Engines for Sale

Monday, October 7, 2013

British Frog 100

British Frog 100

Engine Data from AEROMODELLER October, 1950 by L. H. SPAREY
Displacement: 1cc or 0.06cu
Cylinder Head and Fins: Aluminium Pinoy Díevcast,
2 SBA holding-down bolts to Crankcase. Piston and Contra Piston : Meehanite ground and lapped.
Crankcase: Aluminium Alloy. Die-cast. Integral Fuel Tank.
Front End : Aluminium Alloy. Die-cast attached to crank Case by four IDEA screws.
Connecting Rod : Forged, Hiduminium RRöß. Crank pin Bearing : Plain.
Little End Bearing : Pláin.
Crankshaft : Steel Hardened and ground.
Induction : Crankshaft rotary valve.
Manufacturer craft Ltd., Morden Road, Merton, London, S.w.19.
Type: Compressìon Ignition. Specified Fuel : Frog " ".
Mounting: Radial, upright, inverted. or Sidewinder.
Recommended Airscrews: Free Flight
5X3 inches; Control Line 5X3 inches. Or 6X 3 inches.
Recommended Flywheel : 2% oz.
Cylinder: Steel, hardened, ground and honed.
Complete Article
AEROMODELLER October, 1950
One of the first engines to be tested. when this series began; in 1948 was the Frog " 100 ", so that it is particularly interesting to compare the Frog " 100 " engine of today with its ancestor. This engine was altered considerably both externally and internally.
There appears to be an improvement in performance, as the new engine shows an increase in BHP of over 25 percent. It is dangerous to attribute the difference to any particular factor because the both engines were not tested under the same conditions. One factor was the fuel used in the tests and it alone may have attributed to the performance increase. In any event, it is not highly important to the average user to know the exact cause of the improvement, the salient fact is that from an engine of identical capacity the average aeromodeller’ may now expect a. much improved performance. As with all Frog engines it displays great flexibility, as it ran evenly and steadily at speeds from around 1,000 r.p.m. to 9,000 r.p.m.. In this connection it may noted that the makers claim a maximum of only 6.500 rpm. They also claim. that the engine weigh is heavier than the checked weight obtained by me. In contrast with the exaggerated claims made by some manufacturers­ these days, it seems that it is being realised that a modest claim is even more likely to create buyer confidence than an exaggerated one; and in the long run a much sounder form of advertisement. This tendency has been noted before ín these pages.
The: Frog engine should be of partícular interest to f'reeñight flyers, in view of the. fact that the greatest output lies around the 8,000 rspm. mark. This is the approximately the speed at which propellers usually turn in freefight flying. 1t is also not also a speed not difficult to attain if the propeller is carefully selected. Furthermore, thc speed is reasonably low, so that the engine need not rack itself to pieces in :an endeavor to obtain maximum output at some phenomenal rate of revolution.
An interesting experiment on this motor was carried out in addition to the usual B.H.P. test. The Frog 100 engine is very easily convertible to upright or inverted running, and a series of figures was. obtained for the. engine in both pusitìnns. These results showed so little variation: from one position to the other, that it can be said that the performance remains the same irrespective of the engine's position. I am not aware of any concrete facts on this subject have hitherto been available and from that point of view this information may be helpful. It must be remembered, however, that data. applicable to one type of engine may not neeessariïy apply to other engines of different design and manufacture.

While the Frog diesel engine is of quite pleasing appearance and general proportions, it does seem to be rather on the large side for its capacity. The chief criticism is that, it is too high and on taking thè engine píeces, the reason for this height is easily dìscpvered. The contra~piston is extremely long. This is probably done in order to ensure a good seal in the cylinder.
Test Results:
Test Engine: Frog " 100 " Mk.ll Diesel.
Fuel: "Frog PowerMix." 
Starting : Extremely good under all conditions. 
Running: Shows great flexibility. and :ran well at all Speeds between-about 1,000 and 9,000‘r.p.m. It was not found possible to exceed 9,600 r.p.m.
Power: The curve shows a flat characteristic between 7,000 and 8,000 r,p.m. with a. maximum output of bh.p. at around the 8.000 mark. 
[The Frog 100 " engine testçd in 1948 gave 0.0575 h.p. at 8,100 1. Output declines steadily down to about 1,000 r:p.m., below which a steep drop is indicated. At 9,600 r.p.m. the output is down to 0.05 h.p.
Checked W'eight: 3.75 ozs with tank
Maker's advertised weight: 4 ozs.
Power to weight ratio : 0.304 b.h.p./lb.
Narne : Frog 100 " Mark II. Manufacturers : International Model Air
Remarks : This new Frog engine displays all the characteristics of easy starting, flexibility, and reliability, associated with the range.

Frog family of engines in the collection


 Featured engine with the replacement parts required to get it to run reliably

 Other Frog engines in the collection

 Test run with an APC 7X4 propeller and a fuel mixture containing extra castor oil because of the tight fitting parts.

Published on 2 Aug 2012 Welcome back folks for the rebuild story of wonderfulcat's Frog 100, here on YouTube. It was first thought that the job was only to make a new contra-piston, which is a easy fix. So  said so done, but after second short run the engine lost hot a lot compression, so warm restart was impossible. We then decide to go for a fast and cheap solution. The "growing process"! With this process we heat the object to 850 degrees C and slowly cool down to room temperature. By following this process its possible to increase the diameter by approximately 2/100 mm or more, which should be enough in this case to improve the fit enough for reliable running. Only a quick use of some grinding pasta and a lapping ring will be required after growing should be enough. After doing that, the real trouble arose.The previous almost perfect sealing piston, began to loose its fit after several runs and again low hot compression!  He maybe a runner now, even hot,...but its far from perfect. So I decided to take a closer look inside the nice cylinder. It was very smooth,and shiny...but after measuring...it had almost no tapper. That's,why all the work was a failure. After regrinding the lower part and polishing it to a mirror like finish, I made a new piston. Using my basic material source: old car engine camshafts ( made of high grade cast iron) ....see on my Deezil and MK -16 clips of my piston making techniques.....I always make my piston somewhat lighter, if possible for better performance, and......so I do the same in this case. That will pay off as a later rewarded. The 2,5mm wrist pin was already also somewhat loose in the small end of the rod, but thank God........I have collected needle rollers, size 2,55mm......Lucky me!!..... the perfect size, only a bit reaming needed.....and I am finished. I also straighten the bent spray bar and make a new needle extension,with a nice ball end to it. The compression was very healthy now, but starting was a real problem. I found after a closer look at the carburetor the bent spray bar, also damage to the sealing surface on the outside of the intake, so I made that area flat again and made on my mini lathe 2 copper, very thin rings to get proper sealing there again. Also it needed an alloy washer to prevent the extreme axial play in the shaft, I have seen this many times before on older Frog's MKI's and MKII's. Further there was a need for 2 new gaskets and an internal good cleaning in my ultrasonic bath. But all the effort ...will pay off finally,..after many small setbacks. he now screams very well, for a long stroker from a long time ago. Soon he will be hit the sky in a lovely small freeflight plane again...like in his young days....I really hope that Steve likes the job we did on him...
See it run after a major rebuild.


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