Ryan NYP

[Prosper]

Very interestin'. Re. my previous post about getting an urge to model NX211 every summer, the tiny tailplane is one of the off-putting factors. Bill Dennis has said that he has few qualms about small tailplanes (my interpretation of Bill's words), and I agree with him - I've flown rubber scale models with tailplanes 14% of wing area without problems - but I don't remember if NX211 even attains 14%.

Stephen.

[TheLurker]

Bit of arithmetic and redrawing this evening. 

Running DeLoach's approximation over various permutations of tail plane size and it looks as though a 9% oversized (by area) tail plane will be about right for stability and won't look stupidly large.

I've increased the span rather than chord as I read somewhere (no, I really can't remember where), but it was years and years ago that a span-wise increase was more effective than a chord-wise increase.  Whether that's true or not I think a span-wise increase is less jarring to the eye

For those who need to know these things.

A 9% increase in area increases Vh (TVo) from 0.6 to 0.74 and gives an approx. CG position at 42% root chord.

A 4.5% increase in area increases Vh (TVo) to 0.66 and gives an approx. CG of 40% root chord.

DeLoach advises that for FF scale models that 0.66 <= Vh <= 0.75 is likely to give best results.

Of course this is all "wet finger in the air" with a large dash of haruspexy thrown in, but it's a bit better than wild guesses.

[Jmk89]

I haven't seen 'haruspex' since high school Latin classes (when it was not unusual for me to be thinking about something I'd read in the latest AeroModeller rather than the third declension).

[Prosper]

I haven't seen 'haruspex' since high school Latin classes . . .

I 'ent never sin it before. Forced to go to the dictionary. Lurk, the entrails of which animal did you use to make your divination? Not your pet cat, I hope. Re. the tailplane, I think that there are two things at play here - generally, a higher aspect ratio planform is always better for lifting surfaces; and propwash blanketing the tail surfaces is a bad thing, so extending the tailplane span out of the wash is a good idea. In your case I expect the whole tailplane will still be washed over, but perhaps less so at the tips.

Stephen.

[TheLurker]

...the entrails of which animal...

Weel, chickens are traditional and you can eat the bits not needed for the divination.  Waste not, want not.

[Piecost]

Whilst I agree with Prosper that increased span (aspect ratio) will increase the effectiveness of the tailplane. I respectfully disagree about  propeller slipstream;

At higher power the increased airflow over the tail will increase its effectiveness. But, this tends to be overwhelmed by the destabalising effect of the propeller.

On lower power the propeller may actuallynbe reducing the airflow over the tail (blanking it), but the much reduced destabalising effect of the propeller will be dominant.

So, the stability tends to be lower at high power.

This is generally applicable, but depends on propeller characteristics and airframe geometry etc.

I think that a smaller, faster prop tends to be better than a larger one as it has less destabalising effects and blows more air over the tail. But; I'd be interested to hear other opinions.

[Prosper]

Yes, that's it, as I understand it - the accelerated airflow over the tail surfaces makes them more effective in terms of static stability - but that same airflow from the propeller blankets the tail surfaces from the freestream airflow somewhat, thus reducing dynamic stability - responsiveness to gusts and turbulence generally. There are pictures of piston-engined planes in humid environments - typically on board aircraft carriers - where the helical propwash vortex is visible. I'd like to know whether the vortex diminishes in diameter as it travels aft.

Stephen.

[Piecost]

Good point Prosper; I tend to forgot dynamic stability. It makes sense that the the slipstream blowing over the tail deminishes the the change in downwash that thw tailplane see, as the aeroplane pitches. Yes, the prop wash does contract slightly begind the propeller. I would need to check momentum theory to quantify it. But, this is digressing from Lurkers nice model.

[TheLurker]

I was going to say, "Digress away!" as I do like a thread that rambles, but it's not really a digression.  It has a direct bearing on the likelihood of getting the end result to fly.