oxyfx

Hello,

I built a Northstar from a Balsa USA kit. I put an OS 50 Sx ring engine in it, and instead of the lead in the front I added a camera, 2.4 Ghz video transmitter, OSD etc - this extra weight in the front allowed me to take out most of the lead from the front cone. The gas tank is in the back as recommended by the design.

I originally had a 10x8 prop on it, and it was flying well, however for the takeoffs I needed a very long runaway. I changed the prop to a 10x10 thinking that that will give me the extra thrust - however I was mistaken, the plane got off the ground and just nosed in a few meters later. The damage is not much - broken tail at the ailerons pushrod exits, few cut ribs as the prop cut into the fuselage.

Before I start the repairs, I thought that I do a modification - I will lift the engine higher with abbout 1" to allow bigger - perhaps a 12x8 prop on it. The reason being that I have the same engine in a GP 40 size ultimate and it is running perfect with this 12" prop.

To do this I need to cut off the part which holds the engine and the tank, add 1" thick balsa on top of the tail portion which holds the engine, then glue the whole thing back.

I do not plan to move the engine forward or backward - I will keep it in the same position just higher wiht 1". My question is - how is this modification will affect the CG? What can I expect? The CG will move forward or backward - or it will not nove at all?

Thanks, OX.

Bart[man]

Oxyfx, How's it going? I'm flying mine with a 10-7 and am getting plenty of speed with a .46AX in a 7 lb aircraft. There are a few poasts in the Arrow topic, which is basicly a northstar without an elevator. However there is a gentelman who cut a divit into the top of the fuse alowing him to use an 11" prop with great success. Good luck, Bart (man)

jrf

My Feedback: (551)

Raising the engine 1" will increase the tendency of the airplane to pitch down under full throttle and pitch up when you throttle back, as on landing. It will definately require more downthrust in the engine.

More pitch does not increase thrust, as you have discovered. In fact, it is the high pitch that is causing the long takeoff runs. If you could use a 6 pitch, it would take off shorter, but of course you would lose top speed in the air. Engine load wise, a 10 X 8 is about right for the 50SX. I run a 12 x 4 on mine, which is about the same prop loading. If you are running a wood prop or master airscrew you can gain a lot of thrust by just going to an APC 10 x 8.

Another alternative would be to try a 10 x 7 Graupner 3 blade, but you would lose some top speed.

Jim

oxyfx

Hello Jim,

Thanks for the reply. Originally I had a master airscrew and I replaced it with the apc. How do you run a 12x4 on yours if it is a standard design? There is not enough clerance... Or you raised the engine as well? By how much, and how did you modify the enginemount to add moew downthrust?

Thanks, Ox.

4*60

My Feedback: (41)

Quote:
Engine load wise, a 10 X 8 is about right for the 50SX. I run a 12 x 4 on mine, which is about the same prop loading.

On the engine not a North Star, I assume.

jrf

My Feedback: (551)

Yes, on the engine. It is in a 40 size Edge. I only mentioned it to demonstrate the prop loading that works well for me on the 50SX.

Jim

oxyfx

Thanks Jim. That clarifies things... I have a 12x6 on my GP 40 size ultimate which has the same engine and it runs great with it.

jrf

My Feedback: (551)

You should try a 12x4 or 12x5. The 50SX is a high rpm engine.

Jim

Newc

Before changing other things I would go the opposite direction with propeller pitch. Going higher in pitch may increase maximum airspeed, but not the accleration you want to get off the water. Kind of like using first gear to get away from a stoplight instead of the high gear that is used for highway cruising. Try a 10x7 or even a 10x6 APC.

On one of my Northstars I grooved a 'divot' in the fuse to allow for larger diameter props, and I believe that this is a better solution than raising the engine pod. In addition to the other considerations, increasing the height of the pod will reduce its strength. I've included a picture of modifications that I make to my Northstars (four of them) using 1" triangle stock under the pod instead of using fiberglass as is suggested in the plans. This makes the tail fin/pod much more rigid and able to handle the inevitable rough landing much better. Also note the other modifications in the second picture for improved water handling.

ryanpilot

My Feedback: (4)

As Newc stated, try decreasing pitch. It is like changing gear ratios in a car. The farther you go from 1:1 the more bottom end torque you get but you loose top end speed. Same with prop pitch. Given the saame diameter when you increase pitch you pull top end if you can turn it and reduce your bottom end pull. Decreasing pitch will increase RPM but you hit the props efficent range sooner and gain bottom end. If you increase diameter and reduce pitch you get more bottom end. If you go three baldes you get more bottom end. You just have to play and find the right combination for your altitude, flying style, engine and weight of the plane. Just because one guy uses one set up and has sucess doesn't mean you will. Weight, balance and quality of build are critical and will vary from plane to plane and builder to builder.

I have been adding triangle stock to the pod of my Stars also. It makes a huge differance in strength with very little added weight. I never was comfortable with the small glue area of the engine pod and the huge stress applied.

I flew all my stars with a good .46bb engine and a Mac's muffler. I found that where I lived, at sea level, I got the best performance from a 11x6 APC prop. Great combination of low end and high end. I tried a lot of combinations including going diesel and kept comming right back to the 46 and 11x6. When I build another I will put a Jett 46 or a 50 although I do not know why, this plane was fast enough as it was.

I never build my Stars for land use. If I want a delta for land then I will get a different plane. So I do not add all the stuff for a nose wheel and I move all my radio gear to the front on the cabin. I hollow the nose cone out and use lead shot with enough epoxy to hold it in place and I put the battery in the nose. I use a 5 cell, which gives more power and the weight is an asset instead of a penality.

Also you guys that are new to building these need to remember to follow the building instructs when it comes to the leading edge of the wing and the sharp edges of the hull, they are critical.

oxyfx

Ryanpilot,

How did you make it run with an 11" prop when the clerance over the hull accept only 10" props? I have to repair mine anyway, so if there are modifications to be done, now is the moment. Besides, I followed the construction steps from BalsaUSA to the letter, and applied some of newrc's modifications like the triangles under the nacell. As a result after the nose in the tail and nacelle remained intact only the fuselage broke under the wing...

Newrc, do you have a picture of the groove you made in the hull to accept larger propellers?

Thanx, Ox.

ryanpilot

My Feedback: (4)

I did raise my engine pod slightly. I found that I only lacked a little clearance for the 11" prop. I would not build the pod any higher especially like some want so they can clear a 12" prop. If I remember correctly I only had to add 1/4". This was simple and I felt safe because I added the triangle stock to the area that the pod and fin meet and there was no deflection. In fact adding the triangle stock really added surface area for glue and stress contact and stiffened the whole thing up.

You know you built this plane well when all the structure stayed intact except for where yours failed. That area is the weakest part I feel. That plane will take a lot of abuse if built right. We all know there is a clock buried in our planes and it will strike the crash alarm eventually.

Good luck with the rebuild.

Newc

Here are a couple of pictures of the 'divot' I created in the fuse for prop clearance. The covering job looked a lot better about 50 flights ago!