Why are all the flying wings that I see on the market either tapered, swept, or delta? Can you use a straight wing for a flying wing design??? I'd like to make a ducted fan tie fighter with half of a crashed 4 star wing; do you think it's possible?

Thanks,
Chris

You can use a straight chord wing as a flying wing. Al Backstrom did this more than 50 years ago with his full scale Plank sailplane. The trailing edge does need to have a reflex section either by rib profile or by a turned up trailing edge which can also incorporate the elevons.

The tips are back far from the CG, functioning like the tail does in a regular configuration: pushing down.
Swept back also acts like dihedral.

Yes, but it would be less pitch stable and the airfoil must be reflexed.

Only if you add a tail, I believe.

how much reflex would be correct...I have a lifting body aircraft, a facetmobile, and the tailing edge of the elevon matches the angle of the body in the rear 1/3 of the fuselage, would that be about correct for what i want to do?

chris

Reflex is mainly needed if the plane is destined to be overweight and slow. If the plane is light enough and fast enough, the slight amount [almost none] of elevon up trim will fly the plane.
If you build your [novelty] plane with a flat bottom airfoil, a little bit of up trim is all you'll need. Bear in mind that the wingloading and CG need to be correct. Don't discount too much the amount that the portions of a plane like you allude to contribute to overall lift and must be factored somwhow into the CG location.
I would build a down and dirty version of your model for beginners and bring glue, lead and tape out to the field to get the prototype sorted out.....if you are building a "unique" model with a difficult to calculate CG.
The lighter you build it, the less critical the CG will be.
Don't confuse a lack of vertical area aft of the CG for just a CG problem. They can imitate each other at the launch.
Sweep is done for coolness factor, but it does help the designer get the fins back where they are needed without resorting to long booms and lots of structure. Sweep also does a little better than a straight wing when the flight is over and you land in the hay. For combat, sweep is no good, the streamers are more prone to sliding off.

The more rearward the CG the less reflex you need. Reflex acts like decalage on a conventional plane.

If you build a small hand-launch glider that's exactly to scale, you can experiment with different CG locations and amounts of reflex. This will give you a very good idea of how the full-size version will work out.

thanks for all the info. I'll give it a try

chris

A point to keep in mind with plank style (ie:no sweep) flying wings is that the CG must never go back beyond the 25% chord point. If it does and if it's by more than a couple of percent the wing will be unstable in the extreme. The only exception to this is the pizza box flat wings that were popular for a while recently. On those the literally square shape added enough vortex flow that it confused the model into flying in a decently OK sort of stable way. But even those were generally only flown at post stall angles due to their tendency to "trip" over their leading edge and tumble if flown "normally" in an unstalled condition when the speed built. A normal CG position range for a plank wing is in the 18 to 22% range. And even that 22% figure needs to be approached carefully.

Back in the 1970s, Bill Evans designed the "Simitar" series, both slope gliders and power planes. These were flying wings
with tapered wings and a straight leading edge, giving a median forward sweep. They flew well.

http://www.youtube.com/watch?v=JCXq-so68rs