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Help! CG got me puzzled?!

mayan

Well-known member
#1
Today I went out to maiden the last Howard DGA-6 that I built, trying to finalize the plans. The maiden was so so and has left me puzzled about my CG let me explain what happened during the flight because I don’t have it on video.

On this build I added something around the motor to make it look more realistic, basically added 35g of weight to the front of the plane. When I tried balancing it, it seemed to balance with a 100mah 3S battery way back closer to where I assumed the CG is. The plane unlike the first version I built was super hard to control. With anything above the 25% throttle the plane would do a crazy vertical climb, but on throttle completely off had a gentle glide slope which is something that on the first version was very steep.

All this has left me very puzzled and clueless. Any idea what’s happening?
 

Bricks

Well-known member
#4
Easist way to check thrust angle trim aircraft pull straight up give power and watch how the aircraft reacts if it pulls to the canopy then down thrust needs to be added, if pulls left then right thrust needs to be added. Keep making corrections to thrust angle until plane flies straight when going vertical.
 

Piotrsko

Well-known member
#5
Lot of my planes, full up just causes a stall.. On a howard, the thrust line is below the wing so more power tends to rotate the nose up. IIRC there's not a lot of elevator to compensate. You need some noticeable down thrust. As far as CG: generally more or less around the high point of the wing, OR, center of spar although on FT planes sometime that don't apply. Bigger horizontal stabilizer offsets cg placement.
 

Hai-Lee

Old and Bold RC PILOT
#7
Today I went out to maiden the last Howard DGA-6 that I built, trying to finalize the plans. The maiden was so so and has left me puzzled about my CG let me explain what happened during the flight because I don’t have it on video.

On this build I added something around the motor to make it look more realistic, basically added 35g of weight to the front of the plane. When I tried balancing it, it seemed to balance with a 100mah 3S battery way back closer to where I assumed the CG is. The plane unlike the first version I built was super hard to control. With anything above the 25% throttle the plane would do a crazy vertical climb, but on throttle completely off had a gentle glide slope which is something that on the first version was very steep.

All this has left me very puzzled and clueless. Any idea what’s happening?
Firstly may I say that the fact that it glides with a gentle shallow angle is a good thing! Record the CG point as well as the Wing and Tail incidence angles for inclusion with the design/plans.

As for the violent pitch up with throttle there are 2 things to consider. Firstly the thrust angles as mentioned by most other respondents to your OP. Secondly there is a trick if you have a flow through engine cowl which is large with respect to the Prop diameter. It is possible that the motor thrust is being ducted downwards causing an added force trying to lift the nose of the craft when power is applied. Often blocking up the cowl bottom so that no air exits the bottom of the cowl can alleviate that nasty issue.

Also ensure that the cowl angle is the same or slightly above the planes centre line.

If the addition was a Spinner check that the motor and hence spinner is mounted centrally within the cowl or the spinner could be directing more air into the bottom of the cowl than through the top and over the fuselage.

As a summary it appears to me that you have inadvertently altered either the incidence angles or the CG which has allowed it to glide so well and so now you have to make it so that the motor thrust is applied where it is best for level flight.

I hope it makes sense!

Have fun!
 

mayan

Well-known member
#8
Thanks for the tips
Defiantly could be thrust angle.
Could the horizontal stab or wing incidence be off? Climb under power and unexpected glide with no power.
Is the 100mah battery a typo or is this a very small plane?
Typo 1000mah 3S.

Thanks for all the tips! I am assuming that sloving the glide slope issue is a good thing and that now the only thing I have left to do is figure out the thrust angle issue and then I'll have a winner. Or am I missing something?
 

Arcfyre

Well-known member
#9
I'll just add my opinion here. You added 35g to the nose, but you also then moved the battery "way back". I think you may have accidentally flown in a tail heavy condition. Additional evidence is that you said you had improved glide performance, which makes sense. Often times a tail heavy plane will glide well because even with the motor off, the balance point being further back results in a pitching up moment.

I would not fly it again balanced the way it was this time. Instead, return the battery to the location of your previous iteration, and try again. I haven't built your model yet, but just by looking at it I wouldn't expect it to glide too well. I'd say the steep glide is more normal, and the shallow, "better" glide is a symptom of a tail heavy configuration.
 

mayan

Well-known member
#10
I'll just add my opinion here. You added 35g to the nose, but you also then moved the battery "way back". I think you may have accidentally flown in a tail heavy condition. Additional evidence is that you said you had improved glide performance, which makes sense. Often times a tail heavy plane will glide well because even with the motor off, the balance point being further back results in a pitching up moment.

I would not fly it again balanced the way it was this time. Instead, return the battery to the location of your previous iteration, and try again. I haven't built your model yet, but just by looking at it I wouldn't expect it to glide too well. I'd say the steep glide is more normal, and the shallow, "better" glide is a symptom of a tail heavy configuration.
I am super puzzled because when I tried balancing it, it actually had the nose more downward as if it was more nose heavy but then the flight flet like a constant fight for balance. I am lost!
 

Arcfyre

Well-known member
#11
I am super puzzled because when I tried balancing it, it actually had the nose more downward as if it was more nose heavy but then the flight flet like a constant fight for balance. I am lost!
In that case it's probably most likely a thrust angle (as was said) or incidence angle issue.

An edge case you could look at is an elevator problem. For example, did you maybe add a bunch of up trim by mistake? Did the pushrod move on the linkage stopper, giving you a bunch of accidental "up"? Do you have some weird mix enabled that adds up elevator when you add throttle? Unlikely, but hey. Can't hurt to check.
 

Merv

Well-known member
#12
Thanks for all the tips! I am assuming that sloving the glide slope issue is a good thing and that now the only thing I have left to do is figure out the thrust angle issue and then I'll have a winner. Or am I missing something?
This goes to the issue of trimming philosophy. I’m sure there will be different ideas on which philosophy is correct.

I would trim the plane to fly straight and level at full throttle. Then I would fly very high, cut the throttle and put the plane into a straight down dive. The goal is to be at to dive 150-200 feet straight down. If the plane quickly pulls out of the dive right side up, you need less down thrust. If the plane. If the plane pulls out bell side up, you need more down thrust.

Why it works, when you trim for straight and level at full throttle, you trim out any errors in thrust or incidents. When you dive without power, you remove the thrust and yet maintain good speed. This will allow you to see what was trimmed out.

When you change the thrust angle or wing incidents your trim will change accordingly. When you get it right the plane will fly well at any speed.
 

mayan

Well-known member
#13
In that case it's probably most likely a thrust angle (as was said) or incidence angle issue.

An edge case you could look at is an elevator problem. For example, did you maybe add a bunch of up trim by mistake? Did the pushrod move on the linkage stopper, giving you a bunch of accidental "up"? Do you have some weird mix enabled that adds up elevator when you add throttle? Unlikely, but hey. Can't hurt to check.
Nothing that I realize off the bat. I think I will try to add so thrust angles and check it. If the plane pitches up though what thrust angle would I add?

When you change the thrust angle or wing incidents your trim will change accordingly. When you get it right the plane will fly well at any speed.
That's what I am aiming for! I need to fix the things I broke yesterday and give it another go tomorrow morning.
 
#14
Nothing that I realize off the bat. I think I will try to add so thrust angles and check it. If the plane pitches up though what thrust angle would I add?


That's what I am aiming for! I need to fix the things I broke yesterday and give it another go tomorrow morning.
If your plane is pitching up you need some down thrust angle.
 

Tench745

Well-known member
#15
If your plane is pitching up you need some down thrust angle.
I believe the question being asked was more "How much thrust angle should I add," than "Which kind of thrust angle should I add?"

To answer that question: add just enough to solve your pitching problem; experimentation will be needed. Often a thin plywood shim or small washer under the motor mount may be enough to correct your thrust angle. Add some, check it, add or delete as necessary.
 

mayan

Well-known member
#16
I believe the question being asked was more "How much thrust angle should I add," than "Which kind of thrust angle should I add?"

To answer that question: add just enough to solve your pitching problem; experimentation will be needed. Often a thin plywood shim or small washer under the motor mount may be enough to correct your thrust angle. Add some, check it, add or delete as necessary.
I was actually asking about both so thank you both :).
 

Merv

Well-known member
#17
I was actually asking about both so thank you both :).
With the dive test I described, you get a feel for how much you need to change the thrust angle. If your plane pulls out in less than 50 feet, you need a large change 4 degrees plus. If it pulls out in 75-100 feet, you’ll need 2-3 degrees. If the it pulls out is 100-150, 1 degree or less.
 
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Bricks

Well-known member
#18
This goes to the issue of trimming philosophy. I’m sure there will be different ideas on which philosophy is correct.

I would trim the plane to fly straight and level at full throttle. Then I would fly very high, cut the throttle and put the plane into a straight down dive. The goal is to be at to dive 150-200 feet straight down. If the plane quickly pulls out of the dive right side up, you need less down thrust. If the plane. If the plane pulls out bell side up, you need more down thrust.

Why it works, when you trim for straight and level at full throttle, you trim out any errors in thrust or incidents. When you dive without power, you remove the thrust and yet maintain good speed. This will allow you to see what was trimmed out.

When you change the thrust angle or wing incidents your trim will change accordingly. When you get it right the plane will fly well at any speed.
I can see how this works for an underpowered plane but I do not own or fly a marginally powered plane, so I never considered checking thrust angle this way, great explanation, even my Cubs will fly straight up out of sight . :LOL: :cool:
 

mayan

Well-known member
#19
So I tested the things you all recommend and nothing really made a difference. What really strikes me is that it used to fly OK and then something changed and boom it flies faster but has that crazy pitch to it. I took it out yesterday for a few tests and made this video about it. It's long and has me talking on part of it, I would though appreciate if you had a look and tell me what you all think.

 

Arcfyre

Well-known member
#20
@mayan I watched the above video.

Unfortunately, it seems you were only able to capture 50% of the actual flying. Very often times you were out of frame, and it was difficult to observe the airplane. Maybe you can rig something with a hat cam for next time?

On to what I observed. In general, I think you are on the right track with the thrust angle. The first flight, looping without elevator input, is an obvious sign that the thrust angle is off. As you said in the video, it is a similar behavior to what you had with the simple soarer. After you changed the thrust angle, I feel like it flew noticeably better in regards to pitch response to throttle input.

Here is where I run in to issues with your troubleshooting methods. You are changing too many things at once and it makes it hard to see clear cause and effect relationships. By changing the thrust angle AND removing the cowling, you were making two changes simultaneously that kind of cancelled each other out. Granted, the cowl doesn't weigh much, but it is weight in front of the CG, and by removing it, you made the plane more tail heavy. So you'd have down thrust helping to keep the nose down, but less nose weight, pushing the nose up. See what I'm getting at? The down thrust would help with the pitch on power issue, but in general your plane will still be pitch sensitive due to balance being off. Additionally, after changing the thrust angle, make sure you reset your elevator trim to neutral. If you fly it with the old trim setting, it will give you a misleading indication of what the result of your change is.

On your second to last flight, you changed yet another parameter by having a different battery installed. I wasn't able to pick up if you changed the thrust angle again for that flight, but the battery came loose and the results from that flight are essentially invalid.

Based on very limited observation, I would say that your plane is suffering from three issues:

1. Your established C of G is incorrect. It needs to be moved forward to eliminate tail heaviness. You removed the cowl and the landing gear, both items that help move the balance point forward. I would add those back on to get the plane to balance better.

2. Your stock thrust angle is too high. Your second attempt had much better response to throttle (some pitch up under power for a high wing model is normal) I would leave it at "one washer down" like you had, and try to correct the CG before adjusting it more.

3. There is something wrong with the motor or prop installation. You are getting some nasty vibration above what sounds like 50% throttle. Make sure that when you put the washer on for down thrust, the motor is still securely attached to the firewall. You may need to use more or different washers to set this correct angle, but a motor that vibrates like that is not ok. It will drastically impact handling. Also check that the prop has no broken blades and that its centered on the prop adapter.

I know you're frustrated, but I think you are very close to having a nice flying airplane. Thrust angle down a bit, and move that CG forward.

And tape your phone to your forehead or something so I can actually see you fly. :ROFLMAO::LOL: