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-   -   Bad idea # 15 (http://www.rcuniverse.com/forum/q-40-racing-155/6877845-bad-idea-15-a.html)

ecoliburger 01-09-2008 12:03 AM

Bad idea # 15
I thought I would post this to save someone with my bright idea some time. As I am new to q40 type planes I am leery to fly with no throttle. So I machine down a Perry carb to fit my old used Nelson. I gave no thought to the big boat anchor brass throttle barrel that perry uses for there carbs. Tonight trying to balance this thing I find with the battery cramed back as far as I can I still need to ad almost 3oz of weight to get the CG right. So if you want to add a throttle to a nelson dont use a perry unless you can account for the weight. In hind sight I should have used a supertiger which was one of the other carbs I was considering. So any suggestions for how to add about 8 pounds to the back of this thing?

garys 01-09-2008 12:50 AM

RE: Bad idea # 15
Something doesn't make sense. The weight of the carb barrel really isn't making much difference. If it needs 3 ounces of lead on the tail, it means there's at least 6 to 9 extra ounces of weight on the front, depending on the moment arm from the CG to the nose compaired to the moment arm of the CG to the tail. You do have the fuel tank empty, correct?

I wonder if you're balancing it at the correct spot? I think Ed gave you a place to try, but maybe it's too far back. Since the wing is constant tapered with the exception of the wingtips, go half way out on the wing and measure back 25%, and use it as the CG and see what it needs. 25% should put you in the ballpark to get started. Another option would be to post on the NMPRA forum to see if anybody there knows where the Nemesis CG should be, as there are several that use that forum but never come to this one.

Something else I noticed that has nothing to do with the CG. I noticed the head bolts and glow plug have rust on them. It would be wise to check the bearings, ect.


skull1971 01-09-2008 01:46 AM

RE: Bad idea # 15
I'd like to know what bad idea #1 was? "I think I want to pylon race." Mine was "I think I want an R/C airplane." Of course in Texas we have the "Hold my beer, and watch this!!" :D

Really cg should'nt have really changed, take the carb back off and confirm correct cg.

ecoliburger 01-09-2008 10:16 AM

RE: Bad idea # 15
I did replace the bearings and I will try your CG suggestions, fuel tank is empty. Bad idea # 1 for me was hey watch how fast I can go around this 90 degree corner on my street bike on wet pavement, hey who put gravel in the corner. At least I got to see what the inside of my tibia looks like.

Ed Smith 01-09-2008 11:01 AM

RE: Bad idea # 15
Where is all of the airborne radio stuff installed. My servo trays are as far back as I can get it with the rudder and elevator servos almost behind the wing T.E. The battery can still be placed behind the servos. Whether the tank is full or empty should make no difference as the centre of the tank should be on the balance point. If the tank is not on the balance point the trim of the airplane will change during the flight. If you think that a carb barrel will alter the balance what do you think removing six ounces of fuel from the nose of the airplane will do? The reciever is the lightest component and can go ahead of the tank.

There is a possibility that if the perry venturi diameter is a lot smaller than the nelson venturi the engine will not jump on to the pipe. If the engine is not running right at take off it could go sick in the air, be aware of this.

Ed S

daven 01-09-2008 11:11 AM

RE: Bad idea # 15
The Venturi should be 9mm if you want to check.

Sounds odd that it would be that far off, but I haven't ever seen one of these Q40's so I don't know their history in regards to CG.

garys 01-09-2008 11:50 AM

RE: Bad idea # 15
Ed, if I remember right from the Nemesis airplanes I saw, he may not be able to put the tank on the CG. I wouldn't really worry too much about tank position, it wasn't until the last few years that putting it on the CG became fashionable. It wasn't long ago that you yourself were cramming tanks backwards up against the firewall. Besides, you're never burning off that much fuel in a heat..two, maybe three ounces at most. Most of the fuel used in a heat is used up while running rich on the ground. I typically put 5 ounces in my airplanes and have about 1.25 to 1.5 ounces left after a heat, so I'm only really using 3.5 to 3.75 ounces between startup, needling, and the heat.

Have you tried checking with the person that sold you the airplane to find out where he had the battery, ect, located? Or if he had any CG issues when he flew it?

ecoliburger 01-09-2008 02:28 PM

RE: Bad idea # 15
Thats a good point about the fuel weight, didnt think about that. I cant get the tank on the cg no room there, has to go up behind the fire wall. The guy I got the plane from had never flown it and knew nothing about it. It could be that it was built tail heavy but it does look like it has been flown. I appreciate all the suggestions, I will check things over some more tonight.

Ed Smith 01-09-2008 03:55 PM

RE: Bad idea # 15
At all costs try and avoid adding weight. It is far better to redisdribute necessary weight. The battery can be realy useful for this. This may sound weird, try taping the battery to a short dowel and laying it in the bottom of the fuselage with the battery at the tail end. if balance is achieved velcro the dowel in place.Ensure there is no interference with push rods.

I know of one racer whose battery was four cells arranged end to end. The battery pack slid into a tube glued to the top inside of the fuselage. The battery pack was slid backwards or forwards to help balance.

Ed S

garys 01-09-2008 04:03 PM

RE: Bad idea # 15
Don't worry about the tank location, the fuel burn really won't make more difference than a click or two of trim from a full tank to bone dry..no biggie. The old Formula 1's used 11 ounce tanks mounted up in the nose, with longer noses which would make a bigger difference, and people didn't have issues with them. Have you ever seen a difference in the trim changing on any of your other airplanes? These really aren't any more critical.

Does it look like it's been crashed? Was there evidence of the battery or something being glued in the back of the fuse to help with CG? If not....since it's obviously been flown before, I'd be tempted to leave it as is and just go fly it. Maybe put some extra elevator on high rate just in case it's a little nose heavy, but it really shouldn't be that bad. It probably wouldn't be any farther off than any other guestimate of where the CG should be.

There's not much "building" to be done on these to where the CG would be so far off so as to need that much tail weight. The Nemesis was fairly popular in the 1994-1996 timeframe, and didn't have any reputations for the CG coming out way off. The Miss Ashley's from Rusty Van Baren tend to come out nose heavy due to the length of the nose and swept wing, but even then it's just a case of putting the battery behind the servos. Usually if any racer is heavy or something, they tend to come out tail heavy (since most of the airplane is behind the CG) and need weight in the nose.

Ed Smith 01-09-2008 04:27 PM

RE: Bad idea # 15

You might not like it but logic is on my side. One fluid ounce of glow fuel weighs approximately one ounce. Please do not tell me that removing over a QUARTER OF A POUND of weight from the nose of a 4 pound airplane will not affect the balance. Some may not notice it but at 170 mph I will stay with a balanced airplane.

Ed S

garys 01-09-2008 05:01 PM

RE: Bad idea # 15
Ed, if you want to deal in logic, it doesn't make sense that any of us are in this hobby anyways......haha

About half the fuel burned in a heat race is burned prior to launch. Therefore you're down to about 2 ounces making a difference. Yes, it can be noticeable, but realistically it's not within most people's flying skills to truely make a difference (including mine). Maybe I'm just not any good?

If you run the tank dry you'll see a bigger change in trim, but if you compare the trim within 9 laps (lap 2 to 10, the first lap's much slower, so its hard to compare), it's much less of a difference.

Most people's airplanes aren't trimmed within a click or two of perfect anyways, which further reduces the difference of fuel burn.

Although if you're q40's only going 170, it would take you longer to finish a heat, so therefore you'd be burning more fuel in flight......so therefore you'd have more of a change :)

HighPlains 01-09-2008 05:26 PM

RE: Bad idea # 15
Gary, the reason I didn't have trim problems with my F1 with a 10 oz. tank was because I retrimmed the elevator through out the race. My old Kraft transmitter had cross trims, so the elevator trim was right under my left thumb. But Ed is right, it was a pain in the ..., well, you know. We were also burning about 3 oz of fuel an minute, and unlike 15% fuel, 65% is a whole bunch heavier. I even tried a two tank system to get rid of the cg change, with a 8 oz tank on the cg and a 2 oz header tank at the firewall.

garys 01-09-2008 06:38 PM

RE: Bad idea # 15
First of all, I have to appologize to Ecoliburger for us hijacking his thread.....as I said before, if it doesn't look like it's been crashed, or had a battery or something glued in the back of the fuse, I'd just go fly the airplane.

Ok Bob...F1's burned 3 ounces a minute...so taking away the first lap as they're slow and untimmed due to the reduced speed, they burned about 3 ounces during a heat. What's the difference in the specific weight of 65% versus 15%? Basically there wouldn't be much over 3 ounces lost during the heat.

How much did your trim change in a heat? Now....reduce that amount by 1/3 because a q40 burns less fuel, and by about another 1/3 because the moment arm from the tank to the CG is about 1/3 less (and q40's tend to have longer tails compaired to the nose length than F1's did). Therefore the trim change would considerably less than your F1's.

I have never said there is no trim change due to fuel, I've just said it's not a big issue that's worth worrying about.

Any wind change is going to make a bigger difference in how the airplane flys on the course than the change in the CG due to fuel burn off.

We haven't gotten into the fact that many people (not me, however) trim their racers so they dive slightly when they let go of the elevator stick. They then must hold in some up elevator all the time. Do you really think these people can tell the difference if their elevator trim changes 1 or 2 clicks?

Like I said before, I must not be any good.

HighPlains 01-09-2008 06:46 PM

RE: Bad idea # 15

Like I said before, I must not be any good.
When I'm good, I'm very very good. But when I'm bad, I'm better! - Mae West

ecoliburger 01-09-2008 08:11 PM

RE: Bad idea # 15
No worries about hijacking I never mind a good argument...er I mean discussion. I just quickly measured the wing and found that half way out and 25% back ends up being about 1.9", which looks like it will balance there without too much shifting stuff around. If this is too far forward of a cg will it just make the elev less effective when landing or would there be other problems? If we get some decent weather this weekend I will give it a go and let you guys know how it flies.

skull1971 01-09-2008 08:21 PM

RE: Bad idea # 15
Please remember nose heavy you can live with, tail heavy will be a real handfull.

garys 01-09-2008 08:29 PM

RE: Bad idea # 15
Yes, if the CG's too far forward it'll just take extra elevator and speed on landing. I think you'll be fine.

kommander 01-09-2008 10:17 PM

RE: Bad idea # 15
Hi Bob, Did you ever frequent Leary's hobby shop in the Irvington district of Fremont? I think I saw you in there once with a QM15 long before I ever started flying R/C. Bruce

HighPlains 01-10-2008 12:19 AM

RE: Bad idea # 15
I don't know of any shop called Leary's in Fremont. I did spend a bunch of money at Ron Sheldon's, Pec's, and San Antonio Hobby. I did very little with Q15, mostly just flew it to kill time at the Nats when I was flying F1. A few times I flew the plane against quickies the month ahead of the nats at a few local races. It was slightly faster than the Rossi powered quickies of the time, so I would race a few laps then sandbag a lap, then race a few more...

BUDMAN27 01-11-2008 08:18 PM

RE: Bad idea # 15
Man if thats the only thing that gose wrong with your pylon racing than I would count your lucky stars above. ;)

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