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Incidence on a PT 22

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Incidence on a PT 22

Old 08-18-2008, 04:17 PM
  #1  
Richard m Crapp
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Default Incidence on a PT 22


Hi Guys,
I have built a 33% PT22 and it is nearly ready to fly. One thing still puzzles me is the Engine-Wing -Tail incidence on the real one.
I have set mine the same but why is it like this.
The Engine is 0 deg to center line, the wing 3 deg positive but the tail is 3 deg negative (Up)
This was a 1940's trainer and Mr Ryan must have known what he was doing.

It has 4 deg Sweep on the wings and 4 deg Dihedral, Scale section something close to Clark Y. The tail is not trimerbol but it dose have reasonable trim tabs.

The building plan and other models i have seen have the GC at 33% of the ROOT. not what you would expect from the swept wing. The PT20 that had a straight
wing also had the tail at 3 deg neg. The sweep was apparently because the US Army wanted something a bit more demanding or dangerous.

All the aircraft i have ever seen or studied to build models have the tail at 0 to the center line or something close to it but perhaps I am wrong, But why the 3 deg on the PTs.?

Picture if it appears is the subject aircraft.
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Old 08-18-2008, 06:12 PM
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HighPlains
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Default RE: Incidence on a PT 22

The wing sweep was most likely to offset the different weights of the engines used. The aircraft with the lighter engine would have the sweep.

Incidence of the wing is so it has the right angle of attack at flying speed for the fuselage to be level.

I suspect the tail incidence offsets both the wing sections negitive pitching moment and downwash from the wing.

While this particular decalage may work well for the full size, they may not be right for a scale model.

I have actually had the chance to fly from the front seat of a Kinner powered PT-22 some years back. I was at an airshow in California looking the airplane over when the owner asked me to sit in the front and hold the brakes and flip ignition switches while he hand propped the engine. Then he offered me a ride to his home airport. Well the Kinner is a pretty low RPM engine, something on the order of 2000 rpm at full power. We taxied onto the main runway, and did not really take off, but it seemed like we levitated instead.

After we left the airport airspace, he turned the controls over and I headed toward Livermore over pastures and gentle hills. Since the top cylinder blocks your view forward, an occasional heading change clears the airspace ahead of each course change. The great thing about the Ryan, is that it is built like a tank, with very thick skins on the fuselage. The sounds of the Kinner chugging along (5 cylinders), slight buffetting of 100 mph breeze, and the smell of flowers growing 1000 feet below kind of sticks with me.

Well the 30 miles to his home base was entirely too quick, and we landed with a soft touchdown and taxied back to the hanger. After the engine shutdown, we both climbed out and discovered about a third of the fuselage was covered with engine oil. Somewhere over the course of the flight, something knocked a hole in one of the rocker arm covers on a lower cylinder. While the engine never changed sound, we might have been very close to an off airport landing.

Anyway, it's a great airplane from the 40's, and I would love to see the model.
Old 08-18-2008, 06:48 PM
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da Rock
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Default RE: Incidence on a PT 22

Well, first of all, 4degrees LE sweep really doesn't turn a wing into much of a swept wing aerodynamically. The wing actually looks like it has a bit of taper but that's not really very much either.

Highplains got the rest covered, but I'd like to stress something he mentioned. Models really don't usually have the downwash from the wing their full scale counterpart has. Our models have nothing like fullscale wing loading. So the model wing creates less of a downwash angle. Matter of fact, the model wing won't operate at the same AOA. It'll usually need less incidence as well as producing less downwash. The negative incidence of the horizontal stab won't be needed as much on the model for pretty much the same reason the full scale wing incidence isn't needed.

(edited.... Thanks Highplains. I rewrote this to save others from wasting time.)
Old 08-18-2008, 08:29 PM
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HighPlains
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Default RE: Incidence on a PT 22

I believe that the wing is of a constant chord design. The entire wing panel is swept back 4 degrees, 10 minutes. It is thought that this was to correct the balance, since the radial Kinner mounted to the firewall had less moment than the earlier inline engines used on the Ryan design.

For instance, many of the surplus PT-17's had their upper wing moved forward about 10" when the small engines were replaced with R-985's to maintain the proper CG.
Old 08-19-2008, 08:35 AM
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khodges
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Default RE: Incidence on a PT 22

Richard, I can offer no recommendations about setting the correct tail incidence for your PT-22, but I will echo what daRock and HighPlains said about having it negative. I'm currently building a Cessna L-19 and noticed on the plans (Vaillancourt) that the tail incidence was set at zero to +1, and the fullscale is about -2. When I e-mailed Roy Vaillancourt about this difference (as well as other deviations from true scale), his reply was that the tail incidence was changed to make the plane flyable. His exact words were, "Don't change the tail incidence from the plans, or you'll only fly it once".

I suspect it would give the plane a tendency to pitch up, similar to a tail-heavy trim, and you might not have enough elevator authority, especially at slower speeds, to overcome it.
Old 08-19-2008, 09:04 AM
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Richard m Crapp
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Default RE: Incidence on a PT 22

Thanks guys,
High plains, wonderful story.
I will try and answer some of the points above.
The wing has no taper or change in section.
I believe the sweep back was demanded by the US Army purely to make it a more demanding trainer although their was also an engine change to the Kinner.
I have seen model plans such as the Sig PT20 where the tail is 0 to the center line, still 3 deg on the wing.
Picture below is A Swedish PT22 at the resent World scale championships in Poland. You can quite easily see the 3 deg tail incidence. My model is almost identical.

Is the down wash explanation sufficient to explain the 3 deg tail and would this necessitate the 33% cg at the root, given the swept wing.
If it is nothing to to with the swept wing why is this set up so unusual.

Their is a on line build for my model at http://www.rcscalebuilder.com/forum/...et=last#221056

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Old 08-19-2008, 11:31 AM
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Tall Paul
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Default RE: Incidence on a PT 22

Modest amounts of sweepback were commonly used to fix c.g. problems.
It can be easier to do than to redesign the fuselage structure for a more aft wing installation.
The Short Sunderland for one.
Old 08-19-2008, 11:55 AM
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hogboy52
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Default RE: Incidence on a PT 22



All wings have downwash, which is what makes the plane go up. The negative stab would offset the forward cg which together would make the plane more stable in pitch recovery. It might also compensate for what appears to be a high drag landing gear design.
Old 08-19-2008, 07:58 PM
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da Rock
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Default RE: Incidence on a PT 22

Generally, the incidence of the horizontal tail provides a trim function. At the design speed and payload the wing will create a predictable pitching moment. The AOA of the tail will need to be set to deal with that pitching moment. The incidence of the tail is basically trimmed to suit that. In the same way the AOA of the wing at design speed and payload decides the incidence of the wing to the fuselage, the tail is done. Only in the case of the tail, the downwash is an additional consideration.
Old 08-20-2008, 09:05 AM
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Richard m Crapp
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Default RE: Incidence on a PT 22

Thanks for all the replies, Explains it nicely. It would seem it will only be in trim at one speed. out of that range it will need retrimming for straight and level.
Explains the big trim tabs.
Old 08-20-2008, 09:26 AM
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da Rock
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Default RE: Incidence on a PT 22


ORIGINAL: Richard m Crapp

Thanks for all the replies, Explains it nicely. It would seem it will only be in trim at one speed. out of that range it will need retrimming for straight and level.
Explains the big trim tabs.

You're spot on.

Look at most commercial birds. The big ones have trimmable stabs. It's cleaner to trim the stab/elevator as a unit, keeping the elevator to stab deflection at 0, so they do.

If you read a lot of aviation history, you'll run across a mention every so often of the changes that're done to prototypes. It's not uncommon to read how the vertical stab was move a bit to deal with takeoff effects on overpowered fighters. And how they changed the horizontal stab's incidences to better work when they discovered what the actual cruise speed was going to be. There are also numerous small planes that have ground trimmable stabs. The cheaper and simpler the full scale the more apt they are to be factory built in, but a step above those are the ones with ground trimmable stuff. You also see it on types that offer different horsepower engines (different speed envelopes) from the factory.

It's pretty much a trim deal for sure.
Old 01-09-2015, 08:02 AM
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RickVB
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For historical completeness, the thread over a RC Scale Builder of Richard Crapp's PT-22 build is actually here. Note that you need to register over there before you can see it.

Last edited by RickVB; 01-09-2015 at 08:05 AM.
Old 01-09-2015, 09:03 AM
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Richard m Crapp
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I got the notification the thread had a new post.
The PT22 thread is nearly 10 years old!
The model still flies and has actually been on a trip to USA last year.
It still has a sharp stall particularly if to much rudder is used and if the wing has not been ridged with the correct wash out.
A Wessex Ti-Motor is resting for the winter while a Gladiator is built. All on RC Scale Builder.
Old 01-11-2015, 08:08 AM
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I appreciate that this thread is very old and found it interesting to read nevertheless. My simple addition is that I'm happy the plane yet survives and has been enjoyable. As well, I'll note that looking at the undercarriage, it must have inflicted a fair degree of drag couple affecting the pitch.
Old 04-15-2015, 06:37 AM
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jossurf
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Default Re. PT22

Originally Posted by Richard m Crapp View Post
I got the notification the thread had a new post.
The PT22 thread is nearly 10 years old!
The model still flies and has actually been on a trip to USA last year.
It still has a sharp stall particularly if to much rudder is used and if the wing has not been ridged with the correct wash out.
A Wessex Ti-Motor is resting for the winter while a Gladiator is built. All on RC Scale Builder.
Hi there Richard!

Searching on the net I found this ancient discussion on the tailplane incidence of the PT-22...

I have a 90"+ one-off copy of the likewise one-off 1941 ST3KR Super Ryan and it flies like SH**!
It has absolutely no stability and will not restore from any initiated attitude, both in pitch and roll. Needless to say mine too is very delicate on the approach at lower speed and it too tends to "tip-stall".
It's constantly tail heavy and needs a significant amount of down-trim to keep level. I've considered to add more weight but since it's powered by a 7-cyl. 70cc Seidel radial and therefore short nosed it would need a lot of lead up front.

So right now I'm in the process of re-rigging the wing in order to give it more downwash.

Any tips you might have for me pse?

Will try to add a shot of the real thing and one of the model..

Jos Frusch

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Old 04-15-2015, 07:53 AM
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AA5BY
 
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A lot of room for discussion about your plane. Is the elevator sensitive? You could run the numbers using the aircraft CG calculator using a 15% static margin and compare that with where you are currently balanced. If the CG is in the zone, the significant down trim suggest an incidence problem and they often manifest differing trims for powered level flight and the landing glide slope.

Some ideas. Can the wing or stab incidence be adjusted to make the elevator more neutral? If the ailerons are on separate channels, re-flexing them up 3/16 at lower throttle settings would likely reduce tip stalling.
Old 04-15-2015, 02:31 PM
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Richard m Crapp
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Originally Posted by jossurf View Post
Hi there Richard!

Searching on the net I found this ancient discussion on the tailplane incidence of the PT-22...

I have a 90"+ one-off copy of the likewise one-off 1941 ST3KR Super Ryan and it flies like SH**!
It has absolutely no stability and will not restore from any initiated attitude, both in pitch and roll. Needless to say mine too is very delicate on the approach at lower speed and it too tends to "tip-stall".
It's constantly tail heavy and needs a significant amount of down-trim to keep level. I've considered to add more weight but since it's powered by a 7-cyl. 70cc Seidel rotary and therefore short nosed it would need a lot of lead up front.

So write now I'm in the process of re-rigging the wing in order to give it more downwash.

Any tips you might have for me pse?

Will try to add a shot of the real thing and one of the model..

Jos Frusch

There are a few things going on with the PT22 . If you imagine a fuselage Centre line with the engine on zero the wing should be + 3 deg and the tailplane at - 2 deg. (Trailing edge up) This is partly to counter the gear tredels that give a nose down force. Also because of the very foreword C/G. I don't know why it is but perhaps the 4 deg wing SWEEP contribute or the foreword position of the wheels.

If your model flies badly I would put the CG at 25% at the root and give at least 1 deg WASH OUT. (twist it in with the wires) Add a bit more with the ailerons.
Be prepared to run out of up, or flair, on landing.
If you do, move the CG slightly back until you can flair then stop.
On my model the CG position is on the back of the strut fillet from the Fuselage to gear . It MUST be their or it will never fly well!
To help add weight try a solid steel spinier nut or/and a huge fuel tank.
Hope that helps, yours looks a lovely model. I would like to here more about it.

Last edited by Richard m Crapp; 04-15-2015 at 02:34 PM.
Old 04-15-2015, 04:02 PM
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I'm a spectator to this thread, but I gotta wonder about five degrees of decalage... that is a rather large number for models. It is not surprising that a lot of down trim is needed and not perhaps because it is tail heavy, but because of the decalage number. It seems to me that such a number if used on the full scale was to counter a heavy engine and forward CG and should not have been emulated on a model. If that is the case, following that scheme on a model would produce poor flight qualities especially given that models are very often given more tail surface than full scale for the purpose of added stability.

.
Old 04-15-2015, 05:04 PM
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Originally Posted by AA5BY View Post
A lot of room for discussion about your plane. Is the elevator sensitive? You could run the numbers using the aircraft CG calculator using a 15% static margin and compare that with where you are currently balanced. If the CG is in the zone, the significant down trim suggest an incidence problem and they often manifest differing trims for powered level flight and the landing glide slope.

Some ideas. Can the wing or stab incidence be adjusted to make the elevator more neutral? If the ailerons are on separate channels, re-flexing them up 3/16 at lower throttle settings would likely reduce tip stalling.
Thanks.... You're right. I've used the CG calculator and it comes out at roughly the same. I will however increase NP-CG margin from 10% to 15% using a solid brass spinner and see how that works. Cannot alter incidence of either and yes I have ailerons slightly up when selecting 1/2 flaps or more to simulate downwash.
I've just finished readjusting the bracing wires to twist in more downwash. It was a handful but hope that does the trick.
Old 04-15-2015, 05:29 PM
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jossurf
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Originally Posted by Richard m Crapp View Post
There are a few things going on with the PT22 . If you imagine a fuselage Centre line with the engine on zero the wing should be + 3 deg and the tailplane at - 2 deg. (Trailing edge up) This is partly to counter the gear tredels that give a nose down force. Also because of the very foreword C/G. I don't know why it is but perhaps the 4 deg wing SWEEP contribute or the foreword position of the wheels.

If your model flies badly I would put the CG at 25% at the root and give at least 1 deg WASH OUT. (twist it in with the wires) Add a bit more with the ailerons.
Be prepared to run out of up, or flair, on landing.
If you do, move the CG slightly back until you can flair then stop.
On my model the CG position is on the back of the strut fillet from the Fuselage to gear . It MUST be their or it will never fly well!
To help add weight try a solid steel spinier nut or/and a huge fuel tank.
Hope that helps, yours looks a lovely model. I would like to here more about it.
Thanks... As mentioned in my reply to 5BY you can see I'm sort of following your advice. Will fly first see how it goes. If not I will consider adding Gurney flaps on trailing edge.

Thanks guys glad somebody came thru on this old thread.

BTW The real Super Ryan is owned by Richard Wayne Hamlin of Wellington, Kansas. See his short video: https://www.youtube.com/watch?v=ZlBlTCyQhdA
Old 04-15-2015, 05:44 PM
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jossurf
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delete

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Old 04-17-2015, 10:07 PM
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robert waldo
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I use to have a PT-22 so I can appreciate what Mr. Ford did. The Kinner will teach you to always look for a place to land. The PT-22 unlke it's civilian counterpart didn't have any washout. That combined with the swept leading edge and large gaps in the control surfaces resulted in a particularly nasty snap stall that has killed many a pilot. It's a very easy airplane to fly and land but will bite if you get it slow and load the wing with the ball off center. I good demo was to roll it into a standard rate turn at 90 mph while holding a constant bank angle and altitude, reduce the power and let the airspeed start to bleed off. The only warning you would get before the snap would be a slightly less stick force that most pilots failed to recognize before it was to late. As a matter of fact in 40 years I can't think of another airplane that tip stalls like that little mother. Except a DC-3. But that Old Gal doesn't stop flying until your well below 60 knots. You'd have to be a ham handed moron to stall a 3.
Old 04-18-2015, 06:41 AM
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Originally Posted by robert waldo View Post
I use to have a PT-22 so I can appreciate what Mr. Ford did. The Kinner will teach you to always look for a place to land. The PT-22 unlke it's civilian counterpart didn't have any washout. That combined with the swept leading edge and large gaps in the control surfaces resulted in a particularly nasty snap stall that has killed many a pilot. It's a very easy airplane to fly and land but will bite if you get it slow and load the wing with the ball off center. I good demo was to roll it into a standard rate turn at 90 mph while holding a constant bank angle and altitude, reduce the power and let the airspeed start to bleed off. The only warning you would get before the snap would be a slightly less stick force that most pilots failed to recognize before it was to late. As a matter of fact in 40 years I can't think of another airplane that tip stalls like that little mother. Except a DC-3. But that Old Gal doesn't stop flying until your well below 60 knots. You'd have to be a ham handed moron to stall a 3.
Thanks Robert. That figures... As a model it certainly is not a beginners plane. Can't say I haven't been warned!

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