A flying wing pusher airplane design

[David Lewis]

Hello all, This is a two-place airplane I've designed. It will use a 3-axis artificial stabilization system. The wing airfoil is not reflexed. The horizontal stabilizer provides the down force to counteract section pitching moment. I welcome your comments.

[Bill Berson]

I would be concerned about the horizontal tail down load creating wing tip positive twist couple. Could cause tip stall or worse.

[David Lewis]

Thank you for pointing that out, Bill. The wing would need to be stiff in torsion.

[Birdmann]

hey Dave,
Flying wings are trickey, Historically there have been several of them but they didnt do too well in the market.Before spending alot of money on this make sure you study past models. Personaly i like them but i dont want to own one,
Jeff

[Bill Berson]

Takeoff can be tricky. Read the Sport Aviation archives for "Dean Delta". A fatal crash on first takeoff. The thrust on takeoff pushed the nose down. It wouldn't rotate with no prop blast over the tail. But when the pilot aborted at high speed, the lack of nose down thrust then caused a pitch up and it went out of control.

[David Lewis]

I appreciate your excellent input, Bill and Birdman! All important points. Two features that make an airplane suitable for low time pilots is plenty of stability, and low stalling speed. Two problems with flying wings is lack of stability, and the reflexed airfoil which reduces lift coefficient. I've been impressed, however, with the model radio control flying wings I've seen, which are steady and easy to fly. They use a gyroscopic stabilization system adapted from quadcopters (drones).

[Bill Berson]

Are these models not reflexed?
I have flown some of these powered wings brought out by others. I don't know if they have artificial stability. They were all hand launched with no wheels so the problems of takeoff was not a factor.

[David Lewis]

I assume the airfoil would have to be reflexed on these model wings to decrease center of pressure travel but have not examined it up close. One disadvantage of a conventional flying wing is that up elevon is needed to flare for landing. This changes the shape of the wing in an unfavorable way just when maximum lift is needed. Hence I mounted the elevons on horizontal stabilizers. I can further increase the lift coefficient by moving the CG back because it reduces stabilizer down force. The elevator servo would need to be very fast in such a system because of the short period of pitch oscillations.

All the flying wings, and most of the conventional fixed wing model airplanes I've seen use artificial 3-axis stabilization. It's becoming almost a standard feature.

[Bill Berson]

I doubt these models have abandoned natural stability. They likely still have a normal positive stability margin.
But I will do some inspection at the RC field.
Operating a full size aircraft without natural stability is beyond my comfort zone.

[FlyingRon]

First off, you do not have a flying wing. A flying wing is a design without a prominent fuselage. You don't even appear to have a "tail-less" aircraft. Just a novel design to hang the tail off the wing tips.

What I see of challenges here:

As mentioned, the structure needed to support the yaw and pitch movements way out on the end of the wings like that (there's good reason that they normally attach elevators to the fuselage).

The pitch control is going to be interesting. It appears you have the CG way ahead of the center of lift. This is going to mean you're going to have to generate a lot of downforce with your "elevator winglets" to keep the nose horizontal. This is going to decrease efficiency as well.

Configuations like this often have some fun with aerodynamic things like flutter due to the flexibility of the components and the fact that you've got parts of them likely in the slipstream of the pusher.

[David Lewis]

I doubt these models have abandoned natural stability. They likely still have a normal positive stability margin.That's correct. They possess inherent stability. If you turn off the artificial stability, unlike a quadcopter, they will keep on flying (albeit requiring more pilot skill and attention).

The most common digital stability augmentation system is the Spektrum AS3X (Automatic Stability 3-Axis), which has rate gyros and a heading hold gyro. It also has a separate function called SAFE (Sensor Assisted Flight Envelope) protection. This adds self-leveling and roll & pitch limiting.

[David Lewis]

It appears you have the CG way ahead of the center of lift.The dashed line on the wing depicts the mean aerodynamic chord. My intention is to have the CG a short distance ahead of the center of lift, and increase horizontal tail volume and electronic stability augmentation to compensate for the small static margin.

[Birdmann]

Dave i can feel for your interest for these tings, i do too, so many RC planes are flying wings.
In the subject of safety and usability, why not use a canard like the Velocity or Berkuit, they are essentially flying wings with an elevator stablizer in the front.
These are already developed and are very safe and useable. If you want to get more bang for your buck this is the way to go, It sometimes takes many years to develope
a new model like this, you are risking your money and your life.
jeff

[David Lewis]

Thanks Jeff, Ron and Bill for valuable advice and suggestions. I never understood the advantage of canards over ordinary (tail-in-the-back) configurations. Can you enlighten me?

[Frank Giger]

Very cool that you're thinking outside of the box - at the minimum you will learn what won't work, which is just as valuable as knowing what will.

A question:


The horizontal stabilizer provides the down force to counteract section pitching moment.

I'm not exactly sure what you're driving at here, but you may have just answered your own question in regards to canards (other than canards up front are out of the way of prop blast streams of air and reduce drag, along with other things more educated folks will share).

You could, if you wanted to make things simpler in one way and more complex in another, ditch it entirely. Have the ailerons do double duty, as it were.

I think your sweep on the wings may be a tad aggressive. The overall shape of the design does not imply great speed.

[Birdmann]

Dave here is a video on flying wings

https://www.youtube.com/watch?v=dUCFVXqondU&feature=youtu.be

Horizontal stabilizer on the tail of an airplane produces negative loads to balance the airplane on the wings. It changes with flight speed.
A canard has similar function but uses positive loads. The velocity is a well developed airplane, carrys 4 people at reasonble high speed, reasonably easy to build and very safe.
My favorite airplane. It will be useable when you are done, you wont have to risk your life and money like . just my opinion
Jeff

[David Lewis]

The overall shape of the design does not imply great speed.That's true, Frank, this is a maximum fun sport airplane. I went with a helicopter style canopy design. The cabin is roomy, which would make it too slow to serve as practical transportation. It's a high lift coefficient and low wing loading airplane. The ability to fly slowly is good for touring and sightseeing, and short field performance.

[Dana]

What you're showing is not a flying wing, it's a short coupled conventional design with very small (almost certainly too small) tail volume. Placing the pilot and passenger so far forward means a dramatic CG shift when flying solo or with a passenger... note that most successful designs have the payload (people, fuel, baggage) close to the CG. As drawn, the CG will be very far forward, probably well beyond the ability of the elevator to provide enough downforce for a landing flare.

As others have pointed out, your basic configuration is much more suited to being a canard, then it would be a canard of more or less conventional proportions.

[martymayes]

I never understood the advantage of canards over ordinary (tail-in-the-back) configurations. Can you enlighten me?

There really are none because as you point out, the stabilizing surface does the same thing whether it's in front or in the back. A canard on your design will block out a large chunk of the visibility you are seeking. Now, three flying surfaces..........that has possibilities.

[Dana]

There really are none because as you point out, the stabilizing surface does the same thing whether it's in front or in the back. A canard on your design will block out a large chunk of the visibility you are seeking. Now, three flying surfaces..........that has possibilities.

In theory a canard can be more efficient because the forward stabilizing surface is lifting, decreasing the load the wing must carry, while an aft stabilizer exerts downforce, increasing the load the wing must carry. In practice, the forward stabilizer of a canard must be more highly loaded to insure manageable stall behavior, causing extra trim drag and negating much of the advantage.

[rwanttaja]

In theory a canard can be more efficient because the forward stabilizing surface is lifting, decreasing the load the wing must carry, while an aft stabilizer exerts downforce, increasing the load the wing must carry. In practice, the forward stabilizer of a canard must be more highly loaded to insure manageable stall behavior, causing extra trim drag and negating much of the advantage.
Years ago, a college student wrote a program to optimize the canard size. The final result was that the canard should be five times larger than the wing.....

Ron "Think About It" Wanttaja

[martymayes]

In theory a canard can be more efficient because the forward stabilizing surface is lifting, decreasing the load the wing must carry, while an aft stabilizer exerts downforce, increasing the load the wing must carry. In practice, the forward stabilizer of a canard must be more highly loaded to insure manageable stall behavior, causing extra trim drag and negating much of the advantage.

Any perceived advantage is offset by not one but several disadvantages. One designer seemed to have worked the pro/con formula out to perfection but I notice the industry didn't follow which they would have had there been any real advantage to a canard configuration. In fact, there would have be a very rapid shift to canard airplanes had there been a worthwhile improvement in efficiency.

[Jim Heffelfinger]

check this out............ http://www.marskeaircraft.com/

[Jim Heffelfinger]

First off, you do not have a flying wing. A flying wing is a design without a prominent fuselage. You don't even appear to have a "tail-less" aircraft. Just a novel design to hang the tail off the wing tips.

What I see of challenges here:

As mentioned, the structure needed to support the yaw and pitch movements way out on the end of the wings like that (there's good reason that they normally attach elevators to the fuselage).

The pitch control is going to be interesting. It appears you have the CG way ahead of the center of lift. This is going to mean you're going to have to generate a lot of downforce with your "elevator winglets" to keep the nose horizontal. This is going to decrease efficiency as well.

Configuations like this often have some fun with aerodynamic things like flutter due to the flexibility of the components and the fact that you've got parts of them likely in the slipstream of the pusher.


Ron - are you thinking of a blended wing ? http://www.marskeaircraft.com/

[FlyingRon]

He doesn't have that either.

[tmnolte]

Takeoff can be tricky. Read the Sport Aviation archives for "Dean Delta". A fatal crash on first takeoff. The thrust on takeoff pushed the nose down. It wouldn't rotate with no prop blast over the tail. But when the pilot aborted at high speed, the lack of nose down thrust then caused a pitch up and it went out of control.

Bill - I found one of the articles from 1963 but couldn't find anything else. Not to divert the thread but it is interesting given the level of effort put into designing and building the plane that he would have has such a misunderstanding of it's handling characteristics, doesn't it?

[Bill Berson]

Well, test pilots can't think of everything. That's why they call them test pilots.
I was a test pilot of my own design. It pitched up unexpectedly to 12 feet on a high speed test run. I had no intention of flight but it caught me unexpectedly. It was tail heavy and I was planning to add more nose weight the next day before the first flight.

[tmnolte]

@Bill - hope i didn't imply anything derogatory about the designer/pilot by the way. But you're correct...unexpected events can happen to the best of us.

[David Lewis]

Thank you to all members who have offered valuable advice and suggestions. I have incorporated all the ideas into the revised design.