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F18 SM Gravity Center

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F18 SM Gravity Center

Old 09-15-2005, 06:56 AM
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etibedossa
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Default F18 SM Gravity Center

Hello,
I am beginning to build my SM F18C Blue Angel.I am a little bit anxious because the aircraft seems to be very tail heavy although the engine is not in place yet.What is the exact position of CG ?Does this airplane need a lot of lead for balance ?
Thanks.
Old 09-15-2005, 07:16 AM
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Heatseeker_Hill
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Default RE: F18 SM Gravity Center

are you balancing with gear down or up . as it does make a difference. i followed the recomendations and had no problems
Old 09-15-2005, 09:32 AM
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B777
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Default RE: F18 SM Gravity Center

I fly mine with CG right on the wing joint spars. Gear down and empty tanks. I have three battery packs up front and full scale cockpit for a no-lead balance. No problem flying...just too much fun.

Tips: pull the wings out slightly so you can see the spars and use the joints as CG balance support. You can hang the model from this point while shifting accessories for balance.

B777
Old 09-15-2005, 03:23 PM
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Speedbird66
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Default RE: F18 SM Gravity Center

Hi there,
just to back up B777 regarding the batteries in the nose. Use batteries of high capacity because you will need the weight in the nose. In total I ended up adding 1200g in the nose (all 3000mAh NimH cells). But don't worry the F-18 can handle the weight.
Best of luck/Johan
Old 09-15-2005, 10:28 PM
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pilott28
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Default RE: F18 SM Gravity Center

I started with my CG on the wing spar, but have successively moved it back. I found that the plane was difficult to rotate, was nose heavy in turns and somewhat unpredictable in the flare with the CG that far forward. I am now flying mine with the CG 1 1/4 inches in front of the rear hatch line on the top of the fuse, or about 3/4 to 1 inch behind the spar. I have a friend who flies one too, and that is where his is set. Most other SM F18 owners with any time on the aircraft have moved theirs back as well -- there are some other threads on that topic here on RCU.

There is no lead left in my nose, though both the ECU and flight pack are up front, and I have a farily heavy Just-the-Pits pilot and cockpit kit in the front.

In this kit, 1oz of lead in the nose moves the CG by 1/16 of an inch, so you can use this formula to adjust the balance point once you do the initial CG determination.

It is a very good looking plane in the air and on landing, and the Blue Angels scheme is very visible. I think you will be very happy with the plane. Good luck!

Old 09-16-2005, 03:45 AM
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etibedossa
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Default RE: F18 SM Gravity Center

Thank you to everyone for the help !
Old 09-16-2005, 12:58 PM
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Default RE: F18 SM Gravity Center

Keith,
thank you for this information. I will move the CG further back as you have stated. Some questions:
What is your elevator throw with the CG 1 inch behind the spar?
How much flap do you use on landings?

/Johan
Old 09-16-2005, 10:02 PM
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pilott28
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Default RE: F18 SM Gravity Center

I set the throw to the recommended vaules in the manual, don't remember off hand. I don't use a lot of flaps, as they tend to blank the elevator a bit on this plane. 30 degrees would be a close approximation. I added flap throw gradually from flight to flight.

If you get the nose up and carry some throttle all the way to touchdown, you can make a high AOA approach and landing that looks great. I was also surprised to find very little pitch change when I deployed the speed brake, which is also somewhat effective on approach.

I don't try to hard to flare and float this plane down the runway, as the tail will move around if you get too slow. Just fly a nice gradual approach right to the ground. Some guys like to bring the plane in high and chop the throttle, but I prefer a lower pattern with a more gradual descent, carrying power right to landing.
Old 09-19-2005, 02:57 PM
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Ali
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Default RE: F18 SM Gravity Center

etibedossa:
Sorry for my slow response, but I was away from the office for 4 days at a jet event.
Anyway i think all your questions have been taken care off. But here is my views.
Yes the F-18c is always going to be a hard model to balance without nose weight. I think my first prototype model had 2kg to get the balance where we thought it would be. Slowly we removed weight until the balance point was set at 10-15 mm behind the front spar bolt holes. this I found best for me as it gave a nice solid feel in the air. You can move back even more if you want. i think the max I went was 1.5 inches but found it not so stable on the pitch response.
The model feels especially nose heavy when on it's wheels, but dont worry when the wing starts to work it Will lift off great. I use quite a lot of take of flap. Maybe 20-25 mm as I find it reduces the leap off the ground.
If you have any further questions please feel free to email me direct on
[email protected]
Old 09-19-2005, 05:46 PM
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David Searles
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Default RE: F18 SM Gravity Center

Ali,

I'm just finishing my F-18E. Any comments regarding it's balance point?

David
Old 09-20-2005, 02:52 AM
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Ali
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Default RE: F18 SM Gravity Center

David Hi,
I should be able to let you know in about 8 hours. We are just off to test our 18 E in the next hour or so.
Old 09-21-2005, 09:30 PM
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David Searles
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Default RE: F18 SM Gravity Center

Ali,

What's the word?

David S
Old 09-22-2005, 04:10 AM
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Default RE: F18 SM Gravity Center

We only got to make one test flight yesterday with the balance point set to where the instructions suggest. It was fine. I prefer my F-18's to be a little more nose heavy but thats just my preference.
The flight went really well with the model tracking well and sitting solid like F-18's do. Didn't get much of a chance to do any hi Alpha or high G stuff. We experienced a strange engine problem that stopped flying for the day and caused a bit of damage
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Old 09-22-2005, 09:46 AM
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The boys Back
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Default RE: F18 SM Gravity Center


ORIGINAL: Ali

We experienced a strange engine problem that stopped flying for the day and caused a bit of damage

Thats an understatement!!!

all fixed now just a stripped aileron servo to change and all is good

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