Building a fwd swept wing design but I have concerns over the directional stability.

I foresee the main wing/canard blanketing the rudder, is this an issue, any advice on sizing & placement? I foresee rudder effectiveness being impaired by the wing at low speed high AOA any advice?

Thanks in advance
HO-229

do you have a drawing or pic to show to give us something to go on?

I have attached a file of the concept. no aircraft built at this time for obvious reasons.
I believe what is drawn is correct in theory but feel free to comment if I am not understanding things correctly

In the July 1947 MAN there is a discussion of free flight swept-forward flying wings. The recommendation was for a rudder equal to 15% of the wing area, and a sub rudder with 5 % of the wing area. I have plans for a rubber powered free flight and a towline glider which do not have the subrudder. The glider doesn't have that large a rudder, and needed special rigging to overcome instability in tow.

Here is alink with pictures of the XFG-1
http://modelbouwforum.nl/phpBB2/view...53dbbee98305ae

I would speculate that the prop being a pusher would aid the directional control of the rudder, anyone care to comment on a figure of merit?

I wouldn't say a pusher config would aid in rudder control. Prop wash from a tractor config would make the rudder more effective at times of high angles of attack. If you are using a pusher, a sub fin and rudder would be a good idea I would think. You may have diminished control over the rudder but your elevator and ailerons should remain responsive. I think you should be ok. What is your canard area versus wing area look like? The canard looks a little small from just eye-balling the pic unless you are making it to where the entire canard fin rotates.

The attached image is nothing more than a sketched that outlines/defines the separation/interactions of a forward swept wing.

There are several books & online articles that outline the basic proportions & locations of the wing/canard for the attached sketch.

The one thing I have not found is rudder area/design

If you could be so kind please explain how the tractor prop config. Is a better choice for fwd swept wings/canard.

In a tractor config, the prop wash travels over the rudder ensuring some airflow. However, it will be more of a challenge to balance out your model correctly if the motor is in front. In my opinion, it isn't worth the effort to place the motor in the nose. At high AoA, aileron and elevator are more critical. I really think your design is fine as you've laid it out. I'll dig in my books for rudder area proportions, but the 15%-20% that Jim mentioned sounds about right.

Not to be argumentative but I do not see a tractor configuration as advantageous…
My reasoning is as follows:
a.) the separation/turbulent flow amplitude cause & effect of the canard/main wing coupled with turbulent flow of the propeller wash is a big question mark.
I would have grave concerns about deep stalling utilizing a tractor prop
b.) I would speculate the pusher configuration will gather the turbulent flow & direct it across the rudder giving an effective rudder control increase @ high AOA

But the question still remains, how big

Please feel free to correct me if I have misspoken or my understanding is incorrect.

I see the point you are getting at. However, I feel that the airflow behind the prop would be "recharged" allowing the canard to operate more normally at a high AoA versus if it were entering clear air. That same charged air would also flow over the rudder as well. If the motor weigh was a hypothetical "zero" then I would put it in the nose. The only reason I wouldnt is because of the possible difficulties in getting it to balance out correctly. I think you are right about the turbulent airflow gathering and passing over the rudder in a pusher config, but I still feel like the airflow from a tractor config would be more energetic even after blowing past the canard and fuselage. It's really just an academic argument because the pusher config is the best option for your application all things considered. I've got a book by a guy named Lennon that will have the answer you're looking for. I'm going to dig though it tonight and report back to you.

My buddy, who builds, but does not fly any more-he's 85, and I are on our 8th canard, one of which was an SFW. Several years ago I also built 3 conventional SFW planes.

The conventional designs fly very well. They are good in cross winds. I did use a slightly larger vertical area. Normally, I would use 10%-12%, but went to 15% and had no problems.

Two of our canards are tractors, one with the engine in front and the other is a twin with the engines on the wings. We did convert the tractor canard to a SFW design for a try. I managed to flip it on take off and my buddy didn't want to fix the wing so we never actually flew it other than a 3D type tight loop.

There are 2 problems with a tractor, SFW canard. First, the prop blast is directly on the elevator. This gives you very sensitive pitch control. You have to watch it on take off unless you are slightly nose heavy. Second, sweeping the wings forward effectively shortens the plane. You end up with a very short coupled plane with the engine blowing directly on the elevator. In addition, I think the plane was balanced more to the tail than the straight wing version. Since I have flown 3D planes with the CG way back, I felt I could handle a half inch to the rear. I got a big lesson in not messing with the CG Gods. I rolled down the runway, put in my normal amount of up for take off and it did. But it kept on going and filpped around. I nearly made it back to flat, but managed to get a wing tip.

For a canard, find yourself a good CG program and balance it conservatively. I have one if you need it. I would also figure out how far you put the wing from the tail in a straight wing layout, then find the MAC of the SFW and adjust the fuselage length to match this.

Ed,
You nailed a few of the concerns I had.

a) The effective increase in pitch authority/lift caused by the proximity of the powerplant in relation to the canard.
This effect has the ability to cause delayed canard stall (not good).

If you don't mind commenting, what was the flight like with the undersized rudder, I would speculate it was a dog at low speed/high AOA

May i sugjest you install two fins on the bottom of the fuselage, they will help keep the nose of your plane heading strait and will realy speed up roll/spin recovery

Also, when you say "high AOA" do you mean well past stall angle? if the main wing has not stalled yet the airflow will "stick" to the top of the wing and flow past the rudder without a problem

Having never flown a canard equipped aircraft I do not know what AOA can be achieved with good directional stability.

I have read that 45+ deg AOA is not uncommon with real aircraft.

Yes… concerning your stall AOA question & that ties back to the proximity of the powerplant to the airframe/flight surface.
Just as the tractor powerplant can delay stall at the canard I would speculate the pusher has the ability to delay/reattach/redirect flow for a lack of better words the turbulent flow on the main wing.

I would speculate your suggestion of installing a rudder below the wing would be somewhat helpful

The twin tail plane was the one with the fins too small. This is when I learned that 2 fins fairly close together are like biplane wings without a wide spacing, they aren't as effective. I calculated the size of the fins & rudders and split it in two.

The plane took off OK, but slid sideways in turns rather than tracking around. When doing a stall turn, you could do a 360, turn, although you could also get into a flat spin. In level flight at half power, you could do a tight, flat spin/turn. It would swap ends with full rudder.

We added 1 1/2" to the top of the fins and an inch to the leading edge and also added end plates to the fins. This cured all the problems.

Deep stalling isn't a common thing in model designs except on very special layouts like deltas or extremley low aspect ratio designs. In your case I strongly think you're worried about something that won't happen. By the time it fully stalls the model will be flopping around from the loss of lift and not from the loss of directional control.

Our models can't achieve the higher angles of attack that the full sized stuff can. The Reynolds number effect limits it. We're lucky if we can get up to around 8 to 10 degrees before it stalls.

I also think you're right in that the pusher prop would suck in a lot of the turbulent airflow at lower speeds and it should help out a little. Also the prop disc DOES add to the fin area but I'm not sure by how much.

What you do is build an airplane with a too big rudder. You test fly it, cutting the rudder down until it becomes unstable and crashes. Then you build another one with the rudder the right size.

Bruce,
Thank you everyone for the wonderful feedback.

8-10 deg AOA is awful conservative, I am currently flying several pusher configurations that have to be close to 40+ degrees… granted they have extremely large elevons (oversize) & to achieve these high AOA's requires some power & control finesse. If it's windy that always helps to…


From first hand experience (as you stated), I know delta wings have the ability to achieve extremely high AOA's (greater than 45 deg) with the proper application of power and control so my question is a valid one.

The 8 - 10 is for more or less normal wings of course. And nothing prevents the idea of post stalled maneuvering. 3D models are doing that all over the place. If you accept that a "stall" is where a separation bubble has no re-attachment then I think you'll find that the 8 to 10 degree rule applies. Having a big engine or motor to keep the model flying in a post stall region hides a lot of evils.... If it was a glider then it would stop flying and fall.

Ed,
What did you start with for your static margin? I am modeling/utilizing 20% is this to high/to low?

Being this is a new planform for me, I am ok with a higher static margin, but I do not wish to have a unresponsive airplane at low speed.

Any info would be of great help.

I normally balance 30%-33% back on the MAC.

If you like, I have a CG calculator I wrote that runs on the PC. Send me your e-mail and I'll send it to you.

If you need a readily available CG locator application that figures static margin, try http://www.geistware.com/rcmodeling/cg_super_calc.htm

It's as available as the internet and works really well. And to find out the location for any static margin you wish, simply type it in.

Let me address your question from a different perspective, however.
What makes an airplane unresponsive that pertains to the static margin, isn't what makes an airplane more or less stable. It's sorta related, but only a little.

When you move the CG way forward, the horizontal tail has more work to do simply from having to balance the weight shift. And it uses up some elevator deflection carrying that load. What it uses up simply carrying the "imbalance" is no longer available for pilot requests to pitch the airplane. So the CG shift is actually reducing the effectiveness of the elevator. How do you solve the problem when you elevator is not effective enough? Give the elevator more throw. Up to a point that works.

One thing a lot of us interpret as effectiveness and interpret as stability is how much our model pitches in response to our stick movement. Unfortunately, what a lot miss is that there are two parts to that equation. We almost always interpret any difference in pitch response as being a change in the stability of the airplane. And we usually haven't bothered to re-rig the elevator throw after changing the CG.

We often move the CG forward, feel the longer stick movement that results, and pontificate, "yes, I certainly made it more stable with that new CG, ain't it boys". Or we move the CG back, instantly feel quicker pitch response from our original stick movement and assume the airplane is less stable. What we're feeling in the TX stick is a more or less effective elevator. If we had followed the CG movement with the appropriate change to the servo arm or elevator horn connection points, we'd have understood what we'd actually accomplished.

I've found 20% static margin to be very safe for flying most anything because it takes the elevator effectiveness out where it's power probably can't stall the wing easily, if at all. But I've found that much margin isn't needed or wanted for any airplane after the trim is found and neutral set and deflection amounts roughed in. I usually use the larger static margin to safely work out where I want the elevator neutral trim and get a feel for whether or not the deflection is enough to stall the wing the way I want. And I might find out that the elevator isn't moving much at all to get the stall characteristics I want, and then the sucker gets re-rigged for sure. And then the CG gets tuned back, knowing that the elevator isn't going to feel too quick or too sluggish with the CG movement.

BTW, I've talked a lot about pitch because static margin really isn't a major player in the rudder effectiveness envelope. It does play, but very little. Whereas, if you're talking about static margin, you're REALLY talking about pitch for sure.

Da Rock,
Thank you for the great explanation. I must apologize… I should have started a new thread for this question.

The freeware that I am utilizing to model this design (the same freeware that is in the aero basics link asks for the static margin).

I modeled the design with a 20% static margin, but I may increase the margin because the design is utilizing a horizontal stabilizer vise an elevator & Ed has pointed out he is using 30+
I would speculate the stab has more authority than a elevator & I will have to be very mindful of my throws

I am very in tune with the subject of wing/tail loading. As everything in aero... nothing is free.
You have a gift in they way you write. Very easy to understand the subject being conversed.

I am truly excited to try this design.

Not sure what to expect in performance but I'll soon find out. I am hoping the aircraft has excellent pitch & roll authority.

After I get my feet wet with this airplane, I plan to start playing with the loading/balance to make it a hot potato but I need a good foundation to build from.

I have a few delta I fly that I know for a fact that are barely forward of the neutral point & I truly love flying them as they can pleasure to fly. Very unstable, but fun

Regards.