Author Topic: Trim question. (Read 3021 times)

Perry Rose · « **on:** June 02, 2019, 10:26:07 AM »

I have a Brodak P-40 ARF electric that stalls/mushes through the bottom corners of outside maneuvers. Metal flap horn, plastic elevator horn, carbon tube pushrod, 20 degrees up and down elevator movement and 15 degrees up and down flap movement. I've been lessening the flap movement from 30 degrees up and down with very little change in the mushing. Only on outside corners at the bottom corner. 5.1 second lap times. 62 and 61 foot lines eye to eye. Balance is at the plans/booklet location. Leadouts are set to 3/16" per foot nose down. Wing and stab incidence are zero. I don't have the thrust line to check the motor up/down thrust but it should be close. Plane is level after motor cut off.

P.S. what happened to the "At the handle" column?

Howard Rush · « **Reply #1 on:** June 02, 2019, 10:41:58 AM »

You mentioned both inside and outside. Either way, tape the flap hinge line if you haven’t yet. Then look for ridges or anomalies like wrinkles in the covering on the bottom of the front part of the wing (if the problem is on outside corners), or the top of the front part of the wing (if inside corners).

Dan McEntee · « **Reply #2 on:** June 02, 2019, 10:56:11 AM »

What do you have for a push rod? If it is wire, do you have a fair lead on it to control flexing? Like Howard mentions, tape the hinge lines. To check angle of attack and thrust, get three Robart incidence meters. Put one on the wing like normal, one on the stab, and they have a bracket for mounting to the engine instead of the prop. Set the wing at zero and see what the other two read. They are expensive new but keep your eye on eBay and swap meets and they come pretty cheap that way Well worth having three or four.
Type at you later,
Dan McEntee

Paul Walker · « **Reply #3 on:** June 02, 2019, 12:36:18 PM »

Sounds like the CG is too far aft. The CG on the plans is for IC motors. Move it forward 3/4 to an inch farther forward for electric. Leadouts for this should be 2 to 2 1/2 inches aft of the new CG.

Ken Culbertson · « **Reply #4 on:** June 02, 2019, 12:47:32 PM »

What Howard Said on sealing. That it itself may cure it.

My experience is that mushing occurs when you are flying too slow and you try and turn too hard. Not quite a stall, it just seems to lose all lift. The airfoil on that plane may not do well if you try and "bang" a corner. Might try opening the control movement up a bit if you can (20 degrees is not enough for my taste and this isn't a Ringmaster) and flying it faster and more nose heavy. If that doesn't work, get a Twister.

I've been at it a long time and so have you apparently. Sometimes we get a ship that just plane won't listen to reason.
They become Hangar Queens very quickly.

Ken

FLOYD CARTER · « **Reply #5 on:** June 02, 2019, 01:53:48 PM »

Make sure your pushrods are not bending. That would affect turn one way (pushing) but not the other way (pulling).

Perry Rose · « **Reply #6 on:** June 02, 2019, 02:17:23 PM »

I made a correction to the inside it's outside only. I'll check the covering, put some weight on the nose and re set the leadouts. The pushrods are not flexing but I wasn't sure about the plastic horns which is why I made a metal one for the flaps. I'll try it again in the morning.

Perry Rose · « **Reply #7 on:** June 03, 2019, 09:30:45 AM »

After the first few seconds of the first flight I could tell that the changes I made wasn't going to work. The mush on the outsides was replaced by a normal loop size radius. Attempts to get a better turn were in vain. After 3 flights I gave up. At home I removed 4 of the 5 ounces of nose weight and a half ounce of tip weight and reset the leadouts. I checked the thrust line and found one degree of down thrust so while I was at it I removed the shim and used it for one degree of right thrust. Why not? Tomorrow is another day.

Paul Walker · « **Reply #8 on:** June 03, 2019, 10:53:00 AM »

5 ounces!!! Wow.

What does the plane weigh??
What is its wing area?

Perry Rose · « **Reply #9 on:** June 04, 2019, 05:32:27 AM »

Quote from: Paul Walker on June 03, 2019, 10:53:00 AM

5 ounces!!! Wow.

What does the plane weigh??
What is its wing area?

I hadn't weighed it before now. Not much you can do with an ARF. Anyway it weighs 60.8 ounces with a Hyperion 3300mAh battery. wing area is advertised to be be 560 sq. in. which I figure is 15.6346 ozs./sq.ft.
A bit on the porky side. The balance is on the recommended spot with a 1.1 oz. spinner that was part of the nose weight I added. I'll try again in a few hours.

Perry Rose · « **Reply #10 on:** June 04, 2019, 05:34:49 AM »

Quote from: Motorman on June 03, 2019, 11:03:47 AM

I think this has a flap mounted horn? Is the connection of the push rod from the bell crank forward of the hinge point? If not you could be getting more flap one way or the other?

Motorman

I replaced the plastic horn with an aluminum one and was almost careful to get the pushrods over the hinge line.

Perry Rose · « **Reply #11 on:** June 04, 2019, 08:30:28 AM »

The stall/mushing problem seems to be gone but the outside squares at the bottom corners is a large loop at best. I made a handle adjustment and noticed a lot of right thrust in the motor that wasn't there before. The plywood plate that holds the motor came loose during the last flight. Repairs need to be made. I may drill an elevator pushrod hole aft of the the flap hinge line for more down than up. If that don't work it's off to the raffle.

Ken Culbertson · « **Reply #12 on:** June 04, 2019, 10:24:49 AM »

Quote from: Perry Rose on June 04, 2019, 08:30:28 AM

The stall/mushing problem seems to be gone but the outside squares at the bottom corners is a large loop at best. I made a handle adjustment and noticed a lot of right thrust in the motor that wasn't there before. The plywood plate that holds the motor came loose during the last flight. Repairs need to be made. I may drill an elevator pushrod hole aft of the the flap hinge line for more down than up. If that don't work it's off to the raffle.

IMHO you don't have enough elevator area for that wing loading. I hate giving up on any plane but some of them just won't listen. For some strange reason. my ships always start out life flying better inverted than upright. Probably a character flaw.

Ken

Perry Rose · « **Reply #13 on:** June 04, 2019, 11:01:43 AM »

I epoxied the plate back in place and added two balsa gussets behind it. It may have been loose all along but I don't think so. Anyway I have to start all over with the settings on the plane now.

Steve Helmick · « **Reply #14 on:** June 04, 2019, 04:23:49 PM »

Be careful not to complain to Brodak Mfg., or John will send you a second one.

It would be a good thing to FIRST check for any up thrust in the motor mount. Sounds like it to me. If that is ok, you might try putting some differential into the elevator to fix the reluctance to turn outsides. Since the thing is on the porky side, I'd start with 1:1 controls, and would be reluctant to try more flap than elevator, tho somebody will probably say that it works great for them.

Steve

Ken Culbertson · « **Reply #15 on:** June 04, 2019, 06:51:30 PM »

I should probably be arrested or at least banned here but one of the things I always do with a new ship once I have enough line tension is a "one line" loop in both directions. I don't really go to one line and I do it way up there but I do peg the controls if I can. It is one of the best ways I have found to detect up/down thrust. It also gives you an idea what the plane can do it you get into one of those "ah Sh**" situations.

You have to be a bit crazy to do this early on...I qualify.

Ken

PS - I do the outside from inverted. I'm not that crazy.

Howard Rush · « **Reply #16 on:** June 05, 2019, 12:29:00 AM »

So did you tape the flaps?

Perry Rose · « **Reply #17 on:** June 05, 2019, 04:55:53 AM »

Quote from: Howard Rush on June 05, 2019, 12:29:00 AM

So did you tape the flaps?

The gaps have been sealed all along.

Perry Rose · « **Reply #18 on:** June 05, 2019, 07:19:49 AM »

After drilling several holes in the flap and elevator horns to get more down than up, gluing the motor mount securely in place, changing the handle spacing as I anticipate quicker control response, adjusting the right thrust, changing to a white spinner from the red one and scraping the shark mouth water slide decal off the nose I was ready for another test session. First flight was an improvement over the other flights. Second flight I moved the down line closer to the center of the handle a bit and was able to get in a flight with just a hint of mush in the bottom outside corners. No more half loop. Any more antics from the plane and it's raffle for sure.

Mike Alimov · « **Reply #19 on:** June 05, 2019, 04:09:52 PM »

I’m curious, how did you deal with the vertical CG being about 3/4” above the leadouts? Without any drastic changes to the motor and battery placement, it wants to fly outboard wing down.

Perry Rose · « **Reply #20 on:** June 06, 2019, 05:17:32 AM »

Quote from: Mike Alimov on June 05, 2019, 04:09:52 PM

I’m curious, how did you deal with the vertical CG being about 3/4” above the leadouts? Without any drastic changes to the motor and battery placement, it wants to fly outboard wing down.

I never considered the vertical c/g. The wings have been level all along. The battery center line, all 12 ounces of it, is located on the wing chord line. I had built the kit version too, LA .46 powered, and didn't have a problem with the vertical c/g. I could have added some dihedral to the wing but the plans didn't mention it. I'm just happy I have something I can work with now.

Perry Rose · « **Reply #21 on:** June 08, 2019, 11:29:28 AM »

Before flying yesterday I removed the plastic spinner and except for the control geometry it's back to square one. One ounce of tip weight and 2 degrees of leadout sweep. I noticed a marked improvement in the corners but the steady level flight I had is fading. At home I put a Fox .35 prop washer in place of the aluminum one.
After 3 flights this morning I have it in the ball park. The controls are a bit touchy but I ran out of handle adjustment and the control horns only have one hole that is working. At home I made a new horn for the elevator, removed a half ounce of tip weight and added a LA .46 prop washer along with the Fox washer for a total weight of 1/2 ounce. I'm down to 2 3/4" line spacing at the handle.
The dimensions as of today are weight 59.4 ounces. C/G is 3.0 inches aft of the leading edge. The booklet says 2 5/8" aft and being a bit nose heavy at that setting so I think 3 is the spot it should be. I set the elevator pushrod 5/16" aft of the hinge line, the flap pushrod is 5/16 aft of that hinge line and the bellcrank to flap pushrod is 1/4" aft of the hinge line.
I cut a hole in the wing so I could see the bellcrank and it's real close to centered for equal travel both ways.
Just for giggles I checked the control surface travels. Flaps 25 degrees for insides and outsides. Elevator 25 degrees for insides and 30 for outsides. Something just don't make sense.

Perry Rose · « **Reply #22 on:** June 09, 2019, 10:09:41 AM »

The flights went fairly well today. At home I poked a hole in the wing bottom to see how far the bellcrank traveled in the up direction. I have a hole in the top to see the down travel. Right away I noticed that the arm hit the shear web and that the leadout cable was caught behind the arm. This was probably caused when I relocated the cable from over the pushrod to under it where it should have been. I fixed this before assembling the plane. I didn't notice the cable out of position. Now I have one leadout a half inch longer than the other. I made clearance in the shear web for the bellcrank arm, cut the crimp and wrapping it evened them up. Now the controls are off so I had to adjust the flap pushrod to zero them and a turn on the elevator pushrod to zero them. That should do it.
I removed a half ounce of tip weight yesterday, as suggested, and that brings tip weight to a total of a half ounce and the plane flew just fine.

Brian Courtice · « **Reply #23 on:** April 01, 2022, 10:55:53 PM »

Quote from: Paul Walker on June 02, 2019, 12:36:18 PM

Sounds like the CG is too far aft. The CG on the plans is for IC motors. Move it forward 3/4 to an inch farther forward for electric. Leadouts for this should be 2 to 2 1/2 inches aft of the new CG.

I've encountered this advice to fly electrics with a substantial forward CG several times.
I honestly cant make sense of it.
How does the airplane know that it's current and not combustion making the prop go round? It's quite possible to get an electric motor and an IC motor to turn the same prop on the same airplane at the same RPM. With a governor, even rpm changes with pitch change can be replicated.
The only difference i can think of is that IC motors are heavier than electric motors, so an IC model will have more mass closer to the prop thus farher in front of the CG than an otherwise identical and equally balanced electric model.
None of my RC airplanes ever needed a CG change going from IC to electric or vise versa.