Great plane, poor glide, scale Bleriot xii

[KenSt]

I recently finished a scratch built Bleriot xii. It weighs 7-1/2 oz. with a wing loading of 4.7 oz./sq. ft. Wing area is 228 sq. in. and the balance point is at 28% of the mac. Is this too far forward for an undercambered foil? Stab and elevator is 44 sq. in. with a very slight flat bottom airfoil in it, more for looks than anything else, the elevator part is flat. The distance between mac's. is 17 in. The stab is at -4 (leading edge down) and the wing is set at +10 degrees to the motor thrust line. (yes, 10!) It flies quite nicely at half throttle and climbs well at full. The elevator seems to have enough power, but the plane has little momentum for manuevers. It is difficult to get it to stall. Full up elevator at 1/4 to 1/3 throttle gives about 5 mph for as long as I want. This works well for landings, since I can't get it to flare. Does a Bleriot simply have too much drag for its own good? An all out dive is required to get it to loop. Airfoil is an NACA 6409. The reason for all the numbers is that hopefully someone can calculate why it does not glide very well. In fact, full up elevator with no power is not enough to hold the nose level for more than a second or two. It glides like the space shuttle. :confused: The elevator is half of the stab and at about 25 degrees at full up. I'm leaning toward insufficient stab area. If I had more could I decrease the wing angle to reduce drag? (it's adjustable, as is the stab) Don't get me wrong, it is a very enjoyable plane to fly, 45+ minutes with a 1320 ma lipo :cool: Great way to unwind just before dark ;)

Any thoughts before I start adjusting things?

[Ollie]

Poor Glide:
1. Low aspect ratio.
2. High drag
3. High lift but for high drag at speed.

[Purdue Aero Man]

You say your horizontal tail airfoil is a slight flat-bottom. Are you saying that your tail airfoil has camber that would lend itself to producing lift upwards in normal flight, yet the stab is angled downwards? Perhaps the first thing to do would be to get rid of that camber if you can, as it seems to be counter-productive. In your case you want the tail lifting downwards, not upwards.

You seem to be having trouble keeping the nose at a level attitude. Which should tell you that your tail isn't producing enough lift for the current CG. Try shifting your CG back a little at a time and see if that helps any. Once you get the CG back to the proper range, your glide will get a lot better, since a lot of your drag is probably a result of your excessive elevator usage.

[Salto]

KenST,

The heavily cambered NACA 6409 will have a large downwards pitching moment. The forward C/G will also produce a downwards pitching moment. To fly level, this model will need a large downforce to be produced by the stabilizer. This is evidenced by the massive (14 deg.) longtitudinal dihedral that you say it has.

At low airspeed the stab cannot provide enough downforce to maintain level flight so the nose drops, and even at higher airspeeds the stab cannot provide enough downforce to easily pitch the model up into a loop. My guess is that at low airspeeds, the stab cannot generate enough downforce to allow the wing to reach it's best lift over drag ratio

I can't find a picture of the Bleriot XII, but I assume it has lots of exposed structure like other Bleriot designs. This alone means the model will never glide particularly well, but as Purdue Aero Man suggests, you could try moving the C/G rearwards accompanied by a reduction in the longtitudinal dihedral. This will take some load off the stab with a corresponding reduction in induced drag, plus it will allow the wing to get down to a better lift over drag ratio.

Move the C/G back in small increments and observe the pitch stability which will gradually reduce. You can keep bringing the C/G back to the point where pitch stability is as low as you can tolerate.

Again, without a photo or drawing, I'm guessing that this plane has a very long tail moment, so a small stab area should be OK, as long as the C/G is in a sensible position.

One final thought is that if the stab airfoil you describe is scale, this plane may have been designed to fly with a very rearward C/G and an upwards force on the stab.

Graham.

[KenSt]

Thanks guys, I agree that this plane will probably never glide really well. It's nice to get replies, and, they all have the same general advice. I don't know why I typed "xii", I meant "XI". This was the first plane to cross the English Channel. The aspect ratio is only 4.7 to 1 with this plane. The speed range is limited, the tail structure is exposed, and the "bedstead" landing gear has drag written all over it. The original only went 60 mph flat out and the military version wasn't much better.

I now have the reassurance to start moving my CG back. It will also be easy to get rid of the slight hump in the stab, simply uncover the top, sand and recover. I could also replace it, since it is just screwed onto the frame.

I believe the original was set up for a lifting tail as all the pictures I've gathered show many degrees positive angle in the stab, almost as much as the wing. I guess I could set it up like that, but I don't know how to determine the CG that way. Would this work to my advantage or was my original apprehension with this approach well founded?

Ken Stinson

[Salto]

I believe the original was set up for a lifting tail as all the pictures I've gathered show many degrees positive angle in the stab, almost as much as the wing. I guess I could set it up like that, but I don't know how to determine the CG that way. Would this work to my advantage or was my original apprehension with this approach well founded?

Ken,

For a given trimmed airspeed, the stab incidence (or elevator trim) and the C/G are intimately related. If you alter one you must alter the other to return to the same trimmed airspeed.

Since it's the C/G that you are trying to alter (in order to reduce tail load), just keep moving it back and re-trimming the elevator accordingly. When you've got the C/G as far back as you dare, you can alter the actual stab mounting so that it flies with the elevator flat to the stab. In other words, lead with the C/G adjustments and the required stab incidence will sort itself out.

Graham.

[KenSt]

Will do, thanks again!

[Sparky Paul]

Really, I believe your plane flies as well as a replica of the Bleriot could be expected to.
All that incidence, and built-in drag, that's the Bleriot was. A poor airplane way back when, and probably couldn't be made into a good one today without morphing it into a not-Bleriot.

[Purdue Aero Man]

I believe the original was set up for a lifting tail as all the pictures I've gathered show many degrees positive angle in the stab, almost as much as the wing. I guess I could set it up like that, but I don't know how to determine the CG that way. Would this work to my advantage or was my original apprehension with this approach well founded?

Ken Stinson


A few airplanes have lifting tails, example being the B-52. Most arguments about stability say that a downward lifting tail is benificial for stall recovery. You could do a lifting tail if you really want to be scale-like, but if the 6409 has a lot of downward moment to it, as Salto says it does (I haven't looked at the polar), perhaps the best thing to do would be to set the tail up to lift downwars, thereby counter-acting the nose down moment of the airfoil. I think if you are gonna make a lifting tail, you would need to set the CG to a location that would make the main wings stall first. Either way, I think the best results are going to come with you just getting out and experimenting. Luck man.

[KenSt]

I wish it was June, then i'd have more daylight to experiment with my CG. It isn't a diaster by any means. The glide is very controllable. I can be at the edge of the field at 100 ft. high, chop the throttle, let it dive and simply add power and land right in the middle of the field. Dead stick wouldn't be much different than landing with a parachute. They just wait until the right time and put on the brakes. Full up at the end of a fast dive will pull it level and slow it enough to land without any problem. Thanks for all your input guys.