I have a question that has been bothering me for quite some time. Someone once told me that if you want to build an indoor plane (electric slow-flight), it should only have the top of the wing covered and if you make an outdoor plane (gas), it should have the top and bottom covered. Why is this? I would think that if you didnt cover the bottom of the indoor fliers wing, then it would disturb the laminar flow in the front part of the wing earlier than it would be disturbed if it was covered on the top and bottom. Is it just for weightsaving reasons?

Here is two examples so you know what I am talking about.
Indoor - http://www2.towerhobbies.com/cgi-bin...?&I=LXBDF2&P=0
outdoor - http://www2.towerhobbies.com/cgi-bin...?&I=LXAP68&P=0

With very slow speed flight, the flow is laminar. Anything much below a Reynolds number of 20k. Above that, to the upper hundred thousands, the airflow can be turbulent, have hysteresis effects, seperation bubbles.
And indoor planes really don't care about drag.. look at all the bracing strands on the really light microfilm planes.
A "single surface wing" is better at that slow flight.
Outdoors, drag does become important, and the simple single surface can't perform well.
When properly used, most anything works. When it's misapplied, there will be better choices.

To add to Paul's answer (which is spot on IME)...

Properly applied the design in question should not have lots of structure in the way. If you look at models that are designed for single surface type airfoils the leading and trailing edges ARE the spars and only a few ribs break the bottom surface. Any spars used in this method should be flat and in contact with the covering. I've seen models that use rib and spar designs that are meant for two sided covering only being single covered and that, to me, IS a mistake. Even at the lower speeds you're still making a lot of drag compared to the thrust power often used on this sort of slow flying model. If you want to do a single cover job the structure should be modified to suit so that the structural elements are snuggled up close to and in contact with the covering.

The weight is a larger penalty than the drag. (the definitive answer.)

You add the lower surface microfilm you add enough weight thatthe model's performance decreases more fromthe weight than it increases from the lack of drag.

How do I know? I built some and tried covering the bottom.

Awesome. That is what I was thinking. You just made it sound much better.

FH, it was shown in some very early windtunnel work on duration indoor models that the typical sticks at that speed did little to upset the laminar flow. The turbulence from the exposed leading and trailing edge "spars", if you can call anything that small spars , was so little and damped out so quickly at the speeds in question that covering the bottom was a waste of time. Just as you found out using the empiracal method.

Still, it's nice to know that you and Frank Zaic (IIRC) learned the same things...

I got this from a very old magazine article on indoor aerodynamics. It came out sometime back in the late 50's or 60's I think. Too many years ago to remember.

Nobody can tell me anything... I'm like the bumble bee... I gotta try even when someone says "That can't fly."

Amazing what strange stuff will actually fly. Its so easy to make stuff stay in the air... What the heck too 'em so long to do it?[&:] Engine technology... that's what. The Wright Brothers' ENGINE is what made thier airplane a success. Power to weight wins again.

The wright brothers were not successful just because of their engine. They practically reinvented the wing. Langley had a huge engine powering his craft, but as we all know, he was unsuccessful. Yes their engine did play an importaint role, but it was not the deciding factor.

> The Wright Brothers' ENGINE is what made thier airplane a success. <

Oh my, but you really have that quite wrong. Their engine was a minimal affair that just did the job and lost power as it warmed up. The Wright's learned how to fly using gliders, learned how to control an aircraft in three dimensions, corrected the erroneous tables of lift that everyone else had used, figured out that a propellor was a little wing going around in circles and put it all together to work.

He's not that wrong Dave. The Wrights certainly did their homework in so many of the areas you mention but there were a couple of other aircraft that had most of the factors before that and could have at least gotten airborne even if they may not have been succesful. The engine the Wrights developed was primitive but it did have one of the best power to weight ratios of it's day. Something that their competitors had failed to achieve.

The Wrights may have been the first to bring it all together (unless you listen to the Aussies or the Gustave Whitehead crew ) but I've occasionally wondered what would have happened in Lilienthal had had such an engine during his flying program. He had everything else even if it was only as a hang glider. A suitable small engine with enough power could easily have let his efforts take to the sky years before the Wrights.

Didn't mean to start a debate on why the Wrights succeeded.[&:]

Thier engine as mentioned had best power to weight of anything of the time.

The use of the reduction drive to thier VERY efficient propellers was a tremendous aid to thier power conversion into THRUST. (long slow turning propellors are VERY efficient) Again... a power to weight innovation that was crucial to thier success.

Even with thier wonderful engine and reduction drive (and counter-rotating propellers... which aided stability[8D]) They needed a Catapault launch into a certain minimum headwind to achieve flying speed... and that first engine didn't have the power to KEEP the plane in the air long. (they still had more drag than thrust...)

Thier wing design... well... I'd hate to see the way that wing would flutter if you put more power on it. Its a KITE wing. (thats what you get when you use kites instead of a wind tunnel for testing.)

A marvelous achievement... because they did manage to put everything together. (and because they realized that the thrust of the engine wasn't enough is why they built the catapault in the first place...)

Too true. They were amazing. Blending everything they learned into a working craft was awesome. They are some of the best engineers to ever walk the face of this earth.

> He had everything else even if it was only as a hang glider. <

Alas, he lacked any means to control laterally other than by shifting his body, and adding an engine would probably have been a dangerous combination. Remember, it was an unpowered glider that killed him.

The first Wright flyer did not use a catapault. They devised their catapault later for when they needed to fly under conditions where there was not a nice strong headwind.

Jim

Like Hoffman Prairie, a cow pasture just outside of Dayton OH where the brothers lived. The spot is now Wright Patterson Air Force Base

I'd say the reason the Wright brothers succeeded was not because of technology of any sort. It was their approach to the problem. Testing, testing and more testing. While other were strapping feathers to their arms and proving Darwin correct by jumping off cliffs, the Wrights were figuring it out in ways that wouldn't get them killed.

Personally, I think their accomplishments are extremely under-rated.

- Paul