Question regarding the tailsitter

I am currently working on a Tail Sitter plane and I wanted to know few things. It would be really helpful to reach out a person who can help me.
My questions regarding the Tail Sitters are:

  1. Can a tail sitter have cambered or flat bottom airfoils? or is it mandatory to have a symmetrical or semi-symmetrical airfoils?
  2. Currently, I am using PX4 on mission planner and I am not able to takeoff in Q_Hover mode. I have done the basic settings for Frame type and Q-Enable etc. But, have not done any tuning.
    I am using a flat bottom airfoil so my aircraft is tipping to one side.
    Also in Q_Hover mode the motors does not respond to my stick input, they start spinning at 55% and increase till maximum even if I don’t throttle up. I am try to balance using Q_Trim_Pitch but that does not seem to be working for me.

I kindly request any help and suggestions regarding the Tail Sitter setup and tuning in Mission planner using PX4.

Thanks in advance,
Nihal Y.

There is topic dedicated to dual motor tailsitter in development team categorie. This might be the longest topic ever so you are lucky because there are all the information you need :wink:
If you dont want to read almost 2000 posts you can use the summarize button.
Post 1453 show my non vectored tailsitter with all construction details.
Did you read the Wiki about tailsitter, this is the point to start.

No I have not read anything on wiki about the Tail Sitters, will do that now thanks.

I read about the Tailsitter and also few of your posts on the Tail Sitter Blog. But I could not understand most of the things on the Tail Sitter setup.
But I did understand how the airfoils with -ve Cm cannot be used on tailsitters. But since I have already built one is there a way to some how sort things to get the Hover right? We have tested it for the forward flight and it was pretty stable. Now I have given the throttle input of the 2 motors to two separate channels so I am able to control it without tipping the aircraft. Will this setting of throttle_left and throttle_right be a problem for transition?
Also in Q_hover mode the motors are not responding to the stick input and it goes to maximum once a little throttle is given. Can you please help me to tune the throttle parameters for this problem?

Q_Hover is a altitude control mode. It is tying to increase its altitude so uses more and more throttle. For direct stick to throttle control use Qstabilize.

Yeah I understand that the Q_Hover mode is a Altitude control mode, but what I am worried is that instead of holding the altitude its trying to increase the altitude.

Also I would like to ask you a few questions out of curiosity sir,
As you said in Post 1453, “As I said in the former post, the idea is to get the CG at the neutral point of the wing section under the propeller wash and at the same time the CG in front of the quarter mean cord of the whole wing. There are two ways to achieve this objective: a normal swept wing with motors close to each other and a negative swept wing with motors as far as possible from the fuselage”.
A normal swept wing means a swept back wing right? So you suggest having sweep on the TailSitter for good stability. But when you consider Wingtra it does not have any sweep right sir? Please explain more on this.

Before being a tailsiter, it is a flying wing so positive Cm airfoil are more suitable. Nevertheless, I agree that provided the CG is right it can fly very stable. It will be just less efficient, I mean the ratio Cl/Cd will not be good.

It seems to me, this is the normal way but I don’t understand the rest of the sentence. Was it tipping around the plane yaw axis ?

Do not worry, as soon as sensors will detect altitude increase, throttle will be reduced.

This is a very good remark. I suppose they find their solution to get the CG close to wing NP and have a stable FW flight at the same time. Note they use high end fast digital servo not 10$ corona.

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If you look at the first wingtra version the wing had a straight leading edge and a forward trailing edge. Overall it gives a forward swept wing.
The question of static margin is interesting. If you want to reduce the static margin you may use a stable Cm airfoil. Pw51 is very nice for that. JWL065 I have used with success is of the same family. But maybe the best choice would be a symetric airfoil. I never tried. One thing nice with tailsiter, it is still an open field… and I would be happy to learn from your work.

I am very glad that I got to interact so much with you.
I meant the tail sitter is tipping because of the lift generated on the wings. That is made proper by using thrust vectoring.
I just want my tail sitter to hover because the forward flight was a stable.
So please help me with some suggestions to tune the copter mode of the Tail Sitter.

I will make sure I will use appropriate airfoils for the next version.

I don’t think so. Wing lift should be very small when hovering vertical and elevons deflection should have compensated. If your wing fly as FW, elevon deflection is already the right direction so I suppose elevons are too small or not enough deflection or gains too low (q_a_rat_pit_p is the one to begin with)
You said in an other post your wing does not have vectored thrust, I guess you are talking of differential thrust between R et L motors, this is how roll is controlled.
For the moment, as peter suggested, use q_stabilize instead and hold firmly your wing vertical (with EKF perfectly functional, that is very very important) then increase throttle and shake it a little to feel how it reacts. Do not make test indoor or with arming check disable and take care of your fingers.

Yes sir I will do it.
Oh yeah you are right sir I am using differential thrust and not the vectored thrust. So once I am able to keep the wings vertical, I will work on gains and try the Hover mode.


I am tuning Q_A_RAT parameters for my Tail Sitter. I don’t know where to start with. I dont want to crash my plane is there any PIDs gains to start with?
My tail sitter’s AUW=2kilograms and wingspan is about 1.3meters.

You have tether example in posts 705 and 1197 of tailsitter thread.
Post 1525 I write my tuning experience
q_a_rat_pit_ are gains to be tuned first. You can start with default parameters.
After reading the wiki, a good method is to read parameter description in the full parameter list (at least for q_parameter) and gives your brain a little time to compile. :slightly_smiling_face:

okay, I will do it thanks.

I did carry out few flight tests indoor with a tether in Q_Stabilize mode.
I have 4.2 kg of thrust and my tailsitter is 2kgs. Still my aircraft takes off at 75-90% throttle I don’t know why and also it was highly unstable and I need to tune my Q_A_RAT_PIT PIDs any values to start with?
Should I increase or decrease?

Throttle is not direct passthough there is some scaling and expo applied, this could be the reason your seeing more throttle than expected. As for tuneing recommend you follow the excellent guild on the wiki.

I am a little confused with these two parameters while setting up my tail sitter.
Q_OPTION: Not having clarity while checking bit-masks. The description in mission planner is confusing. Please let me know on what boxes to check for Tail Sitter transition.
Q_TAILSIT_MOTMX: Since we assign Throttle Right and Throttle Left to the motors, they are not controlled by any of the Motor 1 to Motor 8 parameter. But again the mission planner description says “Non-Zero indicates that the airframe is a tailsitter…”. This again causes a bit of confusion because for a non zero number I will have to tick Motor1 and Motor2, but the motors are actually controlled by Throttle Left and Throttle right. Please give me some suggestions on these two parameters.

Thanks in advance.

I don’t think any of the Q_OPTIONS are related to tailssiters.

Q_TAILSIT_MOTMX is for copter style tailsitters, it only does anything if your frame class is not set to tailsitter

Oh thanks for the confirmation.
I have set Q_FRAME_CLASS to tailsitter.
So changing the flight mode from Q_Stabilize or Q_Hover to any of the FW modes like stabilize or FBWA should work for the transition right?
I have not worked on any transition aircraft before so I am worried about how the transition will work.
Also please suggest me on a value to start with for the Q_TAILSIT_ANGLE. Right now its set to 60 degress. Should that be fine? OR should I increase or decrease.