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rocketdriver

KR2 eweight and balance

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I've just answered a pm with regard to weight and balance of these aeroplanes and thought it worth repeating just a part of my answer for a wider audience ....

 

Q "I've heard of KR2s being prone to balancing problems when people build them, is that true? "

 

 

A I think that is probably true for all aeroplanes ..... 19 - 7814 is very nicely balanced in that we are virtually care free when loading, just limited in the amount of fuel when solo (no more than about 57 litres for me on my own .. .. tank holds 65 litres) for CoG reasons. This balance has been achieved by moving equipment around, in our case the battery was moved forward to between the passengers legs ... a little restrictive for them getting in and out, but not uncomfortable once seated.

 

On the other hand, the KR2 we have not yet worked on (ROK) will need a little attention in this respect. It has flown, but I am not satisfied that the CoG location when loaded is optimum.

 

Both of these aeroplanes have the fuel tank in the originally designed position, just behind the firewall. Some later build aircraft have wing tanks installed instead. This minimises rearward CG movement as fuel is used up but does require some additional weight up front to compensate .... a bigger / heavier engine or the nose needs extending a little so the engine weight has more leverage.

 

Hope this clears up some of the rumours that tend to fly around!

 

 

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Any aircraft with the fuel tank in that location will have a fair change of CofG as the fuel load decreases. It has to balance empty.This would be the most rearward condition of the Cof G.

 

IF with some loading situation, full fuel moved the CofG too far forward, you would have to limit your fuel load to stay legal.( in balance).You can't use fuel as ballast to stay in balance because the situation of using all the fuel still has to be covered. Nothing wrong if it is understood. With wing tanks, they can be very close to the Cof G so the contents only affect the weight of the plane, and not the balance.

 

If the Cof g gets too far back the situation is more dangerous than if the plane is nose heavy. You can tailplane stall a tail heavy plane, but nose heavy you run out of elevator effectiveness as you slow up, so you can't slow up any more, But the plane doesn't stall, as it will with the other situation. Nev

 

 

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Hi Nev ... yes you are absolutely right. One or two up, the a/c MUST balance wirth zero fuel, because one day thats where you might be ...... 19 - 7814 does this easily with a bit of rearward CG still available. This is desirable with the KR2 as the last 2 inches of rearward CG make the aeroplane much more sensitive in pitch (so it is said ...not been there myself).

 

cheers

 

RD

 

 

  • Haha 1

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The effect on the rudder and elevator function would be that the tail moment arm is reduced, so the turning/pitching effect IS less. Of more concern is that, the plane can become unstable in pitch. Most aircraft are balanced so there is some download provided by the elevators and horizontal stabiliser (in combination).ie they are nose heavy. This adds to the load the wings must carry, because lift required then equals weight + elevator downforce. so there is more drag (induced) than if for instance the tailplane provided positive lift. This is a performance penalty. The advantage is that the plane is more pitch stable. If the speed increases, the nose will rise, and conversely if it decreases the nose falls. This is the normal thing we all experience in our S&L flying at varying speeds.

 

If the tailplane is carrying it's share of the weight, or a bit more, you can get a situation where a speed increase will give proportionately more lift from the tail and the plane will enter a dive, becoming gradually steeper. At slow speeds the tail may likely stall before the mainplanes, causing a pitch up which forward stick will only make worse. Nev

 

 

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The KR2 C/G in Aust was limited to 30% but it can go to 33% rear and 15% forward, If I owned a KR 2 i would reduce the elev.. movement as it is geared to high (like the pulsar)this would feel a lot better.

 

 

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reducing Elev movement can be done by your hand. Making the Horz. Stab. larger is the key I found to keep the plane from pitch sensitivity and that's on a stock KR-2 not the S

 

 

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