How to position landing gear on nose-heavy plane?

[Fraktal]

I have a plane with the following main body, from nose to aft:

• Mk1 cockpit
• Mk1 LF tank
• Three material bays
• 1.25m service bay
  • Radially attached to this are two Mk0 tanks with an intake and a Juno each.
• Rounded nosecone

The problem with this setup is that if the Mk1 fuel tank is at the rear, the CoM moves forward a lot as the fuel is used up, which makes parachute positioning tricky (aside from the fact that I never build a single plane without enough chutes to come back down safely, the plane has the starting landing gears and is already overweight as it is, so the gears explode instantly if I try to land normally). So I put the tank in the front which stabilized the CoM, but now I have a different problem: the CoM is so far ahead in the front that if I put the rear landing gear near enough to it for the plane to even take off (if so, it takes off at ~87 m/s), the plane becomes impossible to land with parachutes when the tanks are full because even though it noses down while hanging from the chutes, the moment the front wheel touches ground, the plane's rear whips down and tailstrikes hard enough to destroy the rearmost material bay.

So then. Depending on where I put the wheels, either I can't even take off or I can take off but need to fly for at least an hour before I can parachute down without breaking anything. For visual reference, this is the plane in question:

The CoM is in the Mk1 fuel tank just behind the cockpit, both when full and when empty; the rear landing gear is attached to the material bay immediately behind the fuel tank because, again, putting it any more backwards makes the plane unable to take off. Fuel transfers from the Mk0 tanks at the rear cause negligible CoM movement.

[Gargamel]

That's a pretty difficult picture to diagnose, given the angle.   A better one would be from the SPH, a side view with the COM and COL icons lit up. 

But basically, you should put your rear landing gear a bit behind the COM, so it acts like a fulcrum when you pull back to take off.   But you have played with that some already.

My only real suggestion would be to remove one of the tanks some of the materials bays and reduce the overall weight of the plane.   Other's should have better ones.    Or if you do require that much range and if you need the chutes to land , add some more chutes.

Edited September 28, 2018 by Gargamel

[overkill13]

Do you need that much fuel?  Flying a plane of that size with just Junos is pretty boring to me, but you might have better tolerance for tedium in order to gather science further away than the desert.  I used a similar plane, in early career, with a pair of science jrs, but it doesn't have the larger fuel tank.  High wings with a pair of Junos, intakes and fuel tanks embedded in the wings or below them if the aesthetics are bothering me that day.  The wings are more central and CoM doesn't move much so parachutes are easy to mount.  If I needed more range I would add additional fuel tanks between the engines and intakes.  I've landed the version of this with the larger liquid fuel tank without 'chutes, but it is certainly tedious.

If you are hell bent on having that fuel tank on there, then put it in between the science jrs

Here's my rework of your design.  Centralized fuel tank so the CoM doesn't move as much.  Lands flat on the chutes when heavy, but slightly nose forward when out of fuel.  Added an extra wheel on the back there to protect the science jrs.  Raised up slightly to allow the plane to pitch up for takeoff.

[Fraktal]

14 minutes ago, overkill13 said:

Do you need that much fuel?  Flying a plane of that size with just Junos is pretty boring to me, but you might have better tolerance for tedium in order to gather science further away than the desert.

This plane is designed to be able to reach both the poles and the badlands from the KSC; haven't yet visited the badlands, but the edge of the polar icecaps, it can reach in about an hour with about 30% fuel left, by which point it can parachute down safely (I'm at low-transsonic speed and 4x warp for nearly the entire flight but drop back to 1x whenever I'm nearing 10° pitch from SAS not following Kerbin's curvature so that I can nose down manually to maintain speed and go back to 4x warp once my altitude has stabilized to get to my destination in a timely manner because Junos are indeed kinda anemic at 10k altitude). So no, I don't need that much fuel. I just didn't want to waste fuel tank capacity and can't mount the Junos and their intakes elsewhere. Dropping the Mk1 tank for several Mk0 tanks is an idea, but I'm deliberately trying to stay within 30 parts and have already hit the limit.

Edited September 28, 2018 by Fraktal

[Lisias]

1 hour ago, Fraktal said:

[cut] but now I have a different problem: the CoM is so far ahead in the front that if I put the rear landing gear near enough to it for the plane to even take off (if so, it takes off at ~87 m/s), the plane becomes impossible to land with parachutes when the tanks are full because even though it noses down while hanging from the chutes, the moment the front wheel touches ground, the plane's rear whips down and tailstrikes hard enough to destroy the rearmost material bay. [cut]

Comercial airplanes have a maximum landing weight that usually are way less than the take-off weight. It's by this reason that sometimes they need to dump fuel before landing when something goes wrong and the pilot has to come back or land early on a nearby airport.

I suggest you trim your craft for landing, then make any adjust needed to make it take off - and just dump fuel if you need to land early.

[bewing]

Yes, the plane is way too heavy for the gear. You could make it workable by removing half the fuel in the Mk1 tank. However, that would severely degrade your range.

What to do? You need a wheesley at the back instead of those two junos. How do you get a wheesley? Do a small number of contracts -- you will certainly get a part test contract for a wheesley very shortly. Then use that wheesley to do your long distance flights. You will be able to fly twice as fast, and use less fuel. If you get the right kind of contract, you may even be able to use the same experimental wheesley several times before you "test" it and complete the wheesley contract.

Additionally, you are highly likely to get a part test contract for some experimental  LY10 landing gear, and that would solve your problem too.

Edited September 28, 2018 by bewing

[swjr-swis]

Sounds like your main issue is trying to keep the plane balanced with different fuel loads. You are already close -the set of Mk0 tanks can be put to good use for this- but your fuselage misses a bit of length and the Mk1 tank needs to shift a bit back too. One more service bay placed strategically can do the trick.

My suggestions, while keeping to your design as much as possible, would be the following (don't read the spoiler if you first want to try yourself):

Spoiler

• Part order of center stack: Mk1 cockpit, 1.25m service bay, Mk1 LF tank, SC-9001, another 1.25m service bay, SC-9001, SC-9001, nose cone.
• Place mirrored Mk0 tank, Juno and intakes. Balance point is approx. on the second SC-9001.
• Change gear to a taildragger config: steering wheel at the very back, main gear a bit ahead of the CoM. Pull main gear a notch more down than the steering wheel for a slight nose up attitude.
• Shift engine nacelles back and forth until the CoM stays put between empty and full tanks. Place main wings centered on the CoM.
• Switch the swept wings for wing connector type B, for a better lift/weight ratio. The control surfaces on the other hand can be smaller, to save weight and drag -> switch tail fins for AV-R8 winglets.
• Add 5 degree angle of incidence on main wings (one fine snap rotation). Add camber. Shift to leave CoL just barely behind CoM.
• Add one OX-STAT panel on the dorsal on the fuselage (I think I can see two of them in your screenshot - sacrificing one for the extra service bay).
• Add 4x Mk16 chutes to the inner nodes of the service bays. The ones closest to the CoM are in their optimal horizontal position already, just shift them a notch up. Rotate and shift the other two to the same horizontal position.

This should total 27 parts. Your post and screenshot do not disclose what parts you have in the bays (other than chutes?) to total 30, but since you mention it's a science mission, I added the other three instruments that are available up to tech tier 4 (therm, baro, goo).

The extra balance and more efficient flight profile extends the range enough to be able to return from the poles/badlands to KSC - this is why I put in two sets of chutes, since you mention needing them for landing. This plane is not too difficult to land when low on fuel though, so it's worth a shot if you want to replace one set of chutes with two other parts.

 

Some pictures to clarify:

Spoiler

I added one more service bay, which allowed to move CoM back, balance fuel, and place chutes better for level descent. Placed gear in taildragger configuration.

I replaced the wings for better lift/weight ratio - they are in the same tech node as the others, so that should be possible. The control surfaces could in turn be smaller.

Plane stays level, or is easily corrected to level, with chutes deployed. Fuel load has no effect on this.

The open bay doors touch ground first and safely absorb most of the impact, even if there is still significant forward speed.

Landing explosions no longer being a worry, you can take a full fuel load for extended range - almost 3/4th of a circumnavigation.

Full album and additional comments: https://imgur.com/a/mnGrYNj

Craft file: https://www.dropbox.com/s/psg184cjx3hbrtl/Fraktal-Plane1c.craft?dl=0

 

[Fraktal]

26 minutes ago, swjr-swis said:

I think I can see two of them in your screenshot - sacrificing one for the extra service bay

If you mean the two dark rectangles on top, those are actually radial chutes offseted into the main body. I don't know if this reduces drag or not, but I'm doing it anyway on the off chance that it does and because it looks more aesthetically pleasing.

26 minutes ago, swjr-swis said:

Your post and screenshot do not disclose what parts you have in the bays (other than chutes?) to total 30, but since you mention it's a science mission, I added the other three instruments that are available up to tech tier 4 (therm, baro, goo).

5 parts are inside the bay: one thermo, one baro, three goos. I'm forced to use the swept wing for lift and the tail fins for pitch/roll because I don't have the part count for elevons.

[Klapaucius]

It would be helpful if you can upload the craftfile so we can look at it. However, there are a few things I do that get me off the ground almost always.  When taking off the main thing is simply getting the nose lifted. Once you have done that, you are home free. Sometimes that is as simple as some very minor adjustments to the wings or flaps. If you have the lift indication in the SPH turned on, you can see it responds sometimes drastically to small changes.  

A few tricks:

1. I tend to give my planes a slight (or sometimes not-so-slight) nose up attitude.  I either use slightly larger gear on the front of just sink the rear gear a bit more into the fuselage.

2. Canards are your friends. Stick a pair on the nose. They do wonders for getting the nose up.

As you can see from the examples below, you can have very nose heavy planes with gear set quite far back and they will still fly, often very well. These four oddball designs are all great flyers and they all take off rather quickly.

 

Edited September 28, 2018 by Klapaucius

[swjr-swis]

24 minutes ago, Fraktal said:

If you mean the two dark rectangles on top, those are actually radial chutes offseted into the main body. I don't know if this reduces drag or not, but I'm doing it anyway on the off chance that it does and because it looks more aesthetically pleasing.

No, it doesn't reduce drag - only fairings, cargo and service bays do that. In every other case the game treats parts as fully 'transparent' to drag, so clipping makes no difference.

On the bright side, this means you can replace the solar panel by one more goo.

 

28 minutes ago, Fraktal said:

I'm forced to use the swept wing for lift and the tail fins for pitch/roll because I don't have the part count for elevons.

I figured as much, which is why I did the same with the one I shared. With the panel swapped for another goo, that means mine is only one goo short of your design. If you make it a one-way trip like you do now, or decide to chance landing the plane normally on the return trip, you can remove two of the Mk16 chutes (which are shielded from drag btw, inside the bays) - that would let you complete the set of goo canisters, and still have room for one more part.

[AeroGav]

Those basic swept wings have half the lift rating of the other wing parts of same mass,  your takeoff and landing speeds look rather high in the screenshot.      

[swjr-swis]

11 hours ago, AeroGav said:

Those basic swept wings have half the lift rating of the other wing parts of same mass,  your takeoff and landing speeds look rather high in the screenshot. 

Is this to OP or me? OP doesn't show any landing or take off, there's just the one screenshot where the plane is already at 799m altitude.

If you're referring to the screenshots in my post/album, they are for an alternative design I'm suggesting that uses the type B wing connector instead of the swept wing. The type B is the best single wing part OP has access to at this point (lighter and more lifting area), and in the same tech node as the swept wing. More lift than those would require a wing of more parts, which OP indicates is an issue (max 30 parts for the entire craft).

[AeroGav]

3 hours ago, swjr-swis said:

Is this to OP or me? OP doesn't show any landing or take off, there's just the one screenshot where the plane is already at 799m altitude.

If you're referring to the screenshots in my post/album, they are for an alternative design I'm suggesting that uses the type B wing connector instead of the swept wing. The type B is the best single wing part OP has access to at this point (lighter and more lifting area), and in the same tech node as the swept wing. More lift than those would require a wing of more parts, which OP indicates is an issue (max 30 parts for the entire craft).

The OP

You can tell quite a bit from his screenshot - the airplane is at about 5 degree angle of attack (difference between prograde and where the nose is pitched up), the closer you are to stall the bigger this gets.  The G meter is only showing 1g, so his AoA is not the result of a harsh maneuver,  that's just from straight and level flight.  I imagine that if he gets much slower than 70 m/s he'd be at >10 degrees angle of attack , at which point most designs would be at risk of tailstrike,   and an outright stall at 60 or less.

For a utility airplane that's not great,  most of my space planes would be under 50 m/s at 5 degree alpha.

[swjr-swis]

5 hours ago, AeroGav said:

most of my space planes would be under 50 m/s at 5 degree alpha.

Most of your space planes also use much bigger wings with lots of parts, which falls outside OP's tech-level and max part count restrictions.

 

@Fraktal, I placed another alternative on my Dropbox, the Fraktal-Plane1a. It uses the same set of parts you used, now that it's clarified. The only part-change I kept from my previous one is the type B wing in place of the swept wings - that change is rather important for a significant performance improvement. Being in the same tech node as the swept wings, that should not be a problem.

Parts have been rearranged to balance the plane so CoM stays put with changing fuel loads. This allowed placing all four radial chutes right above the CoM. Even with a full fuel load, it needs only two chutes to land safely (along with a drastic change in gear spring strength). They are set to open in two separate pairs for two landings, giving your science trips the option to return to base for full recovery.

The radial chutes do add drag compared to the previous one I posted, which results in higher take off speed, and lower top speed and cruising altitude, but still better than what you had. It can still make it to the poles and back to KSC using about 400 units of LF - it just takes a bit longer (about 50-ish mins one-way). Try not to fly too high - optimal cruising with full load is at around 275 m/s and 7000 m (both raising with less fuel).

Oh, when you go in for a chute landing, make sure you are at least about 150-200m above the surface when staging the chutes: the plane does make a pretty hefty seesaw motion as the chutes drain horizontal speed. It will settle to near-level attitude pretty quickly and can be corrected easily from that point on.