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Phil May25/05/2014 13:52:13
1520 forum posts
154 photos

Afternoon all.

I've just acquired an Algerbra 100 flat field / thermal soarer which is in mint condition.


I have no idea what the c of g is for this model and also what would be a good starting point for elevator and rudder throw.

Does anyone have any ideas where to start.

Thanks all


cymaz25/05/2014 14:26:24
9004 forum posts
1187 photos

This might help

Stuart Eggerton25/05/2014 15:17:11
352 forum posts
302 photos

Hi Phil, I have recently acquired a plan for the Algebra 1000 glider, which has a 3 metre span and is the larger brother of the Algebra 100 2.5 metre span). There is a post for the algebra 1000 glider here which shows the cg in mm. My plan has the same information.

Chord of wing section measured directly from my plan (for the 3m span Algebra) is 232mm. So the cg will be between 81mm to 87mm from the leading edge or 35% to 37.5% wing chord from the leading edge of the wing. I don't know if this could be translated to the smaller algebra 100, but don't see why not?


Edited By Stuart Eggerton on 25/05/2014 15:17:55

Phil May03/06/2014 15:14:48
1520 forum posts
154 photos

Thanks guys, sorry I've been slow getting back to you.

I've been doing a fair bit of research on this model and I'm a little confused.


My model is 100" span with veneered foam wings with approx 8cm dihedral at wing tip, fuselage is flat sided and ply this an EMP Algerbra 100 ? I'm not sure.


It had it's maiden on the weekend off a slope in a gentle breeze . I managed to climb enough to give it a dive test which showed nose heavy ( c of g set at 90 mm)


It was porposing when I held level flight, I thought this meant tail heavy...please correct me if wrong.

It is my first rudder/elevator model and I found that it was diving slightly during turns.

Todsy I was bunggee launching off a flat field and added a little up elevator mix with rudder, this only induced a stalling effect while turning. So I changed this to a little down mix and this just, as expected, pushed the nose down gaing speed and losing height quickly.


Where am I going wrong, any ideas guys.







Edited By Phil May on 03/06/2014 15:16:36

Erfolg03/06/2014 19:43:03
11637 forum posts
1294 photos

It is good to see the old Algebra (EMP) still doing service. As for your comments with respect to construction, you will find threads that outline the EMP Algebra. Yours is one example of quite a varity of constructional methods and wing sections that were produced by EMP.

With respect to turning, it is not unusual for a glider to drop its nose when turning, if no up elevator is fead in.

I am not a great advocate of the "dive test", although it can work for many.

As you suggest my experience of porpoising would suggest a (slightly) tail heavy model. Much further back and the model becomes really difficult to fly, as the porpoising, is more like stalling, recovering, and so on, with you always behind the model.

Conversely if nose heavy, the model becomes less lively, in that it needs a positive input to change its heading etc. To much further forward, it again becomes difficult, in that it is stable in doing what it is doing, until, the forces or inputs make a change, where again it becomes stable in that condition. It is awkward. This only occurs when really nose heavy.

Although many say they like a rearward CG, I am not convinced. The reason is that what it is doing may have nothing to do with a thermal. Better in my opinion to be slightly forward of this almost unstable point. You can still see the movement from a thermal or sink, either deviating from straight ahead, or pitching up or down slightly.

Bungee launching is not popular in this era, winches being the favourite. Winches are expensive, so I can see the attraction of a bungee. The issue is, there are bungees and bungees. The ones that were sold by Bowmens, were non preferred, being shock cords, covered in a woven covering. These went solid when extended, yet released the power over a short distance, and power was less than other types. Many favoured a single solid rubber cord, mostly square in cross section, about 5/16" sq. For lighter models, Latex tubing was liked, this can be stretched massively. On top of this there is the monofilament to consider, we used sea angler stuff. Although little stretch, it would, usefully. Always doggy stake the end in the ground. What ever the claims, coming out of the ground it will never reach you, although it could get somebody else on the way. Or on a blowy day, a screw driver may come out of the ground and go floating off with everything.

I guess you will know that some elevator is normally needed, to maintain tension, to restretch the line and rubber. If windy enough for the model to go behind you on launch, then down is needed.

On poor bungee days, you may well resort to weaving across the sky, trying to kite the model the best you can.

Tom Satinet04/06/2014 15:57:49
519 forum posts

Is that Algebra "normal"? I though the Rudder/elevator version was a polyhedral design.

It looks a bit lacking in effective dihedral angle for a rudder /elevator model.

Steve Houghton 104/06/2014 17:10:05
1902 forum posts
128 photos

I use a bungee, it's more forgiving than a winch, and easier to carry when your flying area is a long way, and up a steep slope, from the car park. I have 30m of tubing and 60m of line and manage to get a good height as I'm stretching the tubing a good 100m too. With a little flap dialed in I get a good steep climb too, not that you can do this on that particular Algebra.


Erfolg04/06/2014 17:30:30
11637 forum posts
1294 photos


You have made me think, were we restricted to rudder elevator for 100s? I do remember that airbrakes were not allowed.

My Algebra is a 3m Selig model, it does use ailerons, and straight dihedral.

I must check how long my Bungee is, I do remember it needed a very big field.


I have included a picture of my bungee and towline. I would think that the rubber is no good any morecrying 2 I still have my winch in the Garden shed, I dread to think the condition that the Pb battery is in, Arrrgh!


Edited By Erfolg on 04/06/2014 17:31:09

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