Top Flite P51 - 65" span

[Peter Jenkins]

Thanks Adrian. There was a moment when I almost decided to junk the doors but a little voice said "no way"!

[Peter Jenkins]

After a detour to the dentist to get one of my molars almost out - he had to give up after 1.30 mins of tugging, drilling and heaving - I've managed to get a couple of small jobs done on the Mustang.

The tailplane and elevators have had their tips sanded to shape and the inboard end of the elevator construction, the addition of a coupling of fairing blocks added to the elevator, shaped and then cut off and glued to the fuselage. I'm not sure why this is called for as I cannot see this on the full size. Still, it's done.

The next small job was to glue the rest of the upper fuselage formers in place including the instrument panel that is provided with a gauge to get the angle of tilt correct. Again, not sure why this is designed this way as the full size instrument panel if vertical and not sloping.

[Peter Jenkins]

Things have slowed down a tad so I've not posted till I had made a decent bit of progress.

I've now added the stringers to the front and rear of the fuselage top formers. The next job was to cut the sheeting for the front top of the fuselage from 1/8" sheet. The instructions tell you to glue the bottom of the sheets to the fuselage and, when the glue is dry, to damp the sheeting and bend it over to glue it to the other formers. I decided to cut the sheet so that it ended at the mid point on the stringer at the top of the fuselage before I glued the sheeting in place.

This photo shows all the stringers in place and the 2 forward fuselage sheets glued into place.

and the view from the front end:

Once the glue had set, as suggested by the instructions, I dampened the back of one of the sheets, made sure that it would wrap around and end at the mid point of the top stringer and then glued it in place supporting it with pins:

Once that was dry, I dealt with the other side in the same manner. The next picture shows the two skins glued into place with all the supporting pins removed.

[Peter Jenkins]

The next step was to cut the rear fuselage sheeting to shape and then glue that in place. Again, because of the curvature of the fuselage formers, I chose to glue the skins to the fuselage at the bottom and, when the glue was set, to glue and then clamp and pin the fuselage sides to the formers.

Once set, it was back to the razor plane and sanding block to trim the fuselage skins level with the formers.

A hunt through the remaining wood in the kit box produced the 1/2 inch fuselage top decking. This had to be shaped to fit over the tailplane and rest against the front of the fin. I at last had a decent job for my curved Permagrit sanding block - the one with the grit on the outside! Once that was done and the decking roughly trimmed to the fuselage top, I glued that in place.

Once that was set, it was out with the razor plane to get the bulk of the balsa removed in shavings rather than dust! Razor planes really are worth their weight in dust!

Final shaping was done with a long Permagrit sanding block, coarse first and then finishing with the medium grit.

[Richard Clark 2]

Posted by Peter Jenkins on 02/07/2020 14:41:12:

Nigel, you are right provided that the wood in the kit is designed for a light build. In practice, in these older kits, my experience has been that they were built to survive a crash landing so, those of us who learned how to land, pay the penalty in weight. Today's light weight models with formers fretted out to save weight do not survive a robust arrival and tend to have the u/c plate ripped out early on.

So, you have to be realistic about the issue of what size engine to fit. I flew a friends TF Spitfire with an OS 90. It was grossly underpowered given its weight. You had to dive at full throttle to work up the speed required for anything other than a tight loop. It was not fun flying it as I was always on tenterhooks lest it stall. Even a pull out had to be gentle lest it tip stalled.

On that basis, I decided that the OS 120 was a better bet as I want to be able to fly the Mustang like the full size and carry out large sweeping manoeuvres.. It doesn't need all the power to be used all the time. I fly F3A competitively and learning how to use the throttle properly is something that took me 2 years. OK, I'm a slow learner but when I watch newcomers to aerobatics they struggle just as I did with both throttle, and the other essential control, the rudder.

This is not to say that I think it's a good idea to fit a 30 cc petrol in this model but if proper throttle control is used then it shouldn't be a problem. The problem is, what is proper throttle control! Back to my earlier point.

 

You and Nigel both make valid points, but remember these TF kits were designed for the non-schnuerle ported 60's of the period and flew perfectly well and in a scale like manner. (The  tip stalling you mention may be a 'high speed stall' caused by an over-weight plane  in a high G manoeuvre.)

Though your 120 won't do any harm as the planes are strongly built, as you say. The Spitfire wing excessively so - I actually had to boil the top ones of the two gigantic basswood main spars and pre-bend them while hot and wet to fit.

A purely personal opinion but I don't think these TF kits were very good originally, and the 'Gold Editions' are no better. The Spitfire has a grossly over-thick wing which ruins the appearance (the Spitfire had the thinnest wing of any WW2 fighter) and there are several other severe  construction faults, all of which would greatly  puzzle those without  enough experience to make their own changes..

And pre-warned about the Cessna Skyhawk tearing its wing  off carrying half the cabin with it in a 'sudden'  manoeuvre I greatly modified mine.    

As for 30 cc in the P-51  I think it's barmy

Edited By Richard Clark 2 on 18/07/2020 03:40:43

Edited By Richard Clark 2 on 18/07/2020 04:00:53

[Peter Jenkins]

I thought it would be useful to research some power to weight ratios in response to your post Richard.

The P51D is quoted in my reference book as having an empty weight of 7,125 lb and a combat weight of 10,100 lb.

The same book lists the Packard Merlin as rated at 1450 hp or 1695 hp at war emergency power. ( At 1 hp = 745.7 watts this converts to 1,081,264 watts and 1,263,962 watts)

Let's assume the TF Mustang comes out at 11 lb ( probably a little heavier than I think my example will) and we then need to look at power outputs of the OS 91 FS, OS 120 FS and the DLE 30. These are given as 1.6 hp, 2.1 hp and 3.7 hp respectively. I have assumed that these power figures will not be reached in real life as we will want to pass noise tests. (Converting to watts and reducing to real world figures gives 1,118.5 watts, 1416 watts and 2,500 watts respectively.

As a comparison, my F3A aerobatic aircraft, a Citrin biplane, weighs 11 lb and produces 2,800 watts measured in the air. This gives us 254 watts/lb. The Citrin has unlimited vertical performance.

Doing the same calculation on the P51D and the model gives the following results expressed as watts/lb:

• P51D at combat weight and normal full power 107
• P51D at combat weight and war emergency power 125
• P51D at empty weight and normal full power 152
• P51D at empty weight and war emergency power 177
• TF P51D with OS 91FS 101
• TF P51D with OS 120 FS 128
• TF P51D with DLE 30 227

From this you can see that the OS91 powered model will have slightly less power to weight ratio (7 watts/lb) than a full size P51D at combat weight and full normal power while the OS120 powered model has very slightly more power to weight (3 watts/lb) than a P51D at combat weight and war emergency power. Whilst the P51D empty weight represents the absolute best power to weight for the P51D this would not be realised in real life since no one wants to be flying a P51 that is empty as it excludes the pilot!

I, therefore contend that the OS 120 is the correct engine to represent a P51D performance at combat weight and war emergency power. If you take a weight of 8,600 lb for the full size and use WEP then this becomes 147 watts/lb which comfortably exceeds the model's power to weight ratio. Your comment that the 120 will do no harm is therefore fairly comprehensively debunked I think! It is the power to weight ratio that determines vertical performance and, were it not for real world issues such as the speed of sound i.e. wave drag and compressibility effects that limited the full size's diving speed, the full size would undoubtedly have gone much faster. The model does not suffer the same issue of being close to Mach 1 in a dive so will go relatively much faster than scale. This can be resolved by not diving at full throttle - a bad habit in my opinion anyway.

Now, I agree that the DLE will over power the model, and I said so in my earlier post, but there is such a thing as a throttle that a competent pilot ought to be able to utilise to produce a realistic performance. At full power, this combination is still some 27 watts/lb less than in my Citrin so I would think that even with a DLE 30 the TF Mustang at 11 lb would not have unlimited vertical performance. It would be more than adequate though! Unnecessary without being barmy perhaps?

I don't think that the TF kits were ever marketed as having out and out scale accuracy. I cannot comment on the thickness of the Spitfire wing as I've not built one. It was not my intention to build a 100% accurate scale model but one that was pretty close. That's why I've chosen to have a retracing tail wheel, operating secondary doors and scaleish hingeing of the control surfaces as these can be readily spotted by almost anyone.

Finally, on the point of high speed stalls, as an experienced full size pilot, I'm sure that you realise that the phenomenon is caused by exceeding the stalling angle of attack of the aircraft and can happen to any fighter type aircraft at high alpha regardless of whether the aircraft is over weight. The tip stalling is a function of how accurately the wing was built and the amount of washout incorporated. I have no idea if the Spitfire I flew had washout accurately incorporated. If done properly, the wing root should stall first and not the wing tip.

The only constructional difficulties I've experience with my Mustang have been of my own making i.e. operating secondary main doors and a retracting tail wheel and operating doors. On the latter, the TF manual tells you that you are on your own on this feature. Apart from that, I've not found any constructional errors - so far. As this is my first TF Gold Edition kit I cannot comment on other kits but as far as I'm concerned this kit, provided you follow the instructions, goes together without any major issues. I have chosen to add specific strengthening mods to the main undercarriage area but that's it.

Just my point of view of course.

[Peter Jenkins]

Ron Gray very kindly made me a 3D printed Mustang instrument panel and gunsight. So, as soon as I got the bits home, I trimmed the panel to fit and left the gunsight on top of the cockpit coaming. It will need to sunk so that only the glass reflector is above the coaming.

Thanks Ron.

[Peter Jenkins]

The next job was to fit the ply dorsal fin. This needed some minor tidying up of the die cut part and then cutting out two balsa parts of the same pattern but with an additional 1/4" at the rear to allow the side parts to fair into the fin. The next task was gluing the ply dorsal fin in place making sure it was dead centre!

The instructions recommend shaping the two balsa parts before gluing them in place. This is a lot easier than trying to do so once stuck in place. The first bit was trimming the extra 1/4" of the balsa parts so that they fitted snugly to either side of the fin. Then they needed to be shaped along the top of the dorsal fin to provide a nicely profiled shape. The photo below shows the first shaped part with the unshaped one.

The other side of the shaped part showing the rebate for the front of the fin.

The next photo shows the balsa parts glued into position and clamped up.

And this shows the completed dorsal fin with the clamps removed.

There is still some more shaping to be done but that's covered later on in the build.

[Ron Gray]

It's the little bits that take the time, but coming along nicely Peter.

[Peter Jenkins]

They certainly do, the little blighters!

[Mike Mueller 1]

OK folks.........here is another question!!!

On the instructions, it mentions rudder and elevator counterbaances............I have looked over and over, but can't seem to figure out just what they are......anyone want to educate a dumb builder??? Thanks!!!!!

[Peter Jenkins]

Hi Mike

If you look on the back page of the building manual (page 60) you will see a two view drawing of the Mustang. In the plan view you will see each elevator has a bit extending forward of the hinge line out towards the tip. On the side elevation view you will see the same thing on the rudder. Look at the wing/tailplane and fuselage plans in these places and you'll see some dotted lines drawn and these show where these items need to built. You don't need to do this since their purpose was to reduce the load the full size pilot had to exert on the rudder and elevator. As the control surface was deflected, the small balance tab would stick out into the airflow on the other side from the rudder or elevator. Aerodynamic drag on these bits then reduced the force the pilot needed to exert to achieve the desired movement of the aircraft.

On Page 1 of this thread right at the bottom, I have a photo of the build showing this.

On Page 2, the first photo of 303 Sqn Mustangs shows both the fin and elevator counterbalances.

Page 7, the last photo shows the fin and rudder with the balance in place

Page 8 , first photo shows the fin and tailplane with the balance tab cutouts.

Hope that describes where to find them on the plan and the instruction manual. You need to build a sub-structure so that when you cut away the structure to make the aperture for the balance tab there is structure left behind to carry the aerodynamic loads around this "hole"!

Edited By Peter Jenkins on 27/07/2020 15:08:58

[Mike Mueller 1]

AH!!!!! NOW I UNDERSTAND!!!!!! Thanks so much..........Had NO concept as to what they were.........now I do, and love the look.....I'll be putting them in...........

Peter: I can't tell you how much I appreciate what you have done here..........the thread is almost as important as the instructions itself........I built a Goldberg Jungmann Bachmann, and the instructions were incredible.........I don't find as good with Top Flite, but I have an added "assistant" with your thread!!!! I can tell you that anyone who is building this plane appreciates all you have done to help us!!!!!

[Peter Jenkins]

Thanks for the kind words Mike but remember, in the land of the blind the one eyed man is king! I'm no great shakes at building but I do know a bit about aeroplane structures and how they fly.

[Peter Jenkins]

The next job was to fit the cowl blocks and to do that I needed to bolt the engine back into place. Next job was to tack glue the spinner backplate plywood to the spinner using spacers. As I'm using a flexible engine mount the spacers needed to be 3/16".

Then it was a matter of bolting the spinner to the engine. I used a spare wood prop as the spacer so that the prop nut had something to space it off the unthreaded part of the shaft.

After that, it was a matter of trimming the 3 balsa blocks to fit snugly between the spinner backplate and the firewall. Once the blocks were a snug fit, I used aliphatic to glue them in place.

Then, it was a matter of planing and sanding the top and side blocks so that the remaining 2 blocks, cut from the provided 1/2" sheet into place. Once the glue was dry, it was out with the David plane.

 

and then the sanding block

At this point, I realised that I had not allowed the top block to sit high enough so the spinner sat a tad proud. No problem, I stuck on a piece of 1/16" balsa to add a bit of meat and sanded that to shape.

I took the opportunity to fill a few gaps and the odd bit of hangar rash (already!) and then brought the sanding block into use.

Edited By Peter Jenkins on 28/07/2020 00:19:38

[Richard Clark 2]

Posted by Peter Jenkins on 25/07/2020 00:25:15:

I thought it would be useful to research some power to weight ratios in response to your post Richard.

........etc.

Just my point of view of course.

Peter,

First thing I will say about your very comprehensive post is Wow!!! You certainly look into these things thoroughly

I will cover our points of disagreement but please note I am not disagreeing 'on purpose' merely to be 'contrary' and I am fully aware that it doesn't actually matter to you whether I disagree or not.

I don't think comparing full size and model power to weight ratios means anything as WW2 aircraft flew on the 'wing', never on the engine thrust as many of our models can - sustained vertical or near vertical climbs. No WW2 aircraft could do that.

Also it's wing loading that counts, which depends on relative  wing area. You didn't state it  and I can't be bothered to work it out.  It's  very different from one fifth, one sixth scale,   or whatever.  The Spitfire Mk V   was 27 pounds per square  foot,  vastly different  from our models whereas the power/weight ratio  is quite close.

High speed stall. The aircraft's weight is the mass multiplied by the G force. The AoA required to sustain lift at any given airspeed depends on the weight and a 'high speed stall' can occur at any airspeed depending on the G force. I've deliberately induced one by fairly gently turning (so I have slightly increased the aircraft's weight while simultaneously slightly decreasing the vertical lift vector) near the aircraft's 'never exceed' airspeed and at full throttle and it is very violent.

As I am sure you know It is the 'incipient spin' which is part of leaning to fly but AFAIK civilian instructors never carry it as far as the above.

I'm surprised you 'debunked' my 120 comment. Maybe I was not clear. My 'no harm' remark meant if  I only had a 120 going spare I would use it in these models without hesitation.

Regards.

Edited By Richard Clark 2 on 28/07/2020 07:16:55

[Peter Jenkins]

Richard

Power to weight ratio is one of the few things that is not subject to scaling. I pointed out that the power to weight ratio of the full size was close to that of the model with an OS 120 (P51D at combat weight and WEP 125 watts/lb vs Model with OS 120 of 128 watts/lb). This will not give a continuous vertical climb. I noted that my Citrin aerobatic biplane had a power to weight ratio of 254 watts/lb and that does possess unlimited vertical performance but it's not a scale aeroplane. So, please read what I have actually written and don't then introduce a different argument about model aircraft having a much higher power to weight ratio than full size. I would agree that in general this is the case but not in the argument I was making.

You raise the issue of wing loading. I measured the Mustang wing tip and root and that gave an average chord of 11.8 " which with a 65" span gives a wing area of 767 sq ft. This is slightly larger than in reality since the root chord is increased by the LE sweep so let's say 700 sq in or 4.86 sq ft. Taking the AUW as 11 lb gives a wing loading of 2.3 lb/sq ft. Quite a long way short of the full size Spitfire Mk 5's 27 lb/sq ft. The Mustang was both heavier and larger than the Spit.

Of course, what's missing from a true scale model are the pilot, the cockpit armour including the armoured windscreen, the guns and the ammunition plus we carry much less fuel.

The final point is that we operate our models at a very different Reynolds number. If we take air density and viscosity at sea level, speed as 60 mph and chord length that gives us the Re. At the tip, the Re is approx 340,000 while at the root, the Re is approx 750,000. Now, the transition from laminar and turbulent flow is around Re of 800,000. So our wing is operating in the laminar flow regime and so as soon as the boundary layer experiences an adverse pressure gradient it will separate. In simple English, as soon as the airflow reaches the thickest point of the wing the wing aft of that point ceases to produce any useful lift. It's too late to go out to the shed and check the chord percentage at which the wing reaches maximum thickness but let's assume it's around 30%. That means that roughly 2/3 of the wing produces no lift and that changes the apparent wing loading pushing it up to around 8 lb/sq ft. Still not close to the full size I grant you. The same holds true for the tailplane and fin which is why a smallish warbird with accurately depicted flying surfaces is such a demanding aircraft to fly.

I would describe the high speed stall as being a function of load factor rather than weight although the two are obviously linked. In a fighter aircraft, I don't know if you have flown one in anger, the high speed stall is generally encountered in high G (or load factor) manoeuvres. As my chums were always fond of saying, " pull to the buffet and hold it there". The buffet was aerodynamic warning that flow was beginning to separate. If the aircraft did not display such a warning then the Test Pilot responsible for clearing the aircraft usually asked for it to be engineered into the aircraft's characteristics as without this warning it is easy to go beyond the aircraft's flight envelope. On the D model Mustang, North American introduced G suits and this allowed the pilot to pull more sustained G than pilots without a G suit but this did result in some Mustangs having damaged wings as they came close to the design limit of the airframe. Anyone who pulled beyond the buffet either experienced a violent snap roll or a mush depending on the way the aircraft behaved. Either situation is inclined to ruin your day.

Can I suggest that we now leave this argument alone as I'm sure it's not of huge interest to most readers. If you want to continue it, please start your own thread. I would rather spend my time describing the build of this model than continue an argument for contrary purposes.

Is that OK?

[Peter Jenkins]

For Mike Mueller.

I forgot to add that the aerodynamic balance also has a weight balance function so that the control surface balances at the hinge line and does not therefore have a moment exerted on it during high G manoeuvres. Control surface mass balances can be hidden in the wing or exposed and in a nicely rounded shape. In the case where an aerodynamic balance is needed as well then they usually double as the mass balance.

[Peter Jenkins]

The next task was to offer up the plastic lower cowl moulding and fit it into place. Once the fit was close, I marked the wood cowling blocks with the plastic cowl outline

Ready to start shaping the wood blocks.

Trial fit. It was clear that the profile of the wood blocks and the plastic cowl did not match but the instructions stated that final shaping should only be done once the cowl had been bolted into position.

Before doing that, I took the opportunity to bolt the OS 120 into position and then marked and carefully made the cutouts you can see below to allow the cylinder and exhaust manifold to exit the cowl. The hole for the manifold is so shaped such that the cowl can be fitted and removed with the manifold in place but not tightened. I will cut further clearance to allow the exhaust manifold nut to be tightened with the cowl in place not not bolted down.

The next task was to glue in the cowl mounting blocks. The cowl has a 4 slightly thicker areas which are clearly intended for the mounting bolts to pass through. I dry fitted the blocks, one side at a time, to make sure they did not foul any engine part or prevent access to the engine when the cowl was bolted on. I measured the thickness of the cowl and then drew lines on the airframe to show the cowl block positions. I then used 5 min Epoxy to fit the blocks. You can just see the cowl block alignment marks in the photo.

Once the cowl block epoxy had set, I taped the cowl in position and drilled through the cowl and block with a 1 mm drill. After removing the cowl, I drilled out these pilot holes to accept the securing screws and screwed the cowl in place. This allowed me to carry out a final check of how much more wood needed to be removed from the blocks behind the cowl. After removing the cowl, I reinforced the threaded holes in the cowl mounting blocks with cyano and then set to with the David plane and coarse and medium sanding bars.

This photo shows me checking the slope required for the wood to blend smoothly into the cowl.

The finished job showing the correctly profiled wood block cowling onto the plastic cowl.

[Richard Clark 2]

Posted by Peter Jenkins on 29/07/2020 00:37:07:

Richard.......

........Can I suggest that we now leave this argument alone as I'm sure it's not of huge interest to most readers. If you want to continue it, please start your own thread. I would rather spend my time describing the build of this model than continue an argument for contrary purposes.

Is that OK?

Fine by me and I will continue to follow your interesting thread.

[Mike Mueller 1]

OK......found out that my cockpit is cracked.............any suggestions where I get one? Someone suggested just get one from an ARF...........thoughts?

[Peter Jenkins]

Mike, can you post a picture of your problem as I'm not sure of what you mean.

[Bob Cotsford]

Posted by Mike Mueller 1 on 31/07/2020 01:13:11:

OK......found out that my cockpit is cracked.............any suggestions where I get one? Someone suggested just get one from an ARF...........thoughts?

What about trimming one down from the FMS spare?

[Tim Flyer]

Sarik Hobbies do a number of scale canopies .. I’m sure you can find one there too

[Alan Gorham_]

Mike if your TopFlite P-51 is the old p-51B version, then spares are available from Park Flyer Plastics in the USA.

I fully recommend them as I recently bought a replacement canopy for my TopFlite P47D and it was perfect and cheap, even allowing for international shipping.