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First foam build - Old Fogey - can't get it to fly!

#1
I sure do hope someone here can put me on the right track.... this hasn't been the best introduction to foam building. :(

[CONTEXT]

My wife and I are relatively new to the hobby - she got me an E-Flite Apprentice for Christmas, and we've been flying a ton ever since. She got herself a Hobbyzone Duet and has been loving it.

We decided to try to get into scratch building both because it's cheaper and because we like building things. Unfortunately, this hasn't been quite as smooth a start as we've had with the prebuilt planes.

We wanted something that was as slow and lightweight as possible, and we wanted a speed build kit so we wouldn't have to stress over whether we did a bad job cutting things, so we got the Old Fogey speedbuild. We didn't buy the Power Pack kit because we have a bunch of components (motors, props, etc) we bought in bulk from a local hobbyist who's getting out of the hobby.

I'm very familiar with aerodynamics because I have some background in general aviation. I've also done a ton of reading and research into the hobby over the last few months.

[BUILD]

Our servos (9 gram Hextronic) and control surfaces are working great. Receiver is range-tested and working. Batteries are fine, ESC works fine, etc. The wings have 2 inches of polyhedral on each end (I made a foam guide to make sure they'd be exactly the same height, and I adjusted them to have slightly less polyhedral than the laser-cut kit suggests, because I've seen elsewhere that the standard amount of polyhedral has given a lot of folks trouble with dutch roll. I'm using 9x4.7 slowfly props from Hobbyking, and I've balanced them with tape. The propeller is definitely mounted 'forward' - the plane blows air backwards and pulls itself forward through the air when the throttle is turned up.

[THE PROBLEM]

Not enough lift! Which is clearly due to not enough forward speed/thrust. I've run down a hill holding the plane in my hand above my head, and it strongly wants to pull upward out of my hand. But no matter what I try, I've not been able to get the plane to pull itself forward fast enough to create the lift it needs.

I've tried two different motors. First, a 25g 1400kv Turnigy (think it's a 2822-azj or something like that - you can't really buy them anymore). It was way underpowered. On a 2 cell battery, it wouldn't fly farther than I could throw it before just hitting the ground. On a 3 cell, it was willing to stay in the air, but I measured the current draw using an inline meter on the table and it was WAY higher than the 7 amps the motor is rated for. And it made the motor way, way too hot to actually consider using.

So I decided to try one of our larger motors. I switched to a roughly 50 gram, 1000kv Turnigy (D2830-11). Now, I understand that 1000kv will ultimately produce less thrust than a 1400kv motor at the same voltage, so I immediately recognized that there was no way this would work with a 2 cell. Which is a shame, because they suggest that flying the Old Fogey on a 2 cell will make it more docile, and that has been our desire/plan all along. But I've now tried it with a 3 cell (1300mah), and it still won't fly. From the moment it leaves my hand, it's like it's on a perfect glide slope for a landing - it usually makes it about 20 or 30 feet before finally contacting the ground. At 100% throttle. Frankly, I just can't understand it. I've tried two different motors, 2S and 3S batteries, 9x4.7 and 8x6 props - all of which supposedly are within the ballpark of what this plane needs. But it simply can't keep itself in the air.

All-up weight is right at 500 grams, and without the 3S 1300mah battery it's at 400 grams, so it's right around the expected battery-less weight of 373g/13.3 ounces.

[OTHER ISSUE]

I've had some difficulties getting a prop mount that really works. I've been trying to use the tapered cone-style prop savers as seen here, but I find that over 50% throttle gives me vibration and prop whine. So I've resorted to other, weirder methods of getting the prop to mount properly. I'd appreciate advice on this, but this isn't really my main issue. I am pretty confident that the prop is actually rotating at the speed of the motor, because I know what it sounds like (and what it feels like) when it doesn't.

[SUMMARY]

The Old Fogey was supposed to be a relatively slow flyer. And supposed to be able to run off a 2S on a 1400kv motor. But it doesn't even come close. And I can't even get it to fly on 1.5x the voltage (3S) with a 1/1.4 kv motor (1000kv). Something clearly isn't right. I don't really want to buy yet another motor when I already have two that should at least theoretically work (even if the small one would definitely burn out eventually at 3S/11.1V). Does anyone have any ideas what we might be doing wrong? I'd really appreciate some help, because this has been a surprisingly frustrating intro to scratch-building, and it's a hobby I'd like for my wife and me to stay in for a long, long time.
 
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basslord1124

Well-known member
#2
I'm not really too sure what it could be from what you've listed so far. But some general things I would look at it (which some you may have):

-Weight
-CG
-Thrust angle of motor
-Control surfaces... make sure they are level with their stabilizers and that it looks "true"

Their recommended power pod for the Old Fogey is B, so I'd stay close to that if you could...at least that can guarantee you can fly it with that...then make mods as you need. Do you have a wattmeter?
 

Gazoo

Active member
#3
A picture is worth a thousand words... A video is worth...ummm...like more than a thousand or something...

I think my first Fogey had the B-pack with an 850 2s. If flew OK. After getting rid of the landing gear it flew great. Can you send a pic?
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#4
Try calibrating the ESC throttle range. If badly set the resultant lack of thrust can be staggering.

Also match the prop to the motor not the plane. Over propped motors get hot and do not provide as much thrust because the motor is overloaded trying to swing the prop and so turns far slower than the Kv rating might otherwise indicate.
 
#5
EDIT: weight: ~1575g all up~ 500 grams all up
CG is right where specified - the apex of the undercambered wing.
thrust angle is perfectly in line with power pod, which is perfectly in line with the sides of the fuselage and the flat top of the nose/"engine compartment". I've heard of people making very slight adjustments to this on planes the size of the Old Fogey, but nothing big enough to keep the plane from being willing to stay in the air.
Control surfaces - I've added a link to a gallery of some photos - you should see that the control surfaces are perfectly aligned (they don't currently need any trim because I adjusted them using the linkage stoppers to be perfectly lined up when the receiver is powered on and the servos are in their neutral positions).

Photos (ignore the crooked landing gear - those things just can't handle landings of any sort).
http://photos.petergaultney.com/Old-Fogey/n-SsVrRL/
 
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kdobson83

Well-known member
#6
My guess is your motors are not giving you the thrust they are supposed to be giving. I fly mine on a tiny 24gram hextronic motor, the blue wonder equivalent. On 3s 800mah battery's and a 8045 prop, it's fly's nice. No vertical but it'll climb nicely at 3/4 throttle. It flew better with out the landing gear and huge tires I put on it but I like the look and ability to take off and land in grass.
Are your numbers on the prop facing forward? Have you tried any other motors other than those two?
 
#7
Oh, and I do have a watt meter. I didn't write anything down the last time I tested, so I'll need to re-run specific combos if anyone would be interested in sanity checking the numbers, but off the top of my head, depending on prop, motor, and voltage combinations, I've seen anywhere from 4 amps to about 12 amps.

I have noticed that the 25g motor (which is pretty much exactly on spec with what the Speed Build Kit calls for), when running on a 3S, doesn't pull as much power as you'd expect past about 50% throttle for precisely the reason (overheating) that Hai-Lee is suggesting.

I'm not sure how to calibrate the ESC range. I have a DX7s if that matters.
 
#8
My guess is your motors are not giving you the thrust they are supposed to be giving. I fly mine on a tiny 24gram hextronic motor, the blue wonder equivalent. On 3s 800mah battery's and a 8045 prop, it's fly's nice. No vertical but it'll climb nicely at 3/4 throttle. It flew better with out the landing gear and huge tires I put on it but I like the look and ability to take off and land in grass.
Are your numbers on the prop facing forward? Have you tried any other motors other than those two?
I agree that this seems likely, but how to confirm? I do have a watt meter. What kind of wattage would I expect to see on a 2 cell battery with a 1400kv 25g motor and 9x4.7 prop? My understanding is that I'm looking for 1400x8=11,200 rpm, and I've used various calculators on the internet to try to estimate the wattage, but I haven't been able to feel certain that I'm understanding them.

Similarly, what wattage should I be looking for, for a 1000kv 50g motor, same prop, at 12V?

Oh, and yes, I'm certain that the prop is facing forward. It does pull itself forward convincingly, just not (I am thinking) as well as it ought.

I really appreciate everyone's thoughts so far.
 

kdobson83

Well-known member
#9
I'm not good with watts and what not, but grams if thrust in my opinion is a better number to work with. If u have a scale, build a thrust tester and test exactly what your stuff is putting out. There are plenty of DIY thrust tests out there. Worst case, buy a new motor. My Hextronic 24 gram was $10. The FT B pack motor which does twice the thrust as my hextronic is only $15.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#10
Calibration is simple.

Turn on Tx with Rx not powered. Set throttle to max.

Power up Rx

When "beeping starts from plane reduce throttle to minimum.

When all beeping ceases check throttle range.
 
#11
I'll try the calibration thing. For what it's worth, I just did two simple tests with two different ESCs. In both cases, I was able to go from ~5 watts at min throttle all the way up to 110+ watts near max throttle, measured by the wattmeter (on a 3S lipo).

Doesn't 100 watts seem like it would be enough to fly this plane? Maybe not. I'll try the calibration.

Oh, and I did build a simple homemade thrust tester a few days ago. I was able to get about 300 grams of thrust if I remember correctly. Which doesn't seem like enough for a plane weighing ---1575--- (correction - ) 500 grams. I should test this again and confirm.
 
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Hai-Lee

Old and Bold RC PILOT
Mentor
#12
Does your plane have an undercarriage?

If it has try to take off from ground. If it cannot you definitely require more thrust.

300 grams seems very little. The problem just measuring the power used is that the motor uses power in two ways, to generate thrust and to generate heat. If you cannot tell how much of each then the power used is only a rough comparison guide and not an accurate indication of performance.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#13
As for the weight Vs thrust/power used there is an old ratio which I remember and use as a definite guide. For general flying a power to weight ration of 100 Watts per pound.

My Spitfires weigh in at around 600 to 800 grams and I use motors of 200Watt which give good solid flight performance but nothing spectacular.

1575 grams is around 3 pounds so I would consider 300 Watt as a desired minimum. Fit a SF prop if you want more sedate flying.
 
#14
Does your plane have an undercarriage?

If it has try to take off from ground. If it cannot you definitely require more thrust.

300 grams seems very little. The problem just measuring the power used is that the motor uses power in two ways, to generate thrust and to generate heat. If you cannot tell how much of each then the power used is only a rough comparison guide and not an accurate indication of performance.
Yes, it has an undercarriage, and no, it can't take off from the ground. Which is definitely another point in the column of - not enough thrust, but why??

And the point about wattage is well taken. I'll try to re-assemble my very rudimentary thrust-measurement workbench and see if I can refresh my numbers. Is there a good rule of thumb for slow flyers in terms of thrust to weight ratio? Obviously I don't need a >1:1 ratio as I see mentioned quite often for quadcopters and 3D planes, but I don't know how much lower one can reasonably go.
 
#15
As for the weight Vs thrust/power used there is an old ratio which I remember and use as a definite guide. For general flying a power to weight ration of 100 Watts per pound.

My Spitfires weigh in at around 600 to 800 grams and I use motors of 200Watt which give good solid flight performance but nothing spectacular.

1575 grams is around 3 pounds so I would consider 300 Watt as a desired minimum. Fit a SF prop if you want more sedate flying.
Just to throw a wrench into all of this, I'm realizing now that you did the conversion that I've been lying about the weight this entire time! :eek: I've been quoting the weight for my Apprentice, whereas the Old Fogey actually weighs in at 1.1 pounds, or almost exactly 500 grams.

I'm going to go back and edit my previous posts so that anyone else that comes along later doesn't see totally ridiculous numbers for an Old Fogey and get confused.

In any case, it sounds to me like 100 watts would actually be about right according to your rule of thumb. Thanks for providing that!
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#16
The only plane I currently fly with a similar weight is my Ugly Stick and it uses a 280 Watt motor and swings a huge SF prop.

It flys slowly and can seem to hang on its prop at slow speeds, Flaps down, when the throttle is slammed open.
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#17
OK if power is no longer the issue I suggest that you seriously look at the wing profile and incidences.

Also the 100 watts per pound assumes you are taking off from smooth ground. Grassy fields, water and other such surfaces can have drag figures on the undercarriage which keep the plane from reaching take off speed. This addendum is pertinent to light weight models.
 
#18
The only plane I currently fly with a similar weight is my Ugly Stick and it uses a 280 Watt motor and swings a huge SF prop.

It flys slowly and can seem to hang on its prop at slow speeds, Flaps down, when the throttle is slammed open.
Yeah, unfortunately, as you'll see, I've been giving the wrong weight this whole time. I really thought I was giving the right one, but I wasn't. The true weight all up is 500 grams, so less than a third of what I was saying. Which would make 110 watts about right for this plane, especially with its high-lift undercambered wing.

All of this is making me feel like I'm just going crazy somehow, and that it should really be flying. I might take it out to a nearby park here in a bit and try it again, and see if I can get some video.
 
#19
OK if power is no longer the issue I suggest that you seriously look at the wing profile and incidences.

Also the 100 watts per pound assumes you are taking off from smooth ground. Grassy fields, water and other such surfaces can have drag figures on the undercarriage which keep the plane from reaching take off speed. This addendum is pertinent to light weight models.
In my case I've been hand launching, always into the wind (and only in winds of 5 mph or so). But this is really good information - thank you!

I really do think the wing profile should be right - the fuselage defines the angle of the wing quite specifically, because of how the wing shares its cambered shape with the top of the fuselage. And the wing, while undoubtedly not perfect, is definitely at least basically the right shape - we were very careful when we assembled it.

If the winds die down a bit, I'll take this out again and see if it won't fly now that I've changed the way (again) I'm mounting the prop, and now that I've definitely measured this exact motor and ESC and battery combo to draw over 110 watts at full throttle. I really feel like it ought to be flying, and maybe somehow my tests on Saturday and Sunday just were missing some strange component that I'll likely never understand.

I'll report back either way. :) And thanks once again for all of the advice and feedback and help!
 

Hai-Lee

Old and Bold RC PILOT
Mentor
#20
Glad to be of help?

Nothing worse than a plane that won't fly with the possible exception of a plane that only flys to crash every time!

Have fun!