GP Electricub on floats
05-10-2008 | 06:32 PM
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From: Ellicott City,
MD
GP Electricub on floats
Ok, not trying to get myself in trouble for posting the same thing on two forums (Seaplanes) but thought that some of my questions here may be aligned with us beginners.. Anyway, this is also on the seaplane side.. just looking for some opinions.. mostly to the "update" part below!
I have a Electricub that I've been flying with wheels for the last few years.. Put a brushless/lipo setup in it last year so that if I put floats on it, it would have plenty of power.. Anyway, I got a set of Eflite floats (For the J3 that they also make) and they are about the perfect size for my cub.. Mounted easily and look great.. My question is regarding the CG.. I Flew the plane a few days ago and it was VERY "Pitchy".. It balances level on the stand, where as it used to balance with a slightly nose down attitude. I've added a bit of weight to the nose, but due to the pendulum effect of the floats, I'd need to add a considerable amount to get back to that "nose down" attitude.. I'm up to 4lbs 4oz already which is alot for that wing.. (20.8 oz sq ft. wing loading)
My step is located 1/2 inch behind the cg.. I think that it would help the situation to move the floats up a bit so that the step is on the cg.. All the reading I've read says that you should be behind or on, the cg.. Will it make much difference with the way the airplane handles on the water if I move them up a bit? Thanks for any suggestions! I'll try to post some pictures of it on the water..
UPDATE:
Ok, moved floats up a bit, and added a few ounces up front.. Flew the plane again today, and it did fly better.. One thing that I realized was that I didn't spend enough time trimming it out the first flight.. It needed A LOT of nose up trim, in fact I think that I used all that the transmitter had to hold level flight at a decent speed.. I guess it just needed more angle of attack to produce the required lift with the floats on it.. The strange thing that I can't get my brain wrapped around is that when I pulled the power back the nose goes up? It was a bit disconcerting on final approach to bring the power back, and see the airplane start pitching up.. Anyway, anyone know what would cause the nose to pitch up when the power is pulled back? I've flown 6-7 rc airplanes now, and a few more than that in full scale and I've never seen the nose of any airplane go up with a reduction in power?
I have a Electricub that I've been flying with wheels for the last few years.. Put a brushless/lipo setup in it last year so that if I put floats on it, it would have plenty of power.. Anyway, I got a set of Eflite floats (For the J3 that they also make) and they are about the perfect size for my cub.. Mounted easily and look great.. My question is regarding the CG.. I Flew the plane a few days ago and it was VERY "Pitchy".. It balances level on the stand, where as it used to balance with a slightly nose down attitude. I've added a bit of weight to the nose, but due to the pendulum effect of the floats, I'd need to add a considerable amount to get back to that "nose down" attitude.. I'm up to 4lbs 4oz already which is alot for that wing.. (20.8 oz sq ft. wing loading)
My step is located 1/2 inch behind the cg.. I think that it would help the situation to move the floats up a bit so that the step is on the cg.. All the reading I've read says that you should be behind or on, the cg.. Will it make much difference with the way the airplane handles on the water if I move them up a bit? Thanks for any suggestions! I'll try to post some pictures of it on the water..
UPDATE:
Ok, moved floats up a bit, and added a few ounces up front.. Flew the plane again today, and it did fly better.. One thing that I realized was that I didn't spend enough time trimming it out the first flight.. It needed A LOT of nose up trim, in fact I think that I used all that the transmitter had to hold level flight at a decent speed.. I guess it just needed more angle of attack to produce the required lift with the floats on it.. The strange thing that I can't get my brain wrapped around is that when I pulled the power back the nose goes up? It was a bit disconcerting on final approach to bring the power back, and see the airplane start pitching up.. Anyway, anyone know what would cause the nose to pitch up when the power is pulled back? I've flown 6-7 rc airplanes now, and a few more than that in full scale and I've never seen the nose of any airplane go up with a reduction in power?
05-10-2008 | 07:05 PM
#2
RE: GP Electricub on floats
Okay, I'll be the first to hazard a GUESS. Nose pitches up when power is reduced...Maybe the motor is mounted with to much down thrust. Try shimming the motor so it does not point down AS MUCH. This may also be symptimatic of the extra drag caused by the floats. The faster you go the more drag they will produce... Again these are just GUESSES. Good luck.
PS: What is the wing span of that Cub?
PS: What is the wing span of that Cub?
05-10-2008 | 07:45 PM
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From: Ellicott City,
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RE: GP Electricub on floats
Mr. 67Stang,
Wingspan is 58.75 inches.. Wing loading with the floats is 20.8 oz per square foot.. The motor doesn't have much down thrust to it as is.. Might not have any actually as I had to build the motor mount from scratch.. I'll look at that though.. it must be a thrust issue somehow.. Also I looked at my transmitter after I made my last post, and I thought that I had WAY more nose up trim than I did.. Turns out it was just a few clicks.. Power system is a Great Planes Rimfire 35-36-1200 Outrunner w/ thunderpower 3cell 2100 lipo.. The airplane does now weigh about .75 lbs more than the manual says it should (4.25 lbs vs 3.5). Anyway, here is what motocalc has to say (ADD moment Motocalc is really cool..) :
MotOpinion - Untitled
Sea Level, 29.92inHg, 59°F
Motor: Great Planes Rimfire 35-36-1200 (#4620); 1200rpm/V; 1.8A no-load; 0.0415 Ohms.
Battery: Thunder Power TP2100 (ProLite) (15C); 3 cells; 2100mAh @ 3.7V; 0.018 Ohms/cell.
Speed Control: Great Planes Electrifly Silver Series 45; 0.008 Ohms; High rate.
Drive System: 10x5 (Pconst=1.06; Tconst=0.995) direct drive.
Airframe: ElectriCub; 471sq.in; 68oz RTF; 20.8oz/sq.ft; Cd=0.073; Cl=0.47; Clopt=0.79; Clmax=1.3.
Stats: 57 W/lb in; 45 W/lb out; 21mph stall; 26mph opt @ 74% (18:12, 107°F); 34mph level @ 93% (11:44, 123°F); 525ft/min @ 13.1°; -283ft/min @ -7°.
Possible Power System Problems:
The estimated steady-state still-air battery temperature at the hands-off cruise airspeed and throttle setting (greater than 200°F) is higher than the suggested maximum temperature for this cell type (122°F). This could result in battery pack damage unless adequate cooling airflow is provided and/or run times are kept short. A lower current would also decrease the battery temperature.
Current can be decreased by using fewer cells, a smaller diameter or lower pitched propeller, a higher gear ratio, or some combination of these methods.
Power System Notes:
The full-throttle motor current at the best lift-to-drag ratio airspeed (20.8A) falls approximately between the motor's maximum efficiency current (20.6A) and its current at theoretical maximum output (119.1A), thus making effective use of the motor.
Possible Aerodynamic Problems:
The static pitch speed (46mph) is less than 2.5 times the stall speed (21mph), which may result in reduced performance at typical flying speeds and a low maximum speed. This situation is usually acceptable for an electric sailplane or other slow-flying model.
Pitch speed can be increased by using a higher pitched and/or smaller diameter propeller, a higher cell count, or some combination of these methods.
Aerodynamic Notes:
With a wing loading of 20.8oz/sq.ft, a model of this size will have flying characteristics suited to the intermediate pilot, for use in calm to moderate wind conditions.
The static thrust (38.7oz) to weight (68oz) ratio is 0.57:1, which will result in short take-off runs, and no difficulty taking off from grass surfaces (assuming sufficiently large wheels).
At the best lift-to-drag ratio airspeed, the excess-thrust (16.3oz) to weight (68oz) ratio is 0.24:1, which will give good climbs and acceleration. This is a good in-flight thrust to weight ratio for a basic trainer.
General Notes:
This analysis is based on calculations that take motor heating effects into account.
These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.
Wingspan is 58.75 inches.. Wing loading with the floats is 20.8 oz per square foot.. The motor doesn't have much down thrust to it as is.. Might not have any actually as I had to build the motor mount from scratch.. I'll look at that though.. it must be a thrust issue somehow.. Also I looked at my transmitter after I made my last post, and I thought that I had WAY more nose up trim than I did.. Turns out it was just a few clicks.. Power system is a Great Planes Rimfire 35-36-1200 Outrunner w/ thunderpower 3cell 2100 lipo.. The airplane does now weigh about .75 lbs more than the manual says it should (4.25 lbs vs 3.5). Anyway, here is what motocalc has to say (ADD moment Motocalc is really cool..) :
MotOpinion - Untitled
Sea Level, 29.92inHg, 59°F
Motor: Great Planes Rimfire 35-36-1200 (#4620); 1200rpm/V; 1.8A no-load; 0.0415 Ohms.
Battery: Thunder Power TP2100 (ProLite) (15C); 3 cells; 2100mAh @ 3.7V; 0.018 Ohms/cell.
Speed Control: Great Planes Electrifly Silver Series 45; 0.008 Ohms; High rate.
Drive System: 10x5 (Pconst=1.06; Tconst=0.995) direct drive.
Airframe: ElectriCub; 471sq.in; 68oz RTF; 20.8oz/sq.ft; Cd=0.073; Cl=0.47; Clopt=0.79; Clmax=1.3.
Stats: 57 W/lb in; 45 W/lb out; 21mph stall; 26mph opt @ 74% (18:12, 107°F); 34mph level @ 93% (11:44, 123°F); 525ft/min @ 13.1°; -283ft/min @ -7°.
Possible Power System Problems:
The estimated steady-state still-air battery temperature at the hands-off cruise airspeed and throttle setting (greater than 200°F) is higher than the suggested maximum temperature for this cell type (122°F). This could result in battery pack damage unless adequate cooling airflow is provided and/or run times are kept short. A lower current would also decrease the battery temperature.
Current can be decreased by using fewer cells, a smaller diameter or lower pitched propeller, a higher gear ratio, or some combination of these methods.
Power System Notes:
The full-throttle motor current at the best lift-to-drag ratio airspeed (20.8A) falls approximately between the motor's maximum efficiency current (20.6A) and its current at theoretical maximum output (119.1A), thus making effective use of the motor.
Possible Aerodynamic Problems:
The static pitch speed (46mph) is less than 2.5 times the stall speed (21mph), which may result in reduced performance at typical flying speeds and a low maximum speed. This situation is usually acceptable for an electric sailplane or other slow-flying model.
Pitch speed can be increased by using a higher pitched and/or smaller diameter propeller, a higher cell count, or some combination of these methods.
Aerodynamic Notes:
With a wing loading of 20.8oz/sq.ft, a model of this size will have flying characteristics suited to the intermediate pilot, for use in calm to moderate wind conditions.
The static thrust (38.7oz) to weight (68oz) ratio is 0.57:1, which will result in short take-off runs, and no difficulty taking off from grass surfaces (assuming sufficiently large wheels).
At the best lift-to-drag ratio airspeed, the excess-thrust (16.3oz) to weight (68oz) ratio is 0.24:1, which will give good climbs and acceleration. This is a good in-flight thrust to weight ratio for a basic trainer.
General Notes:
This analysis is based on calculations that take motor heating effects into account.
These calculations are based on mathematical models that may not account for all limitations of the components used. Always consult the power system component manufacturers to ensure that no limits (current, rpm, etc.) are being exceeded.
05-10-2008 | 07:46 PM
#4
My Feedback: (1)
RE: GP Electricub on floats
When installing floats on an airplane that is well balanced and set up on wheels. The goal should always be to acheve the exact same CG with the floats installed. To accomplish this the majority of airplanes (but not all) will require a little weight in/on the bows of the floats.
This is a case where weights should be used instead of moving batterys or components around because that will allow the changeover between wheels and floats effortless and no need to rebalance every time you make the switch.
John
This is a case where weights should be used instead of moving batterys or components around because that will allow the changeover between wheels and floats effortless and no need to rebalance every time you make the switch.
John
