R/C KK Invader question

[Jeffrey Hall]

I have recently built a KeilKraft (Ripmax) Invader with 2 channel r/c. I flirted with twin rudders but was never happy with the outcome so I ended up with a single fin (and yes I can hear the boos).

With radio fitted it needed 35gm of ballast to get the recommended cg. First test glides showed it to be nose heavy. There was zero decalage so I altered it to about 3 degrees (lowered leading edge of tailplane). Still dived. Next I remove the ballast, just leaving the radio gear. Now it glides nicely but the cg is at 2/3 chord.

It seems that something is not right somewhere. I would like to launch from a high start but am wondering if it would loop off the top with this setup.

Any suggestions gratefully received

Jeff

[Simon Chaddock]

Jeff

I presume yours is the original 40" span version..

According to the plan there is about 4 degrees total decalage. Remember that although the wing is flat bottomed that is not the incidence chord line which for such a section is about 3 degrees positive. The symmetrical section tail plane appears to be mounted on the fuselage with a degree or two negative compared to the wing under side.

If you have added 3 more degrees and still have a CofG at 2/3 chord suggests something is not at all right. A serious twist in the wing?

Also bear in mind that with a single spar wing structure it is intended that the CofG is pretty close to the spar or the wing will tend to twist.

How much does your Invader weigh with the RC gear? In glider form it weighed 3.8 oz (108 g).

[PatMc]

Can I assume by "high start" you're referring to a what we used to call a bungee line ?
Have you ever used one before ?

I used to use a bungee quite a lot to practice & trim competition thermal gliders.
Although the initial launch can be fierce by the time the model has reached the top the model can be floated off the line.

[paul d]

I've got a invader, flys superbly, bit late now but I'll take a few measurements tomorrow.

[Jeffrey Hall]

Thanks for your prompt replies.

Yes Simon, 40".

I checked the decalage with a home made sliding Vs and digital protractor. However after your comments and having checked the plan I am beginning to doubt my readings, so I will have another go.

I cant find any significant twist in the wing or tailplane and I was careful to pin down during shrinking and doing.

The total weight is 215 mg, r/c conversion added about 95 mg so airframe about120 mg. I did a little beefing up in places so that would account for the extreme weight. Is the weight you quoted including ballast?

Yes Pat, bungee. I have seen tales of folding wings so I added a little spruce to the centre of the spar.

My thoughts so far are that I could be getting lift from the tailplane. I perhaps should mention that I modified the tailplane to have a straight trailing edge instead of leading edge so I could add a one piece elevator. However I can't see that this would make a difference, it is still symmetrical.

Quite a mystery. Must go now because SWMBO tells me I am going shopping.

[Pete Collins]

I built an Invader about a century ago when I was young and I remember it flying well, but this was free flight so is not very applicable. However I have recently built a Topper which has an undercambered wing section but is otherwise a similar setup to the Invader. I have checked the Invader plan on Outerzone and it would appear to have about 2 degrees of decalage as measured from the flat underside of the wing. The recommended cg position is on the spar which is about where I would put it. (some of these old designs used lifting tails and rearward cg's but not this one) That should give you a trim just above the stall, which would be OK for free flight but a bit slow for RC. In FF trim the rudder would be pretty ineffective and, if you did get it into a decent turn it would almost certainly stall as you straightened up. I had all these symptoms with the Topper. Trimmed to fly a bit faster however, it should be fine, but only on calm evenings. It's basically a slow flying airframe. Hope this is helpful, and good luck

[Simon Chaddock]

Jeff

Just a thought.

Is their a possibility that at your higher weight (105 g was the recommended flying weight) the wing retaining bands may be 'giving' a bit so the static decalage is not what you are getting when in the air?

I have a couple of 600(ish) g 3 channel RC powered gliders with the wings held on by bands that certainly do misbehave if the bands are not strong enough.

[Jeffrey Hall]

Simon

I think the bands I am using are quite strong but I will try adding another pair to test the theory when the weather improves. I must admit I am always nervous when fitting strong bands to a tissue covered model.

[brokenenglish]

Jeffrey, I think you're right. A 40" tissue covered vintage glider shouldn't need ultra-strong bands.

[Jeffrey Hall]

Just noticed I quoted mg instead of gm and everyone was too polite to mention it. Must be an ultralite.

[Jeffrey Hall]

Simon

Please can your clarify what you mean by the incidence chord line being 3 degrees positive for that section. Does the line pass through the bottom tip of the trailing edge in which case it would pass about 3mm above the centre of the leading edge where I am trying to measure the decalage from? Perhaps I have misunderstood you.

[paul d]

Jeffrey this my invader, its free flight but it should help you in getting yours trimmed, mine weighs just under 4 ounces, the cg is on the main spar and the only packing it needed was 3/32 under the wing trailing edge.

I do agree with you about using to many strong bands, I only use 2 rather thin ones, I use those the postman drops!

I bungee launch mine using a home made affair consisting of 10 yards of 1/8th elastic and 15 of 6lb fishing line, even at full stretch the wings show no signs of lifting off the fuselage.

The most important thing is to have the hook in the right place mines 3/4" behind the nose skid.

[Jeffrey Hall]

Thanks for that Paul

If you had to pack the trailing edge does that mean it was stalling? That would be the opposite of my problem. Do you know how much ballast you used?

I have 15 feet of tubing for an initial trial and need to look in my fishing box to see what line is available. At the moment I am thinking that with the CofG so far back I will get a very poor climb.

[paul d]

mines a couple of years old now but I think I used around a ounce of ballast??? and yes it did stall at first, move the cg forward, The old saying goes that with the cg too far back it'll fly once, too far forward and it'll fly badly.

love to see some pictures of yours.

[Simon Chaddock]

Jeff

That is correct. With true flat bottom wing section the incidence chord line will pass a bit above the actual leading edge, in fact just about where the leading edge would be if a 'proper' wing section like say a Clark Y was used.

A completely flat bottom wing section is not aerodynamically ideal but is used simply because it is simple to construct reasonably accurately. Whether you would be able to detect any improvement in performance using a another section with such a 'simple build' tissue covered wing is open to question.

Any chance of a picture of your Invader?

[Jeffrey Hall]

Simon

Looks as if that accounts for the difference in our decalage measurements. I have compared my fuselage to the plan and can't see any problems. That still leaves me with the CofG problem. Paul mentioned that he had to pack the trailing edge on his Invader so I wondered if I might try packing the leading of the wing and adding some ballast instead of messing about with stab. The words clutching and straws come to mind. Failing that I will just have to bite the bullet and stick it on a bungee to see what happens.

I'll try to get some pics done today but I warn you it is not a work of art.

Thanks for all your help

Jeff

[Simon Chaddock]

Jeff

If it is flying ok with the CofG that far back then the recommended hook position will be rather too far forward so it would climb poorly.

However with the CofG that far back and with a modest non lifting tail plane it shouldn't be flying properly anyway!

Don't worry about how it looks.

None of mine would win anything for quality of finish but they are light and (most) fly well.

[Jeffrey Hall]

Invader photo as promised.

[Jeffrey Hall]

Simon

Having looked at some of your photos methinks you are being too modest.

In my mind I think I can engineer a third hook using the other two as attachment points without doing any butchering. Having said that I still haven't given up on correcting the CofG.