Let's talk thrust lines - RC Groups
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Aug 09, 2012, 08:51 PM
Euphoric flights
GreenAce92_v2's Avatar

Let's talk thrust lines

OK so my aircraft is not normal, it does not have any vertical stabilizers rather relies on the high sweep angle as well as the Gull shape to fly straight. It is a flying wing.

I had no problems flying it with a 10 x 3.8 SF APC propeller but then when switching to a different higher rpm motor with a 7 x 5 APC E propeller, I had troubles. The aircraft now yaws and wants to turn so I needed to re-do all the trim.

I'm just curious why the yawing effect was so noticeable. Other gull wing models would use the same motor and not have a problem.

It could be the servos I am using / bad receiver / glitches if one of the 6 surfaces is twitching then I could see.

I was able to fly straight with the flaps down.

My reasoning about the thrust lines was the "denser, faster flow of air" from the smaller higher rpm propeller made any slight angles much more drastic than a larger slower less dense mass of airflow from the bigger propeller.

I streamlined this airplane today and I was curious if this may have had an addition to the odd behavior.

Usually I fly it as this ugly stick looking thing and I have no problems with directivity however when streamlining it seems that I lost some of the straight flying stability.

Is it possible that a turbulent central leading area followed by high sweep wings with smooth airflow leads to a straight flying aircraft? It doesn't seem like it would make sense. I mean, rudders are at the back of airplanes not the front, then again rudders aren't really meant to cause turbulence are they?

This is the craft.

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Aug 09, 2012, 11:00 PM
Snappy Title On Backorder....
tsymonds's Avatar
I think it all comes down to the smaller prop having less gyroscopic effect on stability. A perfect example was the Northrop YB-35 which was prop driven and required no vertical surfaces. When converted to a YB-49 with jet power four small vertical fins were added to recover the lost stability. My .02 worth.
Aug 09, 2012, 11:17 PM
Euphoric flights
GreenAce92_v2's Avatar
But the previous designs used this motor and had no problems. I was just surprised of the intense new trim

Thanks for the money ( haha )
Aug 10, 2012, 08:00 AM
Registered User
flypaper 2's Avatar
If it is gyroscopic stability, using a prop with a larger dia., lower pitch, on the same motor, would show the difference.

Aug 10, 2012, 11:25 AM
Night Flying
Ron H's Avatar
Streamlined? It looks a bit draggy to me. The slower combo might be better suited. If the CG is ok with the new motor, try shimming the rear of the motor up slightly.
Aug 10, 2012, 02:42 PM
Euphoric flights
GreenAce92_v2's Avatar
Sorry, all I meant by "streamlining" is I covered up the wholes. Stupid mistake since I starved the electronics of oxygen well airflow anyways.

Actually any attempts at streamlining this aircraft results in crashing, I find that it is best to keep the center area disturbed while leaving the wings free of disruption hence keeping longitudinal stability.

See below: Both attempts on models 1 and 16 for streamlining ended in crashes really weird aerodynamics if you ask me

It is really peculiar, when the aircraft has a "streamlined" pod , in flight the aircraft performs sudden snap rolls, loses longitudinal stability ( predictability of the aircraft in flight ) and will act like a tailed aircraft that lost half of its body.

So the solution is to design it like a brick with sticks.
Aug 10, 2012, 03:24 PM
eye4wings's Avatar
Okay, my take on the problem has everything to do with propeller pitch.
Changing to a higher revving motor naturally meant changing the prop used - quite right BUT you said you have a faster running motor AND you've INCREASED the prop pitch rather than decreasing it as you should to get the same airspeed. With a conventional model rudder trim will usually have to be altered to counter the yaw effect of any prop pitch change, but you have no fin or rudder area. Consequently what has happened is that the yaw effect of the prop now is too much for the side area (gull wing and stubby fuselage sides only) of your model.

How about telling us what the old and new motor kvs are so we can get a better idea of the effect of the changes you made?
If the increase in kv is about 40 to 50% I would have thought a prop nearer to 9x3 would have been nearer the mark.

The troubles you had with the shorter fuselage versions were probably similar but like coming at the problem from the other way. Less fuselage side area to counter the yaw effect.
Aug 10, 2012, 07:01 PM
Euphoric flights
GreenAce92_v2's Avatar
Read and die from boredom

Here are some various films regarding different airframes

From the first model to the 16th model the main changes have been control surface, wing tips, wing angles and internal structure construction plus extended wing mounting.

Anyway, when approaching high speed the Gull Wing was susceptible to vibration and destructive oscillation. So changes had to be made.

Every design is different in one form or another. Sorry to have such a terrible set of examples there is no control however I will try and do some testing of my own with the construction of the 17th model which will be fully fiberglassed so should withstand speed above 70mph ( dive ).

Here is the first model with a Park 480 1020kv with a trimmed 12 x 6 APC E down to 4-6" diameter

A sunny day for some fun (2 min 38 sec)

Here it is with a 10 x 7 E in a windy day 20-30mph wind and 30+ gusts no problem at all tracking

APC 10x7E on the Park 480 aboard the Gull Wing (1 min 33 sec)

Here is the 6th model which primarily improved the control surfaces fixing adverse yaw

This one used a BP Hobbies 1500 kV motor on an 8" propeller though trimmed to about 6"

GT-06 Proof of concept [ 6 control surfaces ] (4 min 5 sec)

In this "film" I show both the GT-08 and GT-09, both use the AXI 2808/16 1800 kV difference being the 08 was meant for wing modularity ( attach anything to the wing and it will fly ) and GT-09 was a Hotliner design, consequently one used a 7x5 APC E and the latter a 6 x 5.5 APC E

Progress report of Gull Wing Flying Wing " Blue Fin " (4 min 39 sec)

This is the 14th model with the BP Hobbies 1500 kV motor with a full sized GWS DD 8x4

Launching a Blue Fin and some windy flying with a lightly loaded Blue Fin ( 23oz ) (1 min 31 sec)

This is the current model that I have been flying, GT-16 with a 10 x 3.8 SF propeller, this film was with the flaps down hence I was able to film it myself. The flaps add directional stability, helped me a lot when transitioning from a Park 480 to the AXI

GT-16, doing slow circles at 400' (0 min 44 sec)

Just as a note, the aircraft lost a lot of its "Gull" characteristics as a result of more stability, you see, the more anhedral there is, the harder the airplane is to fly. When I had the middle models ( 7-9 ) they used fully symmetrical wings without any washout as well as both dihedral and anhedral. So consequently they were hard to fly, you had to be damn good at flying. The other problem with the g-wing design is orientation, I burrowed the bottom stripes idea for the new paint scheme from glider pilots.

Yes this aircraft went through rigorous testing for "advanced" fpv piloting ( Hahaha laughing at myself )

I apologize for the crude-ness of my films, having a crappy computer and friends that show interest in airplanes out of kindness doesn't get you very far in desired results. I also apologize for some of the language
Aug 10, 2012, 09:15 PM
Registered User
flypaper 2's Avatar
That uncontrolled yawing oscilations is why they dropped the YB49 as a bomber. It couldn't make a straight bombing run because of the yawing motion.
Just for an experiment, how about a reversed gullwing like the Corsair, but in your flying wing configuration. Should be more stable going from the anhedral to dihedral.

Aug 10, 2012, 10:10 PM
Night Flying
Ron H's Avatar
When asking Q's in the power system forum, don't neglect information such as trimmed down prop. Trimming a prop normally won't reduce amp draw much. Thrust is lost by cutting the tips off, the only gain is it will fit the plane.
Aug 11, 2012, 12:19 AM
Euphoric flights
GreenAce92_v2's Avatar
I did try a W-wing, you need to redesign the airfoils ( I simply flipped the aircraft over and reversed the channels ) it seemed to "sink" rather than "hang" I think that is the primary difference between a W- and an M configuration.

It's ok. The aircraft flies straight it was simply the change from a large propeller trim to a small diameter trim. What I mean by trim is what I need to trim in order to keep it flying straight, not the propeller trim.

Anyway, sorry to waste your time
Aug 11, 2012, 07:18 AM
Registered User
flypaper 2's Avatar
Not wasteing anyones time at all. I get a great kick out of shooting the breeze with you guys, especially about experimenting with stuff. The guys out at the field get a kick out of the stuff I bring out to play with. Half the time the experiments don't work, but the ones that do are well worth the hassles. Here's one that flew but didn't work to well. Check out the vectored thrust beanie.

Aug 12, 2012, 01:37 AM
Euphoric flights
GreenAce92_v2's Avatar
He's a pretty happy guy for having a motor stuck through his head haha. That's a neat looking creation there. Nice paint job. So all you could do was control pitch?
Aug 12, 2012, 04:24 AM
Closed Account
Sorry, I hated the Unicorn yaw even with the little vertical stabilizers.

Why not have a vertical stabilizer? If it is for FPV, you are mixing two tasks together. Yaw un-stabilised aircraft with steady video. Why would you do that?
Last edited by eagle777; Aug 12, 2012 at 04:43 AM.
Aug 12, 2012, 06:35 AM
Registered User
flypaper 2's Avatar
That was the first version of the Flyguy with elevons for legs. The feet swivel back for landing and faceplants. Later one had the arms swivel for ailerons and the shoes swivel for rudders. He did take off vertically once which was the purpose of the big shoes. I may build a bigger version. Here's another experiment for a reversable, varaible pitch prop. The prop didn't work very well but the plane was a geat sucess. Sort of a slotted prop Pizza Box.


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