Help! Underpowered plane

[horsie_flites]

Hello!

Today we were lucky enough to have many experienced hobbyists at our local field. While the flight may not have been "successful," there was a lot learned and the plane is intact.

First off, the 9g servos we are using are supposedly too weak for such large surface area control surfaces such as ours. They may do alright because we're not doing any crazy aerobatics, but still a stretch for a big plane. The first flight (not shown) was a rather short lived one as the plane veered to the right into the grass. We assumed it was P-factor since all the blades are rotating the same direction, and we added some differential thrust. We also gave it to a test pilot (possibly the most capable person possible) to fly it the second time, and that is the video shown. We came to the realization that something of an "X harness" would be more necessary than just quality of life since if by somehow one battery drained faster than the other, one wing would produce less thrust than the other. The pilot said there was an increasing lack of control as the flight went on, and that the plane only wanted to turn right eventually. If I remember correctly there was around a 0.3-0.5 V difference between the two batteries when we found the plane. This was most likely the cause of the veering during the first flight. Lastly, the hardpoint for the landing gear was weak so we will be adding some paint sticks to reduce the flexing it had during test flights.

The batteries will now be wired up in parallel and get distributed evenly between the four motors as opposed to one battery per two motors before. Other possible upgrades would be finding some stronger servos and finding some 7x5E reverse props. Hopefully the next flight is more successful!

Good news is that the plane has enough power for flight. Not the ideal power system, but probably the best we could do with the nearly non existent budget we got this year from a covid-closed school.

 

[Gnobuddy]

Firstly, congratulations on a successful flight and recovery.

Good job by the test pilot, too. He kept his head under difficult circumstances, and it's thanks to him that the model is still in one piece.

I note that the model did indeed need lots of speed to fly, as WebOCalc predicted.

The fact that either battery discharged to any significant extent during such a short flight tells you the battery capacity is really marginal. There's just not enough stored energy. For a model this big, 4S is really not big enough. 6S batteries would have been better, but of course, require much lower-Kv motors, as well as ESCs that can handle the increased battery voltage. None of this was in your budget.

As you say, you guys were working with a tight budget, and under some of the most difficult global circumstances that the world has had to endure since the Spanish 'Flu a century ago. So a big pat on the back to everyone involved. Nice job!

Good news is that the plane has enough power for flight.

Indeed! And It took off from grass, too. Ploughing through grass on the take-off run requires considerably more power than taking off from a smooth hard-surfaced runway.

Contrast that performance to the first flight, where the model was hand-launched at full speed from shoulder-height, and still struggled to barely maintain level flight.

So the power system is a definite success, especially since you were forced to use sub-optimal motors (Kv too high), a sub-optimal battery (voltage too low), and sub-optimal propeller sizes (smaller than ideal, the small size being forced on you by the too-high motor Kv).

I don't think it was ever mentioned on this thread that you were trying to use tiny 9-gram servos. Yes, those are definitely too small for a large, fast, heavy airframe like this one. Small servos are not only lacking sufficient torque to move those big control surfaces, they also have tiny plastic gears in them, with even tinier gear teeth about the size of a grain of sugar. Those tiny teeth break off easily if overloaded. If this happens in the air, or from bumping the big control surfaces during ground handling the model, you can lose the ability to control the model in flight, causing a crash.

So those little servos are intended for smaller, slower models with smaller control surfaces.

The good news is that bigger servos aren't necessarily more expensive. Not so long ago, 9-gram servos were "sub miniature", and much bigger "standard size" servos were used in glow 40-size and 60-size models. Those "standard size" servos would easily handle the control surfaces on this model, and I've found standard-size servos for as little as ten bucks apiece in the past.

Clearly the project isn't finished, and there is more to be learnt. Let's hope the next batch of students continue to work on those things. In the meantime, you guys did an excellent job getting the ball rolling, and the model flying. Hopefully you also learned a lot, and had some fun, and got some good stuff to put into your college application forms as a bonus.

-Gnobuddy

 

[cyclone3350]

I say congrats! It did demonstrate the ability to achieve controlled flight and barring a few bug to work out, I am sure the next group will have great success.

 

[gnandu]

We fixed a few things and attempted another flight -

We wired the wings and batteries in parallel to make sure equal voltage was supplied to each motor. Also, we replaced the old set of propellers with counter-rotating ones to diminish torque about the roll axis and uneven airflow.

Even though our pilot lost control of the rudder when its packaging-tape hinge tore mid-flight, differential thrust was enough to correct the plane's yaw. Unfortunately, the front landing gear snapped upon touchdown. Maybe a plywood truss structure and some aluminum brackets will better distribute the impact on landing. After two minutes of flying, the motors depleted one volt from each battery (2x 5000 mAh 4s).

Thanks again for all the help, and we are open to any suggestions/comments about the plane's structure and flight.

 

[Gnobuddy]

Gnandu and Horsie_Flights, it's fun to watch the progression of the project.

You may not need metal-gear servos, but you do need bigger servos, that can provide more torque, and tolerate minor handling bumps better.

For landing gear, a very stiff, rigid structure is not the way to go. The instantaneous impact force will be very high when rigid gear mounted to a rigid airframe hits the ground. What works best is to arrange for the wheels to have some travel under impact. This enormously reduces the peak forces, and that lets the structure cope.

Typically you want the wheels to be able to travel slightly backwards, and also slightly upwards, as those are the directions in which impact forces act during a landing.

The simplest way is to make the LG "legs" from something that has some flexibility. Steel music wire (aka piano wire) has been used for model aircraft for many decades, and works well. A wooden block mounted to the bottom of the fuselage accepts the tips of the wire. The wire is bent to sit in a transverse groove cut in the wooden block, running across the width of the fuselage before it bends down. The transverse part of the wire in the groove acts as a torsion spring (it twists slightly), allowing the wheels to move up and back in a hard impact.

This is difficult to explain in words, and astonishingly, I'm having a hard time finding some pictures to show you how it's done (though I found a *lot* of pictures showing you exactly how NOT to do it, with rigidly soldered wire landing gear.)

So I made you an image that hopefully conveys the idea. The wire shows only one music-wire "leg", for clarity. The other one is a mirror-image. The transverse torsion-bar section of the wires sit one behind the other in the groove cut into the bottom of the wooden mounting block. This lets the wires twist slightly on impact. Once both wires are in place (upturned ends inserted into holes in the wooden block, transverse section sitting in the groove), you will have to do something to keep them from falling out. Something like a scrap of plywood placed underneath the wires and screwed into the wooden block will work fine.

By the way, there are expensive ready-made landing gear sold, stamped from sheet aluminium. These are not only expensive, they are also a VERY bad design - the sheet-metal legs are extremely rigid in the backwards direction, so they transfer huge impact loads into the fuselage, and tend to rip out or damage the fuselage if you have a less-than-perfect landing.

If you want more of a mechanical engineering project than simple music-wire LG, take a look at the landing gear Louis Bleriot designed for his incredible Bleriot XI (the 'plane that first flew across the English Channel in 1909). The bottom of the LG struts has a backwards-angled pivoted link that can move upwards and backwards during the impact of a landing. Bleriot added a telescoping vertical strut to hold the angled link in position. I assume there was a coil spring inside the telescoping strut.


-Gnobuddy

 

[abobesh]

Hey everyone,

"The next batch of students" has a final update on this project!

After the graduation of our senior members, I referred to this amazing thread for how to continue improving our plane. This made my transition to leading the club very smooth and I learned how to assume the roles that the former leadership had left vacant. Thank you to everyone who helped and gave us resources (WebOcalc was super useful).


This video shows our successful flight yesterday (10/31/21) with a lot of the changes mentioned in the thread implemented:
-mg995 metal gear servos
-1/16" steel pushrods and linkage stoppers
-monokote wings (both longer and with greater chord: 3-piece assembly for ease of transport)
-larger control surfaces (connected with hinges instead of tape)
-reinforced landing gear
-6S 14000mAh LiPo powering dual 650Kv brushless motors
-motor mounts attach motors a good distance away from the wing's leading edge

Unfortunately, on our second flight, the straw broke the camel's back. We had super glued all the linkage stopper nuts onto the threads except for the elevator's because we were in the process of remaking the tail section of the plane at the time of gluing, which was badly damaged from wear and tear. We never got back to it. During the flight, the nut gave way and the crash ensued. Debris was scattered in a marshy area in long grass and a few inches of mud and water, with some electrical components ejected over 40 feet from the impact site (quite a few were lost or damaged beyond repair). The impact even bent the wing's two square aluminum spars! I laugh at the irony, in hindsight, because although we made sure to use overkill servos, push rods, and hinges to prevent any accident from occurring, a drop of glue - or the lack thereof - brought down the behemoth.

We are taking this opportunity to remake the design of the fuselage from scratch, which we were holding off on doing as long as it was still viable to use the old one. We also hope to upgrade the power system to boost efficiency and carry a heavier load. Furthermore, we will weatherproof our electronics and package them in bright colors to aid in future accident recovery. Overall, this crash has allowed us to start anew and proceed with unparalleled ambition.

This will probably be our last reply to this thread because I feel that it brings closure to the project that was lacking beforehand. Once again, thank you for the amazing replies and suggestions across the board - the ones we implemented worked wonders.

Sincerely,
abobesh

 

[Gnobuddy]

"The next batch of students" has a final update on this project!
<big snip>

And a very nice update it is, too!

That was a very nice first flight in the video. Too bad about the one tiny mistake destroying the model on the second flight, but that's how aviation is - even model aviation. Those of us who've built and flown RC models for any length of time usually have some similar stories to tell, of a sometimes seemingly tiny mistake we made, which resulted in the total destruction of a precious model.

Years ago, one of my favourite models had removable wings, which meant the aileron servo leads had to be plugged into the radio receiver after you mounted the wings on the fuselage. I got distracted at exactly the wrong time one night, and forgot to plug in the aileron leads. Worse, I rushed to the flying field after work the next evening, and in my hurry to get in a flight before sunset, did an incomplete control check on the ground: I checked that rudder and ailerons were working, but completely missed that the ailerons didn't move in response to the transmitter right stick.

Those two successive mistakes cost me a very nice model. It ended up as little toothpicks on the ground.

That turns out to be a very common scenario: most accidents the NTSB studies turn out to have been caused by more than one mistake having been made, each one compounding the problem(s) created by the previous one. A single mistake rarely causes a crash; but two or three in succession usually does.

The attached image is from an early aviation safety poster. The caution on the poster remains true today, a century after it was originally coined.

-Gnobuddy

 

[boogieloo]

Thank you for all the suggestions!

@Gnobuddy The motors are 650 Kv, and the wing has a chord length of 13.5 inches.

Due to approaching deadlines (this is a school project), I attempted a launch to show some progress to my lab directors. This launch had a flying weight of 4kg as I removed the payload and landing gear. Here's the video -

While the flight didn't go too well, this crash gives me the opportunity to step back and implement the redesigns you all suggested. I think flaps are a really good idea to help with takeoff. Also, I will probably use Flite Test power pods to mount the motors so that there is more space between the prop and wing (thank you @CarolineTyler !). I don't have the budget for beefier motors or batteries, but I do have two Flite Test Power Pack C kits lying around.

Unfortunately, I already cut out the ribs for the next wing, so I may not be able to lengthen the wing chord. However, I can look into making the wingspan longer. Would you recommend I proceed with the current power system and use bigger pitch propellers, or should I use the power pack C kits in a 4-motor wing design? Any other design questions or suggestions? Thanks again, this discussion is really helpful to me.

Congratulations. It flew a little while. At least you got the twin motors to start. Was this a school project for the whole class? 3 kg. That is almost like 7 ibs. Just about. 2.205 ibs. to 1 kg. So from there. How long did it take to make this? The airplane look good. But that U turn is a hard one to make. It looks good on the wing area. It doesn't look like you used clear tape for that job. It looks like plastic linkages.

 

[boogieloo]

I was reading up on the radios that they use. I don't think it's the radio giving less i guess radio power to the engine. The radio box sends signal to open and shut to the receiver that informs the servos to turn an arm up or down or left or right. Some people may just get a motor that never never stops. Even if they pull the stick down on the throttle. Was the battery use for pack c factory suggested? They go by ibs. on the similar model made. So that would be the battery you order. Your airplane could have weighed more in ibs. 3 kg is not that much. You ordered two more pack c editions. Wow. I have one more engine if I wanted to start over again. But I was wondering if anyone can over shoot on the voltages and actually burn up the engines from the start. I would think that method would be the most common. Going for the higher voltage reading on your battery. I've seen the airplanes the spitfire series, a guy used Amps and specially higher voltage to start his engine. I'm not sure why he needed specially made batteries. It was connected in parallel and specially made to fit his p-47. And it cost him a lot for that Tattu battery and a big one at that. Amps were higher.

 

[Gnobuddy]

Boogieloo wrote: "Some people may just get a motor that never never stops. Even if they pull the stick down on the throttle."

This can usually be cured at least two ways:

1) If the ESC is programmable, program its throttle range so that the motor remains off when the transmitter stick is all the way down.

2) If the ESC isn't programmable, program the transmitter and lower its "endpoint" setting on the throttle channel. This will turn the ESC off and stop the motor when the stick is all the way down.

-Gnobuddy