blunderbum

Is their a formula to calculate how much tail force is required to rotate an aircraft on takeoff. The reason im asking is, I have desined a new aircraft and it requires 2.4kg of force on the tail plane to get the aircraft nose to lift off the ground, does this sound to high, or do I need to reposition my main landing gear further forward. TIA

rmh

actually rearward- to allow the model to balance on the mains - THEN rotate when lift is sufficient
where did you come up with this "force calculation?"

blunderbum

I measured it using some digital scales, if you move the rear landing backwards then wouldn't you require more force on the tail plane to lift the front.

crasherboy

Make it a tail dragger,then you won't have that problem!

rmh

This is getting out of hand-- Ideally -- the model sits level--- that is with no down AOA OR Positive AOA

as it picks up speed the MAINS should easily be supporting the model-- as a tail dragger -the model should easily have come up to level -- as a trike gear -a slight UP control input should easily lift the nose gear . The idea here ---is to allow the model to easily pick up speed BECAUSE the drag of the wing is at the minimum angle to the direction of flight.
IF the trike setup has the mains too far aft - there is too much weight on the nose gear and steering becomes very touchy.
IF the tail dragger setup has mains too far forward --- the model will ground loop easily as the wing -being too far aft --will not easily come up to a level NO drag AOA
So the idea is to minimize wing drag as soon as forward roll begins and yet make steering as easy aspossible
the rudder NOT THE TAILWHEEL should do the steering - if you allow the tailwheel to do the steering -- the model will tpically jump wildly to one side as flying speed is reached and rudder force is stronger than tailwheel on the ground force.
This problem is VERY common on models which are not properly setup
If yo u note full scale practices -- the first thing you will see is the tal being ligter OR the nose wheel being lifted so that the FLIGHT CONTRLS not the wheels are doing the steering.

pimmnz

Once you have flown the model, are happy with the CG, then place the main wheels just behind (If trike) or just forward (if 2 wheel) of the cg. With a trike the model should just put the nosewheel down if the tail is lowered then released. With a tailwheel model you'll probably find the the wheels will be somewhere close to the wing leading edge. As Dick says, most ground handling with full size is done with the aerodynamic controls, same should be with our models. If it's really skittish on the ground, lock the wheels straight (front or back) and use the rudder and elevator for ground handling.
Evan.

Mike Connor

My Feedback: (5)

It depends on the size of the model but 2.4 kg sounds very high to me. I would reposition the landing gear or it may never rotate. What weight model is it?

Edit - Playing around with foilsim on the NASA web site
http://www.grc.nasa.gov/WWW/K-12/airplane/foil2.html
a typical 40 size plane with a 20* elevator deflection would have to reach 137 km/h to rotate 2.4 kg. How long is your runway?

BMatthews

And you're measuring or calculating this how?

If you are measuring the force at the tail to lift the nose wheel then I'd say that much is not far off for a 1/3 scale model of some form that weighs in at 25 kilos. But if this is a regular sized model of under 2 meter span then you need to much tail force and it's time to move the gear forward to achieve rotation with a lot less force.

I've seen it said that the model should almost sit on it's tail if you push the tail down to the ground for most trike gear planes. This "almost" is relative of course and you have not told us the size of the model but my first impressiion is that there's something odd with this design or it's REALLY big.

blunderbum

The aircraft is reasonably large, 3.6m wing span, 600mm chord and it is 3.5m in length. It has a trike landing gear configuration. Initial takeoff weight will be approx 60kg.
I measured the required down force using a set of digital scales. I haven't flown the aircraft yet but I'm just double checking every thing before it's maiden flight.
TIA

vertical grimmace

My Feedback: (1)

The job of your tail is not to lift the aircraft off of the ground. This will be the responsiblity of your wing. The job of the tail is to keep the wing flying in the direction that you want it to go. Now you must have enough vertical and horizontal tail volume for a given wing area but the position of your landing gear has more to do with your CG than anything else. For the trike, have the mains behind the CG far enough to keep the nose gear on the ground and you should be fine.
Interesting looking design. Good luck with the maiden and keep us posted.

BMatthews

Actually for this size of plane I'd say 2.4 kg may well be a touch high but likely not by much at all. That's a big model and the scale of downforce needed to rotate the plane to takeoff attitude is well within the scope of the model's overall weight.

Tall Paul

Your tail booms look a tad undersized.
There is a large download on the horizontal in flight.
I've had twin boom planes tuck and crash at speed.
And I've have to add vertical surface in the form of keels along the tubes for added directional stability.

rmh

structurally either boom setup -looks weak to me
I would have slanted the booms inward slightly to prevent any side to side movement
also I would have used a tapered boom (ala fishing pole segment)
and yes I have done a few double boom models -aerobatic types.

BMatthews

I have to agree with the others. I almost lost a model due to flex in the tailboom on a pod and boom electric glider.

blunderbum

I'm hoping the boom will be fine and they are 1.5 inch OD, 6061 T6 alloy with 43 thou walls. If you think they will be a problem is there any way I could increase the strenght of them? without changing the outer diameter.

BMatthews

Hope won't cut it.

I'd suggest you lock the forward section in place and load the tail with around 5 kg. If the tail alters angle by more than about 2 degrees then it's too flexible. The numbers may be off in degree but the test method isn't. The key is to be sure you can avoid the tail deflecting more than the control surfaces are set for deflecting.

If you find that they are too flexible then something radical will be needed. Perhaps top and bottom spines of carbon rod or some other alternative.

Tall Paul

This push-pull Kadet has triangular sectioned booms.
A chunk from the rear motor spinner came off in flight and took out the lower longeron. I could feel the plane handling differently, so I landed.
On the low-wing pusher, the elevator pushrod bent under load, and the plane tucked.
Rectangular sectioned booms.
Sears laughed at me when I took what was left of the nose-mounted camcorder back for warranty.

Mike Connor

My Feedback: (5)

Looks like the pusher prop in the center picture is on backwards. [X(]

rmh

no--

Mike Connor

My Feedback: (5)

Are you trying to tell me the sharp edge of the airfoil should hit the air mass first?
Or are my eyes worse then I thought?

Tall Paul

The rear prop is installed correctly.
It's just the angle of view that makes it appear odd.

blunderbum

I tried flexing the booms this morning and found that the boom flex is very minimal, the main wing flex's with the booms. I will put some temporary supports under the booms right next to where the booms exit the wing and see if I can flex the booms that way. The boom wall thickness is 65 thou and not 43 as I stated eariler.

BMatthews

OK, then I'll alter my weight load testing. Clamp the fuselage and add around 3 to 4 kg to the tail. Measure BOTH the wing's incidence outside the boom before and after as well as the angle at the stabilizer. If the wing changes by more than 1 degree then you've got troubles and I'd say there's some serious possibility of flutter to be seen. Similarly if the tail angle alters by more than the 2 degrees I suggested before then you've got issues as well.

If your elevators are driven via pushrods from wing mounted servos also check that the elevator position WRT the stabilizer does not change at all when the booms flex. If they do you need to correct ANY change due to that issue.

If you have any of these issues you'll have control surfaces that are not flying with a fixed foundation. All corrective or command inputs will require more than you would give normally. And the structural elasticity may well introduce harmonic vibrations in the structure at some specific speeds. Especially if the control surfaces deflect in connection with any flexing.

Tall Paul

I had a flutter incident yesterday with my pod-and-boom Gentle Lady camera plane while photographing the poppies.
Motor off, it started to dive.. And at the same time the radio stopped working, as I couldn't correct the dive. Spektrum DX-7 transmitter, AR6100 reciever.
The plane was coming down vertically, both wing tips fluttering in that cute way they do..
I fingered it wouldn't hurt any more, so I pushed full down elevator. After a short period, the plane completed the roundout at the bottom of the outside loop!
Missed the ground by about 20 feet when inverted, and began to climb. I leveled it and landed.
No damage.
Flang it again, but this time didn't let it get over the car, just in case there was a "shadowing' problem at the distance and altitude it had been at. Flew fine.
I've heard those receivers have a "brown out" event, when there's too much load.
Or the boom may have bent as the speed picked up.
Added a lot of uptrim on the refly just in case there wasn't enough incidence between the wing and tail. Prolly oughta make that a permanent fix.

BMatthews

Paul, how fast was it going at first? The way that I almost lost my own P&B glider was when the speed picked up a little and the boom flexed down and from there I could not input enough up elevator to hold it. I did just as you and bunted to inverted and rolled out. From that point I was very careful not to let the speed build at all until it was back on the ground. I "fixed" it for a while with a length of control line cable to the top of the fin. In the fall I did the job properly by laying up some carbon tow and epoxy around the tube to stiffen it so it's pretty much like a golf club shaft now. No problems since.