mr_matt

My Feedback: (10)

Hello,

Does this sound right?

You add 1 pound to a 20 pound plane. All else being equal, the landing speed will go up by 2.5%

So if you landed at 40 mph before, after adding 1 pound, the landing speed goes up to 41 mph.

Sound close?

Thanks in advance,

buzzard bait

I think so. Aerodynamic force varies as the square of the velocity. If you need a 5% increase in aerodynamic force then you would need a 2.5% increase in velocity, all else equal, because 1.025 X 1.025 is just over 1.05. But I'm not an expert on this and may be missing something.

Is that how you figured it?

mr_matt

My Feedback: (10)

Yes you got my thinking. Thanks for the response

Quorneng

The increase in landing speed due to an increase in weight may not sound that significant but remember the kinetic energy of the plane also rises by the square of the speed so a 5% increase in weight also means a 5% increase in the amount of energy in the plane, so up goes the risk of breaking something on any 'arrival'!

The other factor is the increase in weight increases the power required to fly by the same amount. It is amount of 'spare' power that allows the plane to climb. A reduction in the amount of 'spare' will show itself as a reduced climb rate.

pimmnz

Interestingly, the weight increase affects both ends of the speed spectrum, all else equal, weight increase also decreases top speed. And for much the same aerodynamic reason.

Evan, WB12

sensei

What are your thoughts on the effects of stall speed by adding 1 lbs. to a 20 lb. plane with everything else remaining the same?

Bob

rgburrill

Bob, IIRC stall speed is a function of angle of attack and airfoil shape, not weight. Of course it's been 50 years since I took any aerodynamic classes.

Rafael23cc

My Feedback: (6)

Stall is a function of the angle of attack. Stall Speed is definitely a function of weight.

http://www.experimentalaircraft.info...ll-speed-1.php

Rafael

sensei

As I stated, all things remaining the same except weight, as the weight goes up, wouldn't the AOA increase to carry the added weight along with raising the wing loading and changing stall speed?

Bob

speedracerntrixie

My Feedback: (29)

Take a nose heavy 20 lb airplane and add a pound of weight located so that the CG gets shifted back to a neutral CG position and the airplane will land slower.

sensei

Hello Shaun,

I understand your point and couldn't agree more, but let's assume for just a moment that Matt possess an already flying powered plane (Not a sailplane) that is properly balanced with no other changes other than he begins stacking weight on the C/G, 1 lb., 2 lbs. and so on. The wing loading will increase along with stall speed, hence increasing the landing speed as the weight goes up.The AOA will also increase as the wing assumes the weight increase, while adding drag, reducing the overall top speed and killing vertical performance. Anyway you know my take on extra weight, I would move an engine, the on board gear gear, the wing or build a new airplane before I would add a pound of lead to a 20 lb. airplane, but that is just me.

Bob

ahicks

My Feedback: (2)

Bob-
Nope, not just you. The better solution in Shaun's scenario, is to do whatever is required to fix the CG issue without adding weight. Rarely is that not a possibility. Not in my experience anyway. -Al

speedracerntrixie

My Feedback: (29)

As usual you guys are spot on. I avoid adding weight whenever possible as well. Although not the best looking solution I had to move the engine forward 1.5" and move the RX batteries as far forward as I could to get CG on my CARF Extra. Gotta do what you gotta do. If I continue to struggle this season with upline snaps the way I did last season the next step is to ditch the composite stabs and replace them with balsa sheeted foam. This would drop 1/2 pound out of the tail and I could swap out the dual 5000 mah batteries with 3500mah. The airplane as a whole could drop 1.5 lbs.

alasdair

Good thinking, but check your calcs.
Adding 5% to the mass increases weight by 5% (no CG change)
Indeed, stalling speed goes up about 2.5%
Kinetic energy (mass times V squared) increases 1.05 x 1.05 or just over 10%
wing loading is up 5%
Power required increases 5% for the weight plus another 2.5% for the extra wing loading, i.e. 7.5% extra power required.

bunsen

I had not considered that a forward CG would cause an airplane to land faster. Is it assuming that the elevator travel is not adequate to increase the AOA to get a slower speed or something else?

Bunsen

UStik

More elevator travel is needed in that case, indeed, and if it's not there the airplane might not reach stall AoA.

More significant may be the extra load on the wing. A more forward C/G means more lever arm in front of the lift vector, which must be balanced by more stabilizer down force. I have a calculation example, a 84" 12 lbs Extra 300, that needs 0.48 lbs elevator down force to balance. That is 4% of total weight and seems to be typical for a stable balance (16.5% static margin in this case).

If the model is balanced exactly "neutral" (0% static margin), there is even 0.33 lbs stabilizer up force (lift) unburdening the wing by 2.75%. The total difference in wing lift would be 0.81 lbs or 6.75%. Calculation may be not quite accurate but is in the ballpark. Now the same math applies as above.

ahicks

My Feedback: (2)

Here's another way of looking at that. If we move the CG back to the point where the elevator no longer has to push down on the tail to get it to fly straight/level, we reduce wing loading. In this scenario, the wing is acting as a fulcrum. The harder the tail pushes down, the more load the wing carries.

Further, in a tail heavy scenario, where the elevator is LIFTING the tail, the elevator becomes one of the lifting forces in play on the air frame, reducing wing loading even further. The weight of the plane doesn't change, so the weight the downward deflected elevator is carrying comes from somewhere, which would be the wing to my way of thinking. This with the airplane right side up anyway.

Wing loading/AoA is just one of the forces in play in a stall scenario. I doubt it would ever come into play on a certified full scale air frame, but in RC you also need to keep in mind what happens when you get air flow separation over an elevator running at too high of an angle (up or down!). It's not going to matter which happens on short final (elevator or wing stall). Either will point the plane's nose towards the ground in very short order.

Point being, in a slow speed nose heavy RC plane landing scenario, it could be excessive AoA OR excessive elevator throw that re-kits your model.... -Al

AA5BY

Drag caused by a non neutral elevator countering either a CG imbalance or improper incidence will at the minimum degrade flight performance but the greater problem of a non neutral elevator is that it often produces a variance in power on / off trim.

A few years back, an older gentleman who had lost his wife and was desperate for interaction and involvement joined our club. He built a Sig LT-25 trainer and one of our instructors worked with him. All went pretty well until he was on his own and he had a bad time with landings. One day after watching a gentle landing of my grand kids LT-25, he remarked that he wished he could land like that to wit I suggested that his LT-25 was capable of doing it. He didn't think so and suggested I fly it to see if it would.

Holy smokes.... the frigging thing dived for the ground when throttling back. No wonder he was having problems landing. The instructor had trimmed the plane for full power (with a hot OS 32 on what Sig recommended a .25) and it had a lot of down elevator trim to counter power on climb. The balance was good, but the asymmetrical wing climbed excessively when going fast.

I re-trimmed for a good power off glide slope with a near neutral elevator and reduced greatly the high throttle travel essentially cutting the power to a .25 equivalent and the old gentleman's landing difficulties went away and he was far more relaxed with the slower speed and greater reaction time.

Bax

My Feedback: (11)

That LT-25 should have some downthrust added to counter the trim change with power change. A hard nose-up tendency with throttle-up means you need downthrust. Same if you get a hard nose-down tendency with throttle reduction...that's also an indicator you need downthrust. Just shim the engine a bit and see what happens. A few flights should enable you to get it really close, if not dead-on.

AA5BY

Normally good advice but there was better advice in this case. The old gentleman got talked into the hot OS 33 on an airplane that Sig specifically says is intended to have a .25. The instructor who was helping him and advised him to buy that engine liked power... the more the better but this gentleman got into RC in his late 70's when reaction time for many has gone past its better days. The LT-25 has the built in needed down thrust for a .25, the better course for him was to reduce the throttle travel and power it similar to a .25, which brought the plane into trim and slowed it down to fit the owners reaction time.

speedracerntrixie

My Feedback: (29)

You both make good points however, correctly trimmed with the correct CG and thrustline just about any airplane can be overpowered and not have a pitch trim change. In this case, adding down thrust AND slipping the CG back a bit would have most likely solved the issue. I do however 100% agree the a .32 on an elderly gentlemans .25 size trainer was a bad idea. I am notorious for overpowering but with everything properly sorted out my airplanes will hold the same pitch trim when flown between 25% and 100% power.

TCrafty

All great information here. And one of the things that makes this hobby so great is the diversity of planes and those that fly 'em!

That said, you can go to about any field and see similar planes flown in totally different manners. Seeing that this is an LT-25, it's understandable that there would be a LOT of lift. And, if you have a larger engine, it'll theoretically go faster, if you throttle it so. AAB5Y handled that by reducing the amount of throw for the throttle, which appears to have worked in this scenario. But, as with the mass of diversity that is within this hobby, there could be other solutions as well?

Could a lower pitch prop be used to slow the plane? Many flyers use WOT as their cruise control (not judging here ) and if that is the case, then a lower pitch prop would effectively slow the plane if clearance and funds allow?

Another solution might be to lower the throttle by other means. If the gentleman could manage pulling back on the throttle to slow the airframe, then he'd still have all the available throttle potential AND use of the entire throttle spectrum if necessary. Again, it's all up to the flyer. He can choose whether to use this option or not. This also has the benefit of using less fuel (lower rpm during most of the flight) and is quieter in many instances.

I would think that the most beneficial scenario would be to set up the airframe to FLY the most comfortably for the flyer, and then go from there. IMHO, I see many folks adding flaperons, spoilerons, down thrust, incidence and whatnot to compensate for various scenarios that they encounter in flight. The only issue for me, is when those adjustments aren't necessary, or WANTED, but because they are hard-wired into the plane, they occur. I've seen a few re-kits thanks to this scenario. I'd say, get the CG right so it's not twitchy or dives like a mother when the throttle is decreased and then go from there. IF it soars at WOT, then only use WOT when you want to go UP.

In full respect to the gentleman who owns/flies the plane, it's his plane and perhaps it is now set up exactly to how he tends to fly it. If so, I hope he and his plane have a long, wonderful relationship together. No disrespect meant or intended. For those looking to improve their flying skills and broaden their horizons, feel free to move the throttle stick a little bit during flight, its bearings are designed to allow for that!

iron eagel

You are correct a lower pitch prop is a good way to compensate, the top speed will be reduced, and at low rpm it will act as a brake. The neat thing is you will gain a bit of thrust that will alow for flight at higher AOA's so you cand land it like a bush plane. (If you need more thrust and the diameter or clearence is an issue use a 3 or 4 blade prop.)
But as many have already said adding weight to "fix anything on an airplane" is the last resort.

mr_matt

My Feedback: (10)

Sorry, I guess the question was cryptic, I could have re-written it to be "Landing speed decrease with weight decrease?" I would not be adding weight to a plane for the fun of it :-)

I mean all (other) things equal, l mean angle of attack, CG position, wind, etc etc. Everything equal, if I take out 1 pound of weight, landing speed drops by 2.5 percent or about 1 mph in this particular case.

This is actually somewhat real world question, I have a chance to drop my power plant weight by about 1.5 pounds and I wanted to see how much the landing speed would change and if it was worth it.

ahicks

My Feedback: (2)

I think you'll see a very noticeable difference in landing speed.

I put a Giant Sportster on a diet with a US 41 to DLE 30 engine transplant, going from 17.5 lbs to 15.5, and there was a dramatic difference. Not just in landing speeds, but in all around handling.

Lighter plane is ALWAYS better!