The P-51

Is nearing completion after twenty five years.  It is now painted and motor and radio gear fitted.   Discovered however is a balance issue with an often found tail heavy condition. Years ago hardwood 1/4 dowel push rods had been fitted and the balance point was found to be three inches aft of the landing gear mounts whereas it should be one and one half inches aft.  There were actually three push rods with the third for the scale tail wheel location.  A short list of things to do to minimize the amount of lead needed in the nose was made including; 
 I expect some nose weight will still be needed but not as much.  Hopefully many ounces will have been reduced to a few.  I'm guessing that half the weight of the push rods was eliminated.  If all goes well, the plane should fly within a week or so.  I'm not without a bit of trepidation however.  This plane was designed as a scale builders model and they sometimes suffer flight performance as models... very often being heavy.  It also has a foam core wing, almost always heavier than built up.  Hopefully the wing covering used as a compromise will help this.  Light sheeting materiel was chosen and covered with polyester to save weight.  The trade off is that without serious filling and weight buildup from doing so, the polyester leaves a rather matted look to the finish paint.  I felt that with the extra bonding glue weight needed to re-sheet the wing, some caution had to be excercised in the weight department.  Also, a lot of sanding was done on the fuselage to get as much old paint off as possible but it likely came out heavier for having been painted twice. 

The plane has been tentatively weighed and is expected to come in around nine pounds plus any balance weight needed.  After the changes noted above, the 5 lb 10 oz plane now balances one inch forward of CG but the 3 lb 4oz wing balances 1 1/4 inch aft so it looks like the effort to correct the CG issue has worked and no additional weight will be needed.  Interesting is that the plans are fairly well done but the weight line in the spec box is blank.  At nine pounds, loading would be 30.3 oz/ft, not excessive for a war bird so it should fly.

I was very wrong with the initial thoughts that the weight issue had been corrected.  Needed was 9 oz of weight added fully forward in the motor compartment cowl.  The ready to fly weight is 9lbs 9 oz for a wing loading just above 32 oz/sq ft.

The P-51 Is Finished after Twenty Five Years (April 10, 2008)


The P-51 finally took to the air on a cool blustery Sunday evening just before dark.  The wind blew hard all day and I waited till late knowing that no one would show at the field anyway and perhaps the wind would slacken in the evening hours.

As expected, a good bit of right rudder was needed to keep the plane from drifting left and avoiding overcorrecting was difficult.  As well, rotation required significant elevator.  A plus was that power up produced no nose over tendancies.  Roll axis was quick, much too quick for my liking and fortunately the plane flew nearly in perfect trim with only two clicks of roll trim needed.

With the plane high, it was slowed and the 1st flap position set with a resulting strong pitch down.  I'd have to land hot without flaps.  Power off sink rate was significant as expected with the heavier wing loading and with the strong headwind power was kept up to do a hot  wheel landing.  I gave elevator too quickly to hold the tail down and it ballooned.

Both Aileron and elevator throw was diminished to 80% with 50% exponential.  Rudder was also given 50% exponential.  When preparing to mix some elevator up trim with the 1st flap position,  I noted that the standard radio settup all ready had some down elevator mix... it was causing the down pitch and readjusted to no pitch change at the 1st flap position. 

The second flight saw a better take off roll as I had a handle on the need for a good bit of rudder and the rudder was less sensitive given the exponential.  Breaking ground however required a lot of elevator.  Roll rate was much better.   The 1st flap position was tested and no pitch was noticed.  After boring a few holes in the sky, the plane was lined up and flaps engaged and power set for a good glide slope.  When ground effect was reached, up elevator was given and it needed full up to keep the nose up.  The elevator was given some differential with the up throw increased.  The motor had sagged very slightly in loops so it was richened a few clicks, otherwise it ran flawless.  

Another take off and landing and the increased up elevator throw was better and alowed flairing with a 1st position flap landing.  Two of the four landings for the outing suffered from a bit of bounce on landing.  With the brisk wind, I was simply happy to get it down without mishap.  Given the very blustery headwind, we will see how it goes with some smoother air.

The HB-61 seemed well matched.  It is turning 11,700 on a 12x6 APC.  This faster plane might enjoy a stronger pitiched prop, perhaps an 11-12 x 8.  The plane is much faster than most in my hanger and the last flight with the loss of some daylight saw visibility challenging with the silver airplane.  I was also growing cold and hands were getting numb.  With the buffeting winds, I didn't do an inflight balance test.  The plane is not overly sensitive on elevator with quite a bit of throw and it took a lot of elevator to get it off the ground and a lot to flare it so it might be nose heavy and if so I'd be very happy to get rid of some of the nose lead that was added,  though I don't want to experience this plane even close to tail heavy as it does a very wicked snap roll and I believe will have no problems doing spins.  Best I could tell in the strong winds, it tracks through very large loops nicely and climbs without major amounts of right rudder.  It has good vertical performance and will do large loops and cuban eights. 

Rolls seem a bit sloppy and might benefit from some differential but with the single servo on ailerons and the bell cranks locked within the wing, there will be no differential.  Multiple aileron rolls did seem much better after exponential on the elevator was set and it was given differential with more up than down.  A little down elevator will pitch the nose down sharply but a corresponding up pitch requires a lot of elevator.

The results of an inflight balance test will be interesting.  Current balance point is set per plans with the exception that it has the position point above the wing whereas I balanced under the wing though that shouldn't make a difference.  In argument that the plane is nose heavy is the requirment of a lot of elevator to rotate and a lot of elevator to flare the stall but those could be higher wingloading virtues.  In argument that it is not nose heavy is the wicked snap rolls it can do.

P-51 Flying report from outing two

 The second outing was only a few days later and several of the guys were at the field to watch.  I discussed with them the issue of needing a great deal of elevator to get airborn.  They offered that it could be nose heavy but to be very cautious about getting CG aft on a war bird.  My plans were to do an in flight CG test using the 45 degree down glide method an observing for either a curl up or down tuck. 

To my great surprise, the plane lurched into the air as it was gaining speed and was eight to ten feet up before reaction time allowed getting the nose pitched for level flight.  I was fortunate it hadn't snapped.  As altitude was being sought, my thoughts were how could it be that a few days ago it took great amounts of elevator to get airborn and today it lurched into the air before it had obtained full run up speed.  One of the guys hollered, "what did you change?"  "Nothing", I replied.  A couple of seconds later, it came to me that maybe I did change something and the light turned on about what it was.

This plane is the only plane I've owned where the wing hold down bolts are in the front of the wing rather the rear and is this way because the radiator scoop on a P-51 is under the wing aft and this is part of the fiberglass fuselage on this model.  Hence, the wing is tucked between it and the fuselage and the bolts are located near the leading edge in a faired section of the middle of the wing.  What had happened was that on the first outing, I'd not tightened these bolts as much as today and the slight bit of incidence change produced dramatically differing results from the foiled stab.  I loosened the bolts very slightly for the next flight getting the fairing section as even as possible and this proved to be the sweet spot.  The next takeoff was perfect requiring only a bit of elevator to get airborn.
The bird was rung out a bit and a down glide was done to check the CG... No upturn or tuck under meant it was ok.  So, the plane was put through some paces and it will do the dramatic lumcevak tumbles following a snap roll.  These were first done very high after checking first that the plane will pull out of a spin without issues.

On the third flight of the day, something was heard when the plane touched down and was discovered to be a wheel strut torsion arm block had broken loose inside the wing.  Either it had not been glued adequately or the old brittle epoxy failed.   There was no damage otherwise to the plane, which continued a normal landing roll.  The gear on this plane has a generous forward rake and unlike many war birds, has no issues on our rough grass field.

The area of most concern has nothing to do with the flight... I'd forgotten how nasty a .60 size two stroke glow powered plane gets.  I'm spoiled by the gassers when it comes time to clean em up.