flandrumjr

Has anyone given this a try;
Mix up-elevator with the throttleat idle tomid throttle (depending on the plane and power plant). The idea is to add a predetemined amount of up-elevator at lower speedand tokeep the plane levelas you pullback on the throttle during normal flight. Push the throttle forward, the mix comes out and your back to your normal flight. On landing the mix should keep the plane level and at touch-down the tail wheel (tail drager i.e. warbird) should hit the runway sooner and keep the tail down somewhat during low speedtaxi. Take off souldn't be an isse since you would be at or near full throttle. I'm not the type that works the throttle much during flight. I usually set it @ 3/4 or so untill ready to land or flying with 1/2 flap.
I mixed this in with my DX 8 and it works well on the bench. Test flights will no doubt determine the validity of such an idea. I would like to hear if anyone has tried this or advise against it for obvious reasons.

RCKen

My Feedback: (9)

To be totally honest with you, that's something I wouldn't be comfortable mixing in. There are times that my want to give a throttle increase without having the elevator changing. IMHO there are times that could really cause more problems than it fixes. What this really boils down to is flying a scale plane. There are lots of small things like that that we "could" program into the radio, but "should" we? The way I look at it adding in a little up elevator in certain circumstances is simply a part of flying the plane. So in that case you need to weigh in what you was a pilot wants. Since I don't always want elevator with a throttle increase, then for me I wouldn't make that programming change. In your case you are going to have to look at the way you fly and make that decision on your own. If it's something that won't cause a problem for you as you fly, then you can sure try it. And if you don't like it you could take it back out. But like I said, it's not something that I would do for my own flying.

speedracerntrixie

My Feedback: (29)

If an airplane drops it's nose dramatically with power reduction it's usually nose heavy. Moving the CG back in small increments will help the situation and will actually lower landing speeds slightly.

combatpigg

My Feedback: (3)

I teach people to fly with a slight amount of down trim..so they must maintain a small amount of "back stick" to maintain level flight. This gives the student the feeling that they are flying the plane 100% of the time, plus if they get unaware of an impending stall, they can release the stick and allow the plane to self correct instead of pushing the stick foraward into some sort of a "panic mode" reaction.
They are also taught at constant power settings [for similar reasons] until they have proven to have mastered the level of power that they were given.
Learn to fly with your thumbs and your eyes.. You can't "program" your way to competence like it is some sort of a video game.

jester_s1

I've done it on two different planes before. One was a Phaeton biplane that has the thrustline wrong and I didn't want to change it at the field. It would nose up (not just rise in altitude a little but genuinely nose up and continue to rotate until I adjusted the elevator) It took a little bit of tweaking to get the mix right but within one tank of fuel I had it right. I also did it on a foamy Stryker that had the opposite issue. I found that it did solve the trim issue when I stayed at a constant throttle, but the problem showed up with quick throttle changes. If I was going wide open in the biplane and let off the throttle, the plane would balloon up until it bled off some airspeed because of the mix. The opposite happened if I cruised around slow and then gunned it; the plane headed for the ground. The Stryker did the opposite, understandably so given that it had the opposite issue. I found that correcting for the mix was more trouble than just correcting the trim change anyway, but for a constant throttle flyer I suppose that would be far less of an issue. The right solution for me was to get the thrustline adjusted right for both models so that throttle changes wouldn't affect the pitch. The biplane still has a slight trim issue related to speed (which is a separate issue that is partly being a little nose heavy and the rest being in the design itself) but it is only maybe a 15-20 altitude change in maybe 600 feet of flying. The wind messes with me more than that, so it's ok.

jester_s1

To try and solve your issue, do you see a noticeable nose up or nose down when you gun the throttle or when you back off suddenly? If so, you have a thrust line issue which is pretty simple to correct. But if you don't see any pitch change tied to throttle changes but rather a gradual increase of altitude when the plane is going fast and a decrease when it's going slow, you may have a CG or an incidence issue. It also matters what kind of plane you are flying. A flat bottom wing is always going to be speed sensitive, at least as long as you keep a forward enough CG to have any degree of stability. Symmetrical wing planes usually can be trimmed to be very close to perfect in a wide speed range and hold their pitch in all throttle positions.

AA5BY

Mixing elevator trim to satisfy trim needs for power off/on is common. As Jester said, traditionally that has required thrust changes with the engine but it can now be done with mixing, which is especially nice when faced with a thrust issue on a plane with a spinner aligned with fuselage.

What you are proposing if I read correctly is to increase angle of attack of the wing at low speeds to gain lift. The problem I see is that instead of a parallel course where more drag is accompanied by more power to maintain equilibrium, the scheme suggested will see drag go up and power go down and the obvious crossing of the threshold into a danger zone.

What your wishing to do... is usually done by flaps, where lift is created at slower speeds but remember that flaps usually require keeping some power on. On planes with flaps, I'll mix a slight throttle increase with flaps along with proper elevator trim to maintain pitch stability.

jester_s1

It doesn't solve the problem though. It actually creates an opposite problem that is more difficult to adjust for than the original one was. An elevator mix is no substitute for having the thrust line set properly.

flandrumjr

Thanks RCKen.
The mix is set for low to moderate throttle onlyso as you increase throttle the elevator returns to the original trimed position. You also have the option of turning the mix off and on as desired by programing the mix to a switch. I see this as flying the plane at crusing speed by having a little elevator on the throttle in stead of the elevator stick and/or elevatortrim. Kind of like have a high speed trim and a crusing speed trim without the constant manual input of up elevator. For landing you can turn the mix off and use flaps at their usual setting.

Thanks again for your input.

flandrumjr

Thanks combatpigg.
I'm sure you have instilled a lot of compentency in your students with your approach to the skill of RC flight.
My simple approach is to add(1) single element to the RC experience and that is to be able to have and maintain acrusing speedjust by simply pulling back on the throttle.The pilot still has 100% control of the plane but now has a crusing speed to master.

Thanks again for your input.

flandrumjr

Thanks jester_s1

I don't have a thrustline problem or nose heavy aircraft. Just want to experiment with the idea and possibly have a crusing speed trimed in.
I see what you mean about the sudden increase or decrease in power with this methoid, but I think that by applyingup or downelevator on the stickyou should minimize the sudden up or down movement once you have flown a few times with the propermix.I have a symetrical wing and the CG is slightly forward but stable. Good to know someone else has experimented with the idea.

Thanks agin for your input.

flandrumjr

Thanks AA5BY

I see what your saying. As mentioned above I'am hoping to fly (cruse)at a reduce speed but not so slow that I am reaching the point of stalling and without flaps. I do use flaps in 1/2 and full positions with elevator mix. In the case of using flaps, I would probably turn the throttle / elevator mix off.

Thanks again.

AA5BY

Sure it is. A down or for that matter up thrust angle is a counter to an undesirable flight characteristic. For example, a very common problem for a non symmetric foil is the changing lift with changes of speed. Traditionally that undesirable has been countered by down thrust to correct the lift when power is applied. What opposite problem is created by correcting climb with elevator mix as opposed to down thrust?

jester_s1

There are two different issues that can cause pitch changes with speed changes. One I'll call "speed sensitivity" the other "power sensitivity." If the thrust line is wrong, there will be an immediate pitch change when power is applied or taken away, most obvious in a sudden throttle change. The actual application of power is causing the pitch change because the thrust from the engine is pointed in the wrong direction in relation to the drag of the airplane. That's power sensitivity. Some planes though by virtue of their design can't help but gain and lose altitude with speed changes due to their lift characteristics. The worst offenders here are the planes that are designed to make lots of lift at low speeds like trainers and Cubs. The issue in that case has nothing to do with the thrust line and everything to do with the CG and wing and tail incidences along with the wing airfoil shape. Identifying which issue a plane has is the first step in actually fixing it instead of applying a bandaid.

The problem with using an elevator mix with the throttle to fix either of these problems is that changes in the plane's speed lag behind changes in throttle. So if there is a thrust line problem, the plane will react to the power first, and then as it builds speed it will start to even out (not completely, but at least some). So fixing it with a mix produces a plane that responds properly to throttle changes but flies out of trim once airspeed catches up. If it's the speed sensitive problem, you'll actually get the opposite effect- the plane will fly in trim once airspeed catches up but will pitch with throttle changes. Programming in a mix is a fix for the very rare plane that is flown at a constant throttle setting for most of the flight and has an aerodynamic design that simply cannot be trimmed to fly straight at all throttle settings. The OP will be happier with his plane if he ferrets out the aerodynamic problem that's causing his trim changes and fixes it rather than applying an electronic fix to cover it up.

AA5BY

I'm in agreement that if the thrust line has an angle and if it is causing problems... then best correct that rather than mix elevator and throttle.... but that is not my argument. I'm assuming a given that the pitch thrust line is zero and the elevator is trimmed correctly for a power off glide slope but a plane has a pitch trim problem under power. Or... the plane has a pitch thrust angle but needs more correction in that direction. I'm also assuming the plane is balanced properly with proper decalage and I hope that word doesn't engender an argument that decalage only pertains to biplanes (while it once did... it no longer does).

In the above given, it may be corrected either way... by a thrust adjustment to counter the power on condition or an elevator mix to counter the power condition. The TX mix is a great substitute for doing it with a thrust angle and far easier. My personal experience is that I've used such mixes several times and haven't sensed any timing issues though I'd never argue that they don't exist as I take you at your word that you have experienced them.

I will say that when I do the mix, I actually do it the backwards in that I trim the plane for full power and then mix elevator for proper off power glide slope as it seems easier to set the mix that way though it can be done either way. Traditionally, a plane was always trimmed for proper glide slope and then pitch thrust line adjusted to suit power... but that is because we used to have to do it that way because it was the only way possible.

Just like reversing servos, or setting throws... modern radios make life easier for us and one way they do that is we are no longer forced to physically adjust an engine pitch thrust angle for proper power on trim.

jaka

Hi!
I have not tried it or have had a desire to try it! Because it's totally unnecessary if you have the plane trimmed out correctly!

jester_s1

A nice thing about the hobby is that if it works and you're happy with it, then it's good! I haven't been happy with either of my attempts to use mixes to fix an elevator trim issue that was related to throttle setting, either thrustline or speed sensitivity. If someone else does, there's not a thing wrong with it. I'll agree with you on the topic of initial trimming. It's a lot easier to get the plane flying straight and level at full throttle and then adjust things to work at lower settings than it is to get it right at mid throttle and then try to adjust to make everything else work.

flandrumjr

Thanks jaka.
I agree, it is not a necessary element for typical flight for a person, like myself,who enjoys scaleflying and has the plane correctlytrimedat a particularspeed, which is the primary reason for exploring the idea. I would like to have the ability to reduce speed, somewhat, and maintain elevation by having a little elevator automaticly "trimed" in when the throttle is pulled back. I,m notlooking for a way to balance the rate of speed with rate of elevator through the entirerange of the throttle to maintain elevation. That would bevertually impossible I would think,just slow the plane a little without having to constatnly apply up elevator or deploy flaps to maintain elevation.
Correct me if I wrong, but when in "auto pilot" on modern full scale planes (I'm not a "real" pilot), when the power is reduced or increased for that matter (to a certian degree), the elevator and / or flapsare automaticly adjusted without pilot in-putto maintain the desired elevation. Like AA5BY said "modern radios make life easier for us" so why not enjoy andexplore the possibillities.

Thanks for your resonce.

To AA5BY and jester_s1;
Very interesting discussion, I have learned alot from you exchange. As stated eariler, all my plans (scale models), to my knowledge, don't have a seriousthrust line problem and are balanced near the recommended CG andto my particular methoid of flying, just experimenting with an idea. Thanks for your insite.

AA5BY

An observation on a Spacewalker might serve to illustrate. The design has a generous low wing with a high thrust line, and given a more than scale power plant results in a down pitch couple between the drag of the wing and the thrust of the engine. Up elevator is needed to trim the plane for full power to counter the pitch couple but when off power, trim needs returned to neutral elevator.

So... should that couple be countered with up thrust? Sure... it would work... but up thrust looks really goofy plus the Spacewalker has a spinner that fits to a forward cowl boss and adjusting it would malign the spinner and cowl unless the engine mount was moved on the firewall a small amount and that would require slotted mounts and perhaps several efforts to shim and slide the mount after removing/reinstalling the cowl each time. It could be done with considerable physical effort, but why when it can be done far easier electronically within the Tx and we wouldn't be making the plane look goofy.

flandrumjr

Good point AA5BY.
If you have a minor issue with a plane that can be corrected by electronic in-put, go for it Magor issues, such as proper alignment of wing and tail, balance and thrust alignment should be addressed and accomplished initially on the bench. Once you have done your best at that, and trimed the plane for your style of flight, use the advanced technolegy to do the rest including experimenting with "cruse cotrol" I'll call it. After all, why do you think the TX maufactures give you the tools? Someone thought it was a good idea to expand the options and give so many mix applications. A lot of rc pilots must agree cause they are discarding their old TX and buying the advance TX.

speedracerntrixie

My Feedback: (29)

IMO if the airplane is trimmed so that at lower speed it requires up trim and higher speed it requirers down trim then it is not trimmed correctly. I get where you are going with wanting to use mixes to compensate but in reality a mix for your CG being too far forward is not going to work. The problem is that the mix is instant and your airspeed changes are not. The idea would be to set the CG so that anything from about 1/3 power to full power does not induce a pitch change. Right now I'm betting that you carry some up trim. At full throttle the trim is more effective and makes the model climb. At lower throttle settings the trim is less effective and the model dives. Move the CGback some and I'm sure you will see what I am saying.

jaka

Hi!
Agree!
I well trimmed trimmed airplane doesn't need any mixes or to fly straight at any speed!
A trimmed airplane remains in forward flight with just slight elevator input.

Remember that a model airplane should always be trimmed at full throttle/speed so that plane flies in a stright line ....and trimmed (by changing the Cof G) so that it remains flying in a straight line, without loosing too much height, at nearly every throttle setting!

flandrumjr

I don't have a CG issue. The plane is balanced.
Example; The plane is trimedand balanced@top speed and I am fly @ top speed. The plane is flying on rails,all isright with the world. Now without the throttle/elevator mix and asI pull back on the throttleto say 1/2 throttlethe planewill beginto slow, as it slows the plane begins to gradually loose altitude due to the lose of speed.At this point, I could manually apply up elevator and continue flying at the reduced speed, or before flight, I could mixup elevator with and atthe lower throttle speed, then the plane should fly straight and level without the constant manual input of up elevator.Am I missing something????
It will be trail and error to obtain the correct throttle/elevator mix. I think it is no different than mixing used by sport pilots and the mix can be turned on and off. Hope to try it soon as the weather cooperates.

jester_s1

If half throttle is slow enough that the plane descends even though it's holding a straight attitude, then you're right- trimming isn't going to counteract that. But if 1/2 throttle is still fast enough for the plane to cruise you should be able to fix that with trimming. You haven't said what procedure you've been through with trimming the plane, so I'll offer a few things to check if you're so inclined.

Start with the plane trimmed for perfect straight and level flight at full throttle and don't touch the trim until you're done. Do these tests on a fairly calm day.

Make a full throttle lap so the plane is up to speed, then quickly chop the throttle. If the plane continues flying straight and level for 2-3 seconds then gently drops its nose as the airspeed reduces, your thrustline and CG are correct or at least close. If you notice an immediate pitch change, you have a thrustline problem. If the plane doesn't drop its nose but does lose altitude, you're tail heavy. If it drops its nose before slowing down, it's tail heavy. With this test you are checking for how the plane flies as a glider, in other words, you're testing its sensitivity to airspeed. Adjust thrustline and CG to get the results described in the first line.

After doing test one and trimming for the right results, get the plane up high, cut the throttle, and hold elevator until it's nearly in a stall then push to a 45 degree dive. Watch for pitch changes. If the plane pulls to the canopy, you're nose heavy. If it pulls to the gear, it's tail heavy. Here's you're checking the glide again but this time in an opposite way by having the plane accelerate in a dive. This test is more sensitive to CG issues than the first one is. Make note of the results but don't make changes until after test 4.

Get a feel for how the plane lands. If you are able to maintain elevator control all the way through the stall to the ground, your CG is close to right. If the nose drops even with full up elevator you're nose heavy. If the plane flares all on its own as airspeed drops you're tail heavy (also will cause tip stalls on landing with planes prone to them).

Fly inverted at full throttle and note how much elevator is needed to maintain straight and level. You'll want to make a fairly long run inverted to really get a feel for what it takes to hold the plane at a constant altitude. The up elevator should be fairly small, but how small is determined by how much throw you are running. I'd say if it's more than 1/4 of your low rate then it's too much. Excessive up elevator is indicative of either a CG problem or a decolage problem. Adjust CG based on the results of tests 3 and 4 then retrim for straight and level flight at full throttle.

Check decalage (wing incidence): if the elevator is level or very slightly down, your decalage is probably fine. If it is up after all of this, your incidence is too low. Many pilots are only happy if the elevator is level at this point, but I found that some planes will have less speed sensitivity when it sits down just a degree or two, since it raises the tail a little at high speed and lets it drag a little at lower speeds.

Optional for decalage setting: For aerobatic planes, do a knife edge pass. A pull to the canopy or gear MAY indicate a decalage issue, but exactly what it indicates is specific to the exact plane you're flying.

If you work through all of this with most planes, you'll find that pitch/speed sensitivity will be negligible. Some designs won't even out no matter what you do (trainers are notorious for speed sensitivity, but they are designed to be) but most any sport or speedy scale plane will trim out nicely enough that the effects of wind turbulence will do more to cause pitch issues than speed sensitivity will.

jaka

Hi!
How do you know the plane is trimmed out right? Have you checked other CG positions?