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Posted by kenelder | Nov 12, 2019 @ 01:37 PM | 828 Views
Here is an old article concerning wing to tail incidence as well as biplane wing alignment. It is done in Q&A format.

Q: What is incidence?
A: Incidence usually refers to the angle of attack of the wing when compared to the horizontal stabilizer. For an airplane to be stable in pitch, the wing should have a small angle compared to the tail. The angle is called positive when the leading edge of the wing is higher than the trailing edge. The angle is negative when the leading edge is lower than the trailing edge. For aerobatics, the incidence angle is usually zero: the wing and tail are at the same angle of attack.

Q: What incidence angles should be used on a biplane?
A: Model biplanes are usually designed with incorrect incidence angles. It is typical for a model biplane, such as the Sterling Stearman kit, to have slight negative incidence in the forward (top) wing and quite a lot of positive incidence in the rearward (bottom) wing. The wings are not parallel to each other.

This causes extreme flight problems. When the rearward wing is at a higher angle of attack than the forward wing, the rearward wing will stall first. When the rear wing stalls, the airplane will pitch up rapidly and try to enter a spin. This was demonstrated recently at our field on the test flight of a new biplane. Also, at high speeds the wings fight each other and the plane tends to suddenly pitch up and down.

The correct incidence for a scale biplane is to have a slight positive angle of attack in the forward (top) wing compared to the bottom wing. This way the forward wing will stall first and the nose will drop to recover the airplane will be stable.

For an aerobatic biplane, the wings should be set at zero incidence so everything stalls at the same time.
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Posted by kenelder | Nov 08, 2019 @ 08:33 PM | 1,008 Views
We've all heard someone say it: "Nose heavy airplanes fly but tail heavy airplanes fly once". Whoever came up with that doesn't understand aircraft balance. Let's talk about aircraft balance first. An airplane in flight is a balance of forces and the trick is to make that balance stable. Part of those forces are aerodynamic and part are simply weight. All the forces have to balance out and keep the airplane in stable flight, and each airplane has a range of balance where it will fly. You usually see this as the Center of Gravity range (cg range) and most manufacturers' specify a most forward CG and a most rearward CG. If the manufacturer specifies the safe range then your airplane will be stable in flight.

What happens when you balance your plane at the manufacturer's forward cg point? Your plane should fly, but it is going to take some amount of "up" elevator trim to maintain level flight. All airplanes do this when balanced forward. It is not a good idea to try and fly a plane with the balance forward of the manufacturer's most forward cg point. The amount of UP trim needed will actually add to the airplane's wing loading- because up trim pushes the tail down and that down force adds to the weight of the plane. When you go very far forward, it takes a lot more downward push of the tail to keep the plane level. Even worse as you slow down for landing you will have to hold back stick on the elevator to keep the plane from diving. A...Continue Reading
Posted by kenelder | Sep 23, 2019 @ 03:37 PM | 1,110 Views
I've been reading the Eflite Havoc thread for a while and see that a lot of them are stalling at high speed. Yet when you watch the factory videos, those pilots instantly change direction without stalling. So there has to be a way to set up a Havoc to be able to do those high speed turns without stalling. And......there is!!!

A little non tech aerodynamics: When an airplane is in balanced level flight, the wing is producing lift and the airfoil is at a positive angle of attack. Also, the horizontal tail is forcing the wing to fly at that angle in order to stay level by pushing down on the fuselage. The lift needed to fly level is simply the weight of the airplane PLUS the downforce of the horizontal tail.

Here's an example: lets say your plane weighs 4 pounds and the horizontal tail has to push down 1 pound to maintain level flight. The wing has to lift 5 pounds total. In this example that extra pound of down force is 20% of the total lift!!! Now real world the down force is not that bad, but it can be significant. That's a lot of extra weight for the wing to carry, yet there is something that you can do about that. The down force is needed to counteract the CG which is usually ahead of what is called the neutral point. The further ahead, the more down force is necessary. Move the CG back and you reduce the down force. Even if you only reduce a few ounces of down force, you are doing the same thing as reducing your wing loading.

By purposely...Continue Reading
Posted by kenelder | Jun 08, 2016 @ 04:12 PM | 7,146 Views
For anyone interested the attached file is the instructions on how to switch hammer22's spektrum module to the various configurations.

Enjoy!
Posted by kenelder | Nov 28, 2014 @ 10:27 PM | 9,258 Views
Here is information and pictures of the takeoff dolly I use for the F104. This was all designed by the TLAR method (that looks about right) but some principles were considered.

1. Tricycle gear with wheels located close to the F104 scale positions.
2. Low cg: the plane needed to sit as low as possible. I've seen far too many dolly's trip over and spill the plane if they are too tall.
3. At least a 5 deg positive angle of attack. Most airfoils stall around 10 deg and not wanting to take off too slow, I figured 5 deg was about right.
4. Steerable nose wheel: I'm lazy and wanted to taxi out to the runway for takeoff.
5. Had to be easy and cheap to make- thus use of pvc pipe and fittings.
6. End of wing support had to be located right where the scale main wheels were- too long a support prevents the airplane from rotating during takeoff. I'm not so sure this was very important, but I was considering it.

The pics have captions explaining everything. It has worked amazingly well, great to see the plane taxi to the end of the runway turn around and then make a takeoff. After liftoff I give it just a little rudder so that by the time it stops it is off the runway so I can land without worrying about hitting it.

I hope this design helps all those that want to make a takeoff dolly that works!...Continue Reading
Posted by kenelder | Nov 20, 2014 @ 05:05 PM | 8,441 Views
Here are some pictures of my scratch built F104. It is built from plans of a ducted fan version originally designed in Europe somewhere. The plans were reduced 3/4 size to fit a 90mm fan. I have modified the original design; the airfoils have been changed to Selig model airfoils. Original plan airfoils were 10% thick symmetrical section and after watching a friends' model with that airfoil I decided something with more lift would work a lot better. Used Profili, the airfoil software, to compare the original symmetrical section to various other sections and ended up with the Selig foils. Lift coefficient was 2.5 times the original section at the root, 3 times the original at the root.

There were other changes such as moving the elevator servo to the tail, but those were all minor. Other than the airfoils my model is exactly like the plans at 3/4 size. Fuselage is 48" long.

I have not weighed it recently but it carries a 6 cell 2700 battery, 100 amp ICE speed control, and a Wemotec midifan powered by a Mega motor.

I also used my own design takeoff dolly. I did not want to risk a bungee launch withe the new airfoils and all the work put into it. Much to my surprise, the dolly worked perfect from the very start. As I like to taxi onto the runway, the dolly includes its own receiver and nosewheel steering servo.

The F104 lands on its belly on a long strip of fan belt rubber glued to its bottom. This works great as a brake- although it lands relatively fast, it stops in about 10 feet. This lets me feather it down to the runway without worrying about running off the end.

The color scheme is from Darryl Greenamayer's low altitude speed record holding F104 back in the '60s or '70s.