booms and basic crutch of foam sheet with the Solarfilm over ribs or formers.
This ain't scale of course -but to prove a setup -it is cheap and dirty and best yet - it works.
I am going to dig out a photo of these turkeys. -

True, I would love to see your pattern pushers. Please find the photos.

The trouble with the small foameys powered by the brushless outrunners is that they are addictive. It is so nice to fly without a single fear of crashing that it's hard to power up an expensive bit of airplane.

My J21 project suffers from bad timing. It is too early to electrify the plane due to the high cost of LiPo batteries. And it is too late for a foamie as I already have the composite fuselage and booms so only minor changes are permitted.
But the NACA scoops are interesting. Perhaps they could be hidden under the fuselage. Time for me to learn more about them.

These are really easy to make...

Take the NACA drawings, (link posted above by me) and scale them to the size you need, then make a plug, against a flat surface, so you have a glueing flange when you're done. Finish it with glass, or something that will stand up to molding, and you're done.

I first made a female mold, then realized that a male mold would be better, and produce a more structurally sound part. That part of my jet article is at http://www.nextcraft.com/mold_naca_inlet.html

The male "mold" allows the "shiny" or smooth side of the molded part to be the one on the outside, and it also allows you to make a small fillet along the sides, which would be inside the plane...not possible with a female mold. For your reference: To produce 3 sq. in. of intake area, the NACA inlet will have to be about 8 inches long, and about 3 inches tall. (I'm using two, one on each side of the rear fuselage, on my jet.)

The weight issue is a big factor in the 40 sized models that use a pusher configuration. You have to add a lot of weight to the nose, and that increase in wing loading is in my poinion more of a problem than clean airflow. I also would note that as with the full scale Do 335, two stroke pushers suffer from overheating, especially if you daudle on the flight line. A pump of some sort is usually used, as your tank is most likely at the CG point because of weight distribution. Also, remember you will have a longer rollout for takeoff, as you have no prop wash over the surfaces, and you can't jerk them into the air because of prop strikes at the tail. But asside from all of that, a good jet model like the A 10, or Hornet just plain looks so cool in the air without all that mess up front, and forget about cleaning your airframe when you are done- no gunk spewed all over your plane if the engine is in the rear!

Here's the math model for a pusher I'm working on.
45" wingspan, AXI 2814 motor, depron skins. I hope to start a prototype build during the holidays, possible kit release later next year.
Allan

Allan, does "math model" mean the c.g. is figured?
I've found that motors at the extreme rear demand very long noses with the motor batteries up there.

I pretty much know where I want the cg to be but, until I start building, the battery location is a guess. The battery pack will weigh enough to balance it out and there is nothing much up there in the way but exactly where, we will have to see. I hope I don't have to lengthen the nose...
By "math" model, I mean the digital model. Sorry about the confusing jargon.
Allan

Just curious...

In the bottom view, what's the little dome thingy under the nose?

That is supposed to be a gun turret. Maybe a bottom of a plastic cup with a couple of straws sticking out the front? Not part of the basic model but I like to have fun with the prototype.
Allan

Then you'll kindly explain why 'Pushy Galore' was so unbeatable...

I'm currently working on a A/P plane to carry up some video equipment. Eventually I'd want to mount the engine in the back and the camera in the front of the plane. One thing I've been told is 2 strokes run very hot. Placing them in the back of the plane moves them out of the airflow that cools them. At that point they overheat, so a bigger heatsink should be used. Two other suggestions were helicopter 2 strokes with the bigger heatsink, or use a 4 stroke.

I started out with a 4 stroke on my (puller) pattern plane earlier this year and have noticed a difference in tempature. Most of the heat seems to be around the exhaust header, while the rest of the engine remains very cool. The 4 stroke offers better fuel economy, so I'd rather use a 4 stroke than a 2 stroke engine. Placing the exhaust header out in the passing air isn't that big of a deal. I haven't really tried the idea, just throwing it out there. Anyone else played with 4 stroke pushers?

--Scott

A 2-stroke pusher needs to be cowled, with cooling scoops to ensure it gets air. Otherwise it's just as good as a tractor setup.
There's a greater variety of tractor props though. Easier to fiddle with diameter and pitch. Pushers tend to be NNx6 pitch, although there are some others.
Electric motors are better than fuel motors for AP. No mung all over everything. But the cost is higher, and the system installation weighs easily twice as much.

How about a mid-plane position? For example, some of the Northstar, enforcer, type designs... or maybe even ducted fan?

Edit: ooops! missed a few posts somehow. Question in reference to cooling...

Allan, I think you're going to find that the motor batteries need to be a lot further forward into the nose to achieve balance. Even then you may need nose weight for your design as shown. Looks good though.

The ten cell pack shown on the model was in the rearmost location with respect to SPACE issues and its effect on cg was not considered much. You are correct that this location would not balance right. I will use a 7 or 8 cell (1700s) and the pack will definitely be further forward. Based on my measured weights of the motor/mount & tail surfaces, the 8cell pack should fit nicely while the 7 cell pack will be pretty close to running out of room.
After that, I either lighten the tail surfaces (they are now sheet 1/8" balsa) or... lengthen the nose for the kit version.
Allan

How about a 3-cell Lipo? Small enough to stuff pretty much anywhere, and plenty of power. BTW, can I go ahead and place an order? Looks great!