Pusher/Puller
 

Is there any information on flying a pusher puller configuration with regards to matching the props when using two identical engines, say .28 size or larger? It seems to me that the pusher mounted behind the wing needs to have a prop with a higher pitch than the front one doing the pulling. The pusher is seeing higher velocity air.

RE: Pusher/Puller
 

Villa:

The average entry velocity is close enough to the same for pitch variation to be not an issue.

Bill.

RE: Pusher/Puller
 

Bill, Thanks for the reply. I put together a pusher/puller SPAD using .28 Magnums and 9X6 APC props. The same engines and props I had on a tail dragger SPAD with engines in parallel. In the pusher puller I could barely get off the ground. It was so marginal that I doubt if it would get in the air without a headwind. Each plane was about 6 to 6.5 pounds. The P/P is tric gear and the other was a taildragger. I finally put a 9X5 APC prop on the front and all is just fine now. The P/P is a way, way stand-off scale of a Cessna Skymaster. However, since I started in twins with the GP P-38, I feel like naming it my PP-38. A photo of the earlier twin is in an earlier post under twin spad.

RE: Pusher/Puller
 

Villa:

Might work even better with 9x5 props on both engines.

Bill.

RE: Pusher/Puller
 

Bill,
Tower Hobbies only has the APC prop in the 9x6 size. Do you know of a source for 9X5 APC pushers? The rear engine is a screamer in the air. It really unloads. Only the rear engine can be heard. There is no syncronization rhythm change and I miss that. I have not checked it with a tach while the front is running but it probably is up around 13,000 on the ground. I have wondered if some of the screaming comes from the wing acting like a drum. The trailing edge of the wing is just 3/16 inch from the trailing edge of the prop. Wish I had more room there. What do you think? I have a 9X4 to try on the front also. I added winglets to the wing and I think, but am not certain, that I imroved the takeoff performance. Rolls are faster in the air. Next weekend I'm going to put on the 9X6 prop again to see if I can credit the winglets for better takeoffs. I lost the front engine and the rear one just kept on screaming and I could maintain altitude. This one is a keeper. I'll have photos soon.

RE: Pusher/Puller
 

Villa:

I now see the probable cause of the rear prop's noise. You have made an inefficient siren.If you can cut the trailing edge of the center section, put an extension on the rear engine's propellor shaft, anything to increase the gap between the t/e nd the prop blades should help to quiet it down. Even moving the engine rearward on its mount. Or get one of the long mounts to get it further back.

Zinger propellor:

http://www.zingerpropeller.com/Pusher.htm

lists 9x4 through 9x7 pusher props for $2.90 each, pack pf six. Their range of two blade pushers runs from a 7x4 to a 24x16. They also have just about any size three blade pusher you might want.

Hope this helps.

Bill.

RE: Pusher/Puller
 

Bill, Thanks for the prop information. Triming the wing TE to provide more prop clearance is already on my list. I have some structural plywood in that area to support the rear engine weight so I have hated to get in there and cut. Moving the engine back is a no for the moment. This plane has been extemely tail heavy from day one. However, I have managed to remove the 1/2 cubic inch of lead I had on the front engine mount. I have not settled on the best location of the CG. I now have it much further back than expected. My generous horizontal stabilizer apparently gives me a large range for the CG. I do want to end up with enough performance to do one engine takeoffs with either engine. I may trim the TE 1/4 inch and move the engine back 1/4 inch and hopefully get rid of the scream. I'm sure the scream translates into some prop performance loss. I used wooden props when I started in R/C in 1974. Thru the years I have become afraid of wooden props, somehow believing they may fly apart and hurt me. How do you feel about wooden props on R/C. My props touch the grass and ground frequently. The rear prop, though, is well protected from the ground. Only an inverted landing can touch it.

RE: Pusher/Puller
 

Rudy:

Generally speaking, there's nothing wrong with a wooden prop. Other than breakage resistance they are much preferable to the flexible plastic props, and they are less expensive than the rigid composite props.

Aerodynamically, the modern composite props are better, but they are more expensive and usually heavier. If you are in the habit of breaking props the inexpensive wooden ones can quickly become more expensive than composites.

The wooden props are available in many more pitch and diameter specifications than composites. It's pretty cheap to set your CNC machine to run off a small number of wood props, the mold for casting composites is expensive.

My preference? If I can get the prop I want from MAS or Zinger in a composite I'll use it. But on a twin if I have to use wood on one engine the other engine will have wood also.

In your case, if you regularly cut grass this can damage the tips of a wooden prop, one of its few bad points, and composite will be better for long life.

But whatever you put on the rear engine, the front really should be the same except for the reversed pitch.

Bill.

RE: Pusher/Puller
 

Here are some photos of my P/P-38. Engines are Magnum .28. Wing is the SPADTOTHEBONE.COM Dominator with 3/16 added depth for servo clearance.

RE: Pusher/Puller
 

The first photo shows the bottom of the wing. In the wing are the two aileron servos, elevator and rudder servos, flat battery, and radio receiver. The second photo shows a detail of one of the servo access doors. The servos are mounted to the inside of the door w/rubber two sided tape and a tie wrap. Note two 4MM Dia. bamboo skewrs. After the door is closed the bamboo skewrs are pushed into the wing and lock the doors in place. The door is made by cutting three sides and scoring the fourth side from the outside of the wing to provide a hinge side. No cutting at the hinge for added strength. At present performance at take-off is marginal but weekly improvements are being made. One engine out is no problem. Still experimenting with best location of the CG.

RE: Pusher/Puller
 

I missed the detail of the door. Here is the photo:

RE: Pusher/Puller
 

Rudy:

An interesting airplane.

But with the front engine so far out how can the plane be tail heavy? Is the stab made of sheet lead?

I like your servo mounts and doors. Ingenious.

Bill.

RE: Pusher/Puller
 

Bill,
Getting the CG right has been a struggle and very interesting. Everything on the plane except the front engine is BEHIND the target CG. The tail surfaces are made of 4MM COROPLAS which is very light. I have had to trim even that. Also shortened the tail moment. Also changed from aluminum channel for the booms to aluminum angle. Also had a piece of lead that totaled about 8 inches in length (the type sold for models---don't know # of ounces) mounted with the front engine. After about 30 flights have managed to get rid of all of the lead weight. The CG has moved back about 1-1/2 inch and it still loops OK. Very tolerant in pitch. The main gear is just forward of the CG. Hope to move the front engine back some with further test flights. It is a darling in the air. Lost the front engine twice and no problem maintaining altitude. Am trying to figure out why it does not build up more groung speed during take-off. Hope to eventually make one engine take-offs. Presently it struggles to taxi on one engine. If it was balsa I would never take-off. This plane will not be hurt if it is forced into the air and doesn't want to fly, which is usually the case. he take-offs require 110% from the pilot. Per your suggestion I will be putting a Zinger pusher on this weekend.

RE: Pusher/Puller
 

Rudy:

The pictures look like you've used "Cutting board" to make your rear engine mount. If this is so, it should be simple to drill new mounting holes in the fuselage metal and move it to the rear. This would give youo more prop clearance as well as moving the CG back.

And I hope you meant the main gear was slightly behind the cg instead of just forward.

Where is the cg in percentage of the wing chord?

Bill.

RE: Pusher/Puller
 

Bill,
On my P/P-38, I started with the CG at 27%. Flew great but had a hard time getting off the ground. I slowly removed nose weight till I got the CG at 34%. It was still very steady in a loop. I added tail weight to try to locate the rear limit for the CG and flew at 38%. It still handled well at that point. Take-offs are good but could still use more power. Have not tested with a rear 9-5 prop. I hope to move the front engine (w/front wheel) back in increments of 1/2 inch to locate the rear limit of the CG but will have to do some calculation since I will be cutting off the fuselage end. What % is your estimate for the rear limit? You are right on the main gear. I have winglets on the wing tips and they look great. Not sure if they add performance. Very stable at a high AOA while flaring during a slow landing. Any experience with winglets?

RE: Pusher/Puller
 

Here are some more photos of my Pusher/Puller

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Here is a second try on the photos:

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Another photo

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Another photo

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Another view

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Here is a detail of the main landing gear and the front engine.

RE: Pusher/Puller
 

Rudy:

My guess on the aft cg limit? If you are already at 38% my guess is you've already passed it. But if it flies well at 38% it's obvious my guess is wrong. Just keep moving it back until it does what you want.

Are you including the ailerons in your measurement of the chord? If you are not then your 38% would be some distance forward of the point I would call 38%.

The airplane looks nice, bet it's enjoyable.

Bill.

RE: Pusher/Puller
 

I'm really late getting in on this thread - someone told me about it a couple days ago. I have been flying a Push / Pull style plane since 1997 and have come to some conclusions about the sizes of props and their composition. My plane uses two .25 motors swinging 9x6 props. Here's what I found in my experimentations with different props. Please visit: www.dawnaviation.com for more information and photos.

I use the same size props on both ends of the plane. The pitch of these props is also the same. I found that using wooden props (especailly on the rear engine) is not the way to go. The tips of a wooden prop when mounted on the rear engine will dance around fairly violently. I didn't like what I was seeing plus it caused some unwanted vibration. Using a plastic or composite prop STOPPED this problem completely. The tips of the prop stayed in a single arc and the vibration was eliminated. For this reason, the props I use are APC 9x6's on each motor. I will add that I tried a couple different brands of wooden props and they all had the odd tip movement on the rear engine. I also tried using props of different diameters between the two motors but didn't find any advantage to doing this.

Another thing I found interesting is that with both engines running together, the RPM of each engine increases about 500 RPM. For example: If I take a Tach reading on the front engine running by itself, it might read 12,000. I then run only the rear engine and it also runs at 12,000 RPM. When I start both engines and run them up, each engine is now running at 12,500 RPM.

Finally, I see some discussion on the location of the CG. On my plane, the CG is located right at 25%. The plane rotates easily and will fly vertically for as long as I point the nose up.

RE: Pusher/Puller
 

Villa,

What you need is very large four-stroke engines on board. Also try some 15% fuel and never be afraid to ask your fellow club members for help sounds like your a newbie.

RE: Pusher/Puller
 

CHRISTANEAGLE, been at it since 1974. The P/P is flying great now. A number of small changes (moved rear engine back a little, new glow plugs, 15% NITRO, moved CG back further) did the trick. That you, Deprat?