chris de jon

Good Day
I have searched the forum to try and find an answer to this question.
Could someone please spend a little time to advise me,
What are the likely consequences, of flying performance(not build consequences) of lowering the engine say 1/2 inch below the Datum line?
The plans of my model show the engine thrust line along the datum line.
The model is a low wing similar to a Moth minor or PT19 but has a flying tail plane.
The engine if mounted head up, has a lot of ugly motor above the fuselage, lowering it will make a lot of difference visually. I would rather lower the motor than invert it.
Cheers and thank you
Chris De Jon

flynte

On my Kadet Senior kit, I lowered the engine 1/2" below and an extra 1/2 degree down thrust as I went with a .61 instead of the smaller .40 it asked for. I haven't noticed any ill affects from doing it, but with the airframe of the kadet I think you could probably do anything to it. This is my first year in the hobby, so don't take this as a definitve answer. I would seek out further assistance from more experienced ppl and different type of airframes. Just thought I'd share what I did to this plane.

flynte

Tall Paul

Lowering only a 1/2" won't have a significant effect.
Mounting the motor sideways can keep the top of the cowl line clean.

crasherboy

I would agree with Tall Paul. The only thing you might have to change is the thrust angle if you moved it down[or up]. Just look at some of the designs of that have been made. Like the P6E for one. See how low the engine sits in that one. then look at flying boats with their engines on top. Just have to have a thrust angle that will make them fly right. Hope this helps.

mesae

The effect will be minimal. I see no reason not to do it.

chris de jon

Thank you all for your replies. The following is interesting and I would dearly value some input.

I have now finished the model and flown it. I placed the motor 1/2" below the datum.
The maiden flight needed some 15 clicks of up elevator. The model flew nicely, no problems on landing.The wind was strong and variable to do much analysis, so I figured that with the engine thrust line being moved down and having to trim with up elevator that some engine up thrust may be desirable.
I added up thrust:- a washer on the bottom of each side of the engine mount on the firewall - approx 1ml (3/64")
I then had 2 more flights and needed to add another 3 clicks of up elevator.
In these 2 flights I performed only loops and rolls and was not able to adequately do too much serious testing as the motor is still running in.

The model is an 84" low wing, with a flying tail plane.
The engine is an ASP 180, with 16x8 prop. The model weighs approx 14lb and the CG is in the middle of the specified range.
I would appreciate some help
Cheers
CdeJ


The model proba

pimmnz

Change the tailplane to a good symmetric (non-cambered) airfoil, recheck the wing to stabiliser angular difference (1~3 difference, with the wing at the higher angle) and thrust line at 0 deg. to the stabiliser, ie on the same angle. Then fly it again. Oh, and Cg at 25~30% MAC, of course.
Evan, WB#12.

mesae

I agree that the characteristics you observe are caused by the positive cambered tail. It is lifting too much (or not providing enough down-force). That's why so much up-trim.

Raising the thrust angle has the effect of moving the CG backward under power. This frees the tail up to lift even more. Sounds like you need down-thrust, not up, but you need a better tail airfoil even more if you want to do aerobatics.

Incidentally, just because the tail has a positive camber, does not guarantee that the tail lifts. If your CG is at 25-30% MAC, it almost certainly is not lifting, but rather pushing down as is conventional, but with the disadvantage of having a positive camber.

Tall Paul

Up trim says nose heavy.
And you may be trying to fly too slowly.
(I see no reference to a cambered tail in the original messages.)

mesae

Admittedly this was an assumption on my part but I thought the OP meant positive camber (curved on top) by the above remark. I await correction by the OP.

If my assumption was wrong, then my suggestion may also be wrong since it was based on the above assumption.

I was trying to come up with a plausible reason why angling the engine further upward would result in an increase in up-trim required. I think I have one.

chris de jon

Good day,
Thank you all very much for bothering to answer. I don't understand OP. Could you please explain that one to me Mesae. Is the OP me? On rereading your reply I guess it is

By "low wing" I meant that the model has the wing underneath it. Like a PT19.
The section of the wing is the same curve below as on top. That is eliptical isn't it?

The stabiliser is curved on top with a flat bottom. (Would that be Clarke Y, tailplane?)

The model was very docile to fly. It slowed down beautifully to land. However I have not had sufficient flights to test properly. When I get some more time on the engine it will be better to tell.
The up elevator trim in the longer term may not even be an issue but I would like to have enough knowledge to be able to test to see what the likely problem is and then make some changes and see if performance improves or otherwise, and also pick up some useful knowledge of what can happen.
I thank you for your advice.
Cheers
CdeJ

pimmnz

I don't know what `OP' is either but a wing section with the same curve top and bottom is a symmetric (non-cambered) section. your tailplane section is probably just something somebody drew, a `Clarke Y' section is a specific cambered section which is not actually flat bottomed. Flying with up trim means you are reflexing the tailplane section, and lowering the effective camber so that it produces less, or maybe `negative' (downward) lift. If you wish to fly with neutral trim you will have to raise the leading edge of the tailplane. How much will depend on the balance point of the airplane, airspeed and the area of the stabiliser, ie, you will have to experiment. Full size pilots have an elevator trim wheel to adjust the tailplane lift for various stages of flight, model pilots usually don't have this control, but we use our elevator trim function as a coarse control of A/c speed.
Evan WB#12.

mesae

"O"riginal "P"oster.

Your airplane has a very unusual configuration with it's symmetrical main wing and cambered tail. As configured your plane is not meant to be seriously aerobatic, but gentle and slow. Unless you change the tail to one with a symmetrical airfoil, you will not be able to rid it of it's current tendencies, though you might be able to reduce them in accordance with pimmnzs' suggestion.

Airplanes with tails like your airplane's sometimes have the characteristic of nosing down at higher speeds, which I would expect your airplane to do, since the camber alone will cause the tail to rise with higher speed and the symmetrical main wing's lift increase depends only on it's incidence, not it's camber. It's even possible, again depending on configuration, that if flown at a sufficiently high speed, the nose would "tuck" and not recover no matter how much up elevator was applied.

The term elliptical is usually applied to the wing planform (shape as seen from the top or bottom) or it's lift distribution, not the airfoil.

RCPAUL

My Feedback: (1)

What in the original post makes one think the tail is undercambered? He said flying tail.

Paul

mesae

For simplicity, I take the geometric convention, rather than an aerodynamic one, and every time I use the term "positively cambered", I mean greater camber on the top surface of the airfoil than on the bottom.


I never stated the tail was undercambered, nor that I got the idea that the tail was positively cambered from the "original post". The Original Poster, chris de jon's reference to a "flying tail plane" a few posts ago, caused me to correctly assume he meant one with more curvature on top than on bottom. This agrees with his description of the airplane's flying characteristics. Positively cambered airfoils are not necessarily undercambered. A NACA 24012 has a positive camber, but is not undercambered.


I realize that a positively lifting tail does not have to be cambered at all (but might be much more efficient with positive camber), but as I mentioned earlier, a tail with positive camber may lead one to assume that the tail always lifts positively (which I believe many equate with the term, "flying tail" ) , but this isn't necessarily true, since CG and NP must be considered as well.


My real point here is that we really don't know for certain whether the tail of chris's airplane lifts positively or negatively in any flight condition. We only know that the airplane needs a lot of up-trim, and needed still more when the thrust angle was increased. We can say that the tail lifts positively at all times, but more so at high speed, or we can say that the tail lifts negatively at all speeds, but less at high speeds, or some combination. Still, we have a clue toward a hypothesis: If the tail was lifting positively, then raising the thrust angle would increase the nose-up moment, forcing the tail down, requiring down-trim. But if the tail was lifting negatively, and we raised the thrust angle of THIS airplane, it might free up the positively cambered tail to to produce a nose-down moment, which would require up-trim. Therefore, my hypothesis, subject to refinement, correction or abandonment, is that even though this tail has a positive camber, it is possibly lifting negatively at the speeds chris has flown it. This might indicate that the designer did not have a good grip on stability theory, though that is not meant as a criticism.



Chris, If it was my airplane I would at least try going to down-thrust instead of up and see if that helps. I know it seems counter-intuitive but taken with your earlier observation, it makes sense to at least try it. Just be ready to re-trim again.

Tall Paul

With a "lifting tail", the plane becomes a one-speed airplane. It will be trimmed at one speed. To get that trim.. very little elevator deflection, the c.g. will need to go aft.
It's basically a low wing Telemaster.

mesae

Or would be a low wing Telemaster if the main wing's airfoil wasn't symmetrical. That throws off the relationship between the wing and tail as compared to the Telemaster. With the Telemaster, as speed increases and the tail rises, so does the wing, since it's also strongly cambered. No so with chris's model, because as speed increases with it, the main wing doesn't experience as much of a lift increase in proportion to the tail, hence the 15-18 clicks of up-trim needed. Also, raising the thrust angle effectively moved the CG aft under power and that made the problem worse. I would want to know the tail volume, etc, before I moved the CG back much on this particular model, knowing only that the tail has positive camber. It might work fine, but it would be a lucky guess at this point.

mesae

Chris, what is the specified CG for this model in %MAC?

chris de jon

Thank you for replying.
The specified MAC including Ailerons is 30.7%. Either side of this + or - 10Ml is Ok.
I am going to try and attach scans of the wing section and tailplane section. I probably should have tried this from the start as it would make your help a lot easier.
Cheers and thanks
Chris De Jon

buzzard bait

Chris, the Moth Minor is a really attractive, unusual subject--congratulations on the build! Your wing has a "semi-symmetric" airfoil, meaning it is curved on the bottom similarly to the top, but not quite as much.

First of all, don't worry about the thrust line (an insignificant change aerodynamically) or the stab airfoil. The stab is what it is, and you don't need to take it off and rebuild to get a decent flying plane. It's probably quite close to the full scale design, and it flew OK!

Second, get rid of the up thrust. Up thrust is not advisable except in very rare circumstances (generally when the motor is on a pylon, making the thrust line exceptionally high). You need a down load on the stab, which you are getting by adding 'up' trim. That up trim effect (i.e., the down load on the stab) changes with the rate of air flow. More air flow, stronger effect. That means that when you add throttle, the tendency is for the nose to rise. That is the reason for DOWN thrust. Down thrust causes prop wash to hit the under side of the stab, counteracting some of the down load. The more throttle, the more it counteracts. Thus downthrust helps keep an even down load on the stab. Yes, I know that the up thrust helps reduce the amount of up trim, but the problem is that high throttle and low throttle will have big differences in trim with your set up. With me so far?

Third, do flight tests to fine tune the center of gravity. You may be nose heavy. The way to find out is to take the plane up a ways at medium throttle and trim for level flight. Then put the plane in a moderate dive and release the stick. If it swoops up, your plane is nose heavy, and you're having to carry too much up trim to counter it. Move the CG back a little and try again. When the plane recovers slowly by itself from the dive it is just right.

If the plane is not nose heavy, then I suspect you somehow got the incidence of the wing and/or stab incorrect during building. You can check those with an incidence meter, which can be a simple homemade device.

OK, now we want pics!

Jim

old git

I have always understood a flying tailplane to be similar to the Miles M20 or as on many sailplanes, no elevator but a variable incidence horizontal stabiliser.
I have always avoided this because of the difficulty of having exact neutralising of the control surface due to servo innacuracy or poor mechanical connection.

Use the maximum leverage from the servo, ie outermost hole and longest servo travel, the neutral position will be most accurate if maximum servo travel is used.




old git - - - - - - aka John L.

da Rock

"maximum leverage from the servo"..............................
An analogy:
You're going to clean out the gutters on a house. You have an extension ladder. You use it first to clean out the gutters that are over the porch. The ladder is easy to move for that 1 story reach.

Now you extend the ladder so that you can reach the 2nd story gutters. The ladder is still the same weight, but it's much harder to move around.

Now you have to get the gutters in the back of the house and they're 3 stories above the ground. You extend the ladder to reach and the first time you try to move it, you can't keep it from falling over. Why?

The leverage has gotten worse and worse as you extended the ladder. The weight of the ladder has moved away from you.

So now pretend that you are a servo and the weight of the ladder (which never changes) is the aero load (which doesn't change either). Move the servo load away from the servo with a longer arm, and the servo is less able to move that aero load.

If you want to give the servo some help, make the surface control horn longer and the servo arm shorter. This always gets somebody. This time it got the git.....

Rodney

As old git says, a flying tail is usually assumed to be a fixed surface rotating about some axis. The illustration does not show where this axis is and that is very important. In general flying tails present several difficult design problems, primarily getting enough strength in the hinge points to prevent unwanted displacement or bending under air loads plus making sure that the fore/aft placement does not cause serious problems in airodynamic balance.

old git

Sorry I didn't make it clear, outermost hole in control horn, innermost on the servo. This results in maximum servo travel, creates leverage and so greatest accuracy. I guess I was starting from the horn rather than the servo in my poor description.

I am suggesting that the control is most accurate if the full range of servo movement is used. That results in the most accurate neutral point. My experience with servos is thirty years old maybe I am out of date and they operate differently. I will be glad to be corrected on this point.



old git - - - - - aka John L.