Thread Tools
Mar 10, 2018, 11:22 PM
Registered User
Sushmit's Avatar
Help!

Need help with my pusher plane.


Hi there I made a pusher plane and I balanced it according to the Cg but when I through it at full throttle it nose dives. So I shifted Cg to very bottom of the wing so It glides at full throttle but as I reduce throttle it Flips back since Cg is very back. Cg angle is 0
Last edited by Sushmit; Mar 10, 2018 at 11:35 PM.
Sign up now
to remove ads between posts
Mar 10, 2018, 11:35 PM
If it flies - I want one!
Petem's Avatar
Hi Sushmit,
A basic principle of trimming is to set the centre of gravity (balance point) and the angle between wing and tail correctly for a good power-off glide, then try with gradually increasing power and adjust the thrust line of the motor for good behaviour under power.
So, go back to a good balance point (typically around one third of the wing chord back from leading edge) and trim the elevator for a smooth glide from a hand launch - no power.
All setup trimming after that should be with the thrust line of the motor, so if the aircraft dives under power you may need to tilt the thrust line up.
Or you may just need to hold some up elevator on launch until the aircraft gains speed.
Some pictures of your setup and details of your radio system would help.
Regards,
PeteM
Mar 10, 2018, 11:39 PM
Registered User
Sushmit's Avatar
Quote:
Originally Posted by Petem
Hi Sushmit,
A basic principle of trimming is to set the centre of gravity (balance point) and the angle between wing and tail correctly for a good power-off glide, then try with gradually increasing power and adjust the thrust line of the motor for good behaviour under power.
So, go back to a good balance point (typically around one third of the wing chord back from leading edge) and trim the elevator for a smooth glide from a hand launch - no power.
All setup trimming after that should be with the thrust line of the motor, so if the aircraft dives under power you may need to tilt the thrust line up.
Or you may just need to hold some up elevator on launch until the aircraft gains speed.
Some pictures of your setup and details of your radio system would help.
Regards,
PeteM
Uploaded
Mar 11, 2018, 12:06 AM
Registered User
Sushmit's Avatar
Added about 5 thrust angle
Last edited by Sushmit; Mar 11, 2018 at 12:55 AM.
Mar 11, 2018, 08:54 AM
Registered User
Sushmit's Avatar
Since I was applying force parallel to wing it created moment as force applied was parallel to wing but when we apply force perpendicular to wing (or near it) it will reduce moment and the reaction force produced by vertical component will rise the plane and vertical component will push the plane
Mar 11, 2018, 10:28 AM
An itch?. Scratch build.
eflightray's Avatar
Is there some hidden weight in the nose. ?

Looking at the placement of the radio gear, battery, ESC, and motor, I can't see how you got the CG, (balance) correct without extra weight in the nose.

The picture shows where the model should balance , or slightly forward of there to start with, (25% to 33% of wing chord).

.
Mar 11, 2018, 10:34 AM
Registered User
Sushmit's Avatar
Actual after hit and trial is in the pic
Mar 11, 2018, 10:38 AM
An itch?. Scratch build.
eflightray's Avatar
I see the battery is now in front of the wing.


.
Mar 11, 2018, 10:39 AM
Registered User
Sushmit's Avatar
Quote:
Originally Posted by eflightray
I see the battery is now in front of the wing.


.
Yes. I gave some angle to the motor so it glides well on power but still that spark is missing
Mar 11, 2018, 10:50 AM
An itch?. Scratch build.
eflightray's Avatar
Difficult to see, but have you got the propeller on the correct way round ?

Imagine the prop was on the front, (and correct rotation), then imagine sliding it down a long shaft to the rear position.
The propeller should not be turned around between front or rear mounting.

.
Mar 11, 2018, 11:42 AM
Scratch building addict
rotagen's Avatar
Quote:
Originally Posted by Sushmit
Hi there I made a pusher plane and I balanced it according to the Cg but when I through it at full throttle it nose dives. So I shifted Cg to very bottom of the wing so It glides at full throttle but as I reduce throttle it Flips back since Cg is very back. Cg angle is 0
Nose dives at full throttle but I'm assuming flies OK at lower speeds? You have a wing incidence issue You most likely have zero incidence or god forbid a negative incidence angle (front of wing too low relative to horizontal stabilizer) or a trimming issue (you need more up-elevator). Best incidence in my experience is around 1 to 1.2 degrees (front of wing pointing upwards relative to stabilizer). Or if it's a wing (no tail) - you need more reflex in your elevons (point them up slightly). Don't mess with cg.
Mar 12, 2018, 02:22 AM
Registered User
Sushmit's Avatar
Made a new fuselage with force passing through the center of gravity which is 1/3
Last edited by Sushmit; Mar 12, 2018 at 03:37 AM.
Mar 12, 2018, 05:39 AM
Registered User
Sushmit's Avatar
Any Guess?
Mar 12, 2018, 06:35 AM
Registered User
Extreme Sports's Avatar
Looks like you have negative decalage. Decalage is the angle between the mean wing chord and the horizontal stabilizer (or second wing in a biplane). See my black lines on your photo. It is often confused with the angle of incidence, but let's not get sidetracked by semantics.

You want the angle between the wing and the HS to be zero or slightly positive - as shown by the green line. Why? Because the way you have it now, the HS has a slight amount of down elevator built in, so as the plane speeds up, it will nose down, then speed up some more, so nose down even more...i.e. an unstable system. Make the decalage positive (or just add some up elevator trim), and the system becomes stable.

The motor angle may also be a problem - google images of planes like the Bixler and similar. Their motors have a ridiculous amount of down angle built in to counteract the rotating moment of having a motor above the horizontal CG.

Be warned though. These high wing pusher planes can be hard to trim well. The combination of the high thrust line and propwash so close to the tail means that they often need to be re-trimmed for every throttle change. But, as someone has pointed out, don't confuse the CG position with in-flight trimming. CG is positioned at 25%-30% of the chord not to 'balance' the plane but to determine its STABILITY. The change in balance is just a part of the stability dynamic. Further forward = more stable, further back = less stable. This is why your plane became hard to fly when you moved the CG too far back. So set the CG in the 25% - 30% range and leave it there. Then trim the elevator to give a level and stable glide (i.e. plane will nose up slightly as glide speed increases, and nose down slightly if glide speed drops). Now see what happens as you add and remove throttle - and fix any undesirable behaviour by changing the motor thrust angle accordingly.

The other thing I would check is how much flexibility there is in your tail boom. I see you have a CF strip, but I bet the boom still has a lot of up and down flexibility. Unless it is rigid, you really don't have any proper control, and everything I've just said about stability and trimming is meaningless.
Mar 12, 2018, 08:36 AM
Registered User
Sushmit's Avatar
Well went for flight this time it flew pretty well but I had to give full elevator but throttle related diving was negligible. I will shift the battery back so it will fly much better


Quick Reply
Message:

Thread Tools

Similar Threads
Category Thread Thread Starter Forum Replies Last Post
Help! Need help with my pusher wing Sushmit Pusher Prop Jet Models 13 Feb 23, 2018 10:14 PM
Help! Need help with my homemade plane! Sushmit 3D Foamies 11 Nov 04, 2017 08:58 PM
Help! I need help finishing building my fpv plane. Interator Foamies (Scratchbuilt) 3 Jan 01, 2017 05:37 PM
Help! Need help with my first ever plane!! Devinator0363 Electric Plane Talk 7 Nov 30, 2016 03:31 AM
Discussion need help setting up my 250 quad with Graupner mz 12 with a mz 18 flight controller ABWQuads Graupner OPENHOBBY 10 Jan 15, 2016 02:08 PM