That's interesting.

The tip airfoil is symmetrical. Since it will never see a positive angle of attack that makes sense.

One could argue that conventional aircraft also have their tail (stab) always at a negative angle of attack relative to the main wing and use symmetrical airfoils there too. The angle between them isn't 10 degrees though.

It's an interesting design.

Mike

Have you launched it with out power and just glide test it?..you have pretty large prop dia..

Originally Posted by turbonut

Have you launched it with out power and just glide test it?..you have pretty large prop dia..

Nothing to grab onto on the bottom so hand launching would be difficult. The prop pitch is 6, would have to get the plane a fast hand launch to get the elevons to work.
Could not get 3/8" surgical tubing to pull 25 lbs, it would snap even using multiple cords. The plane is 5 lbs, so to get 25 lb pull I had to use to use 1/2" tubing
https://www.mcmaster.com/5234K65

Hey,

Just stumbled across this thread, thought I might be able to help out. There are many erroneous assumptions here.

Spanwise flow is not unique to an aircraft with bell shaped lift distribution such as this. Unless the wings are swept forward, conventional wings experience this as well. This is precisely why winglets were invented in the first place, and also why you also see fences on certain design like the Mig 15. So the magic here is nothing to do with spanwise flow, and everything to do with the non elliptical twist. Don't mistake this twist for washout either, totally different animal.

Fins/winglets etc. are not necessary, you are complicating things by adding them. There is a very good chance that they are hurting more than helping. I'm not sure why you just won't try it as designed. I've never had one divergent in yaw at all.

Very hard to tell whats going on from your video, but suspect a couple things. Your wing loading is a bit on the high side. This plane was designed with a Cl of .6 if I remember correctly. Yours being heavy means it has to fly faster to achieve that. The video to me looks like you are simple flying too slowly. Couple this with the torque of your pusher prop and it could get ugly. You are correct no differential, equal throws both ways. No mixing at all, just simple two channel control.

Also, to hotwire cut one of this is not possible, even with a CNC without special fixtures. Check put post #413 in this thread: https://www.rcgroups.com/forums/show...andtl-d/page28 where it explained. Probably not the most clear description so if you have any questions please feel free to ask.

Red

Originally Posted by p901P901

It has a aspect ratio of 16.6:1.
This is a 2.6m wing and a 627 sq in wing area, loading is 18.4. oz/sq.ft and weighs 80 oz. The Albatros 2.4S/E has a wing area of 846 sq.in and a loading of 9-10 oz/sq.ft, weight is 52-58 oz.. So my plane is heavy in comparison. But I do have a much larger motor and battery installed.

Also, if you are using this as your CG, you are waaaay tail heavy. Start out at 12% and move it aft as you get comfortable. Traditional MAC calculations are not applicable to P wings because it is a straight geometric calculation based on area and an elliptical lift distribution. Since this wing does not use an elliptical lift distribution, the wing generates more lift towards the center, and less at the tip, it drives the CG forward. I know 12% seems awfully far forward, but it will be very nearly in trim there. I suspect that this is your main issue.

Red

Originally Posted by Alien_Tech

Also, if you are using this as your CG, you are waaaay tail heavy. Start out at 12% and move it aft as you get comfortable. Traditional MAC calculations are not applicable to P wings because it is a straight geometric calculation based on area and an elliptical lift distribution. Since this wing does not use an elliptical lift distribution, the wing generates more lift towards the center, and less at the tip, it drives the CG forward. I know 12% seems awfully far forward, but it will be very nearly in trim there. I suspect that this is your main issue.

Red

Red would be the guy to listen to, as he probably has the most RC PRANDTL stick time of anyone on the planet. In this video from 2015, he can be seen at about 1 minute laying down the ground rules for the test flight, and then flying the model until about the two minute mark.

NASA Armstrong PRANDTL-D Interns, Summer 2015 (5 min 48 sec)

He has worked very closely with Al Bowers on this program over the years.

Kind regards,

Stan

Thanks Stan:
Met Red several times at MMM but never really talked about his work. I also know Derek, another team member.

Red
So adding tip fins was just a confidence booster to get the plane flying. They would be removed later. The 2nd flight CG was closer to 12% - 15% because I wanted a larger static margin for better pitch control.
My only other thoughts about control problems was receiver faults, but many ground test proved this was not a problem.

So there has to be some type of stall problems due to the tip fins. Will have to try again without the tip fins, pull the plane with 30 lbs instead of 25 lbs and not roll the plane until speed picks up. Will be at a different location also.

Thanks

Originally Posted by p901P901

Thanks Stan:
Met Red several times at MMM but never really talked about his work. I also know Derek, another team member.

Red
So adding tip fins was just a confidence booster to get the plane flying. They would be removed later. The 2nd flight CG was closer to 12% - 15% because I wanted a larger static margin for better pitch control.
My only other thoughts about control problems was receiver faults, but many ground test proved this was not a problem.

So there has to be some type of stall problems due to the tip fins. Will have to try again without the tip fins, pull the plane with 30 lbs instead of 25 lbs and not roll the plane until speed picks up. Will be at a different location also.

Thanks

15% is too far aft. The range is very narrow. 12% is the number. Larger static margin is less pitch authority, not more. Not sure what you are thinking there. Speed off launch isn’t your problem, you will still have roll control post stall as evidenced by the wind tunnel data and my own experience.

Red

Yes screwed that up about more control with more static margin.
Having a hard time understanding "roll control post stall".

Originally Posted by p901P901

Yes screwed that up about more control with more static margin.
Having a hard time understanding "roll control post stall".

It means the airflow is still attached, and you have roll authority during and after a stall. You don't need a 6g launch to get airborn, your thinking is flawed there. Put the CG in EXACTLY the right spot and try again. With the CG aft, the proverse yaw also goes away. Thinking about it futher, your video exhibits that characteristic as well. It crashed well after leaving the bungee.

Red

Originally Posted by Alien_Tech

It means the airflow is still attached, and you have roll authority during and after a stall. You don't need a 6g launch to get airborn, your thinking is flawed there. Put the CG in EXACTLY the right spot and try again. With the CG aft, the proverse yaw also goes away. Thinking about it futher, your video exhibits that characteristic as well. It crashed well after leaving the bungee.

Red

"Put the CG in EXACTLY the right spot and try again." --- Will do exactly that.

Thanks again and take care

Confused a bit on flying wings with bodies and if my calculations are wrong for including the body.
I assume I want CG 5% ahead of AC?
Inertia is unknown, do not know how critical it is for this design.

Looked at the Z location of AC in a side view and noticed that the thrust line is below.

Originally Posted by p901P901

Confused a bit on flying wings with bodies and if my calculations are wrong for including the body.
I assume I want CG 5% ahead of AC?
Inertia is unknown, do not know how critical it is for this design.

I would ignore the fuselage for CG calcs for now. Thrust line should point through the Z CG for minimum coupling, but I wouldn’t change it at this point.

Red

Compared the MAC line with and without the body, results attached.